Technologies For Rf Systems Terry Edwards

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Technologies for RF Systems
Terry Edwards

Library of Congress Cataloging-in-Publication Data
A catalog record for this book is available from the U.S. Library of Congress.
British Library Cataloguing in Publication Data
A catalog record for this book is available from the British Library.
ISBN-13: 978-1-63081-450-2
Cover design by John Gomes
© 2018 Artech House
All rights reserved. Printed and bound in the United States of America. No part
of this book may be reproduced or utilized in any form or by any means, elec-
tronic or mechanical, including photocopying, recording, or by any information
storage and retrieval system, without permission in writing from the publisher.
All terms mentioned in this book that are known to be trademarks or service
marks have been appropriately capitalized. Artech House cannot attest to the
accuracy of this information. Use of a term in this book should not be regarded
as affecting the validity of any trademark or service mark.
10 9 8 7 6 5 4 3 2 1

vii
Contents
Acknowledgments xv
CHAPTER 1
Twenty-First Century RF Systems and Electronics 1
1.1 Introduction 1
1.2 Abbreviations Relating to Symbols Used in this Book 3
1.3 Antennas 3
1.4 The Challenge of Frequency Bands and Wavelengths 6
1.5 Software-Defined Radio and Cognitive Radio 9
1.6 The Challenge of Noise 10
1.7 RF Receivers 11
1.8 RF Filters 12
1.9 ADCs and DACs 14
1.10 Oscillators, Mixers, and Frequency Converters 14
1.11 Semiconductor Device Requirements 17
1.12 Semiconductor Manufacturing 17
1.13 Diodes and Transistors 18
1.14 Hybrid Circuits and MMICs 19
1.15 The Challenge of RF Power Amplification 20
1.16 Electronic Design Automation 21
References 22
CHAPTER 2
RF Semiconductors 23
2.1 Introduction 23
2.2 Semiconductor Materials 23
2.2.1 Bandgap 24
2.2.2 Drift Velocity 25
2.2.3 Resistors Made from Semiconductors 26
2.2.4 Electron Speed and Transit Time 27
2.2.5 Some Further Important Properties of Semiconductors 27
2.2.6 Semiconductor Manufacturing 28
2.3 Semiconductor Diodes (RF-Oriented) 28
2.3.1 Some Semiconductor Junction Diode Fundamentals 28
2.3.2 P-I-N Diodes 30

viii Contents
2.3.3 Varactor Diodes 31
2.3.4 Noise in RF Diodes 32
2.4 Transistors 33
2.4.1 Introductory Remarks 33
2.4.2 High Frequency Circuit Models for Transistors 34
2.4.3 CMOS and Related Transistor Technologies 35
2.4.4 GaAs and GaN Field-Effect Transistors 36
2.4.5 The GaAs HEMT and pHEMT 37
2.4.6 The GaN HEMT 38
2.4.7 Bipolar RF Transistors 39
2.5 MMICs and RFICs 41
References 42
CHAPTER 3
Passive RF Components 43
3.1 Introduction 43
3.2 Discrete Passive RF Components 43
3.2.1 Capacitors 43
3.2.2 Inductors 45
3.2.3 Resistors 47
3.3 RF Transmission Lines 48
3.3.1 Coaxial Lines 49
3.3.2 Microstrip 50
3.4 Coplanar Waveguide 60
3.5 Substrate Integrated Waveguide 61
References 62
CHAPTER 4
Passive RF Circuit Elements 63
4.1 Introduction 63
4.2 Fundamentals of Directional Couplers 63
4.3 The Lange Coupler 64
4.3.1 EM Structure 66
4.4 Wilkinson Power Dividers 67
4.4.1 Introduction to Wilkinson Dividers 67
4.4.2 Equal-Split Wilkinson Dividers 67
4.4.3 Unequal-Split Wilkinson Dividers 68
4.4.4 Multiport Equal-Split Wilkinson Dividers 70
4.5 Baluns 72
References 74
CHAPTER 5
Switches, Attenuators, and Digital Circuits 75
5.1 Introduction 75
5.2 Solid State RF Switches 75
5.2.1 Some Overall Aspects 75

Contents ix
5.2.2 Reflective and Nonreflective SPDT GaAs FET Switches 76
5.3 Attenuators 78
5.4 Digital Circuits 80
5.4.1 Selected Examples of Logic Gates 80
5.4.2 Digital Signal Processors 81
5.4.3 Electronically Programmable Read-Only Memories 82
5.4.4 Field-Programmable Gate Arrays 83
5.4.5 Provision for Built-In Test and Related Requirements 84
5.4.6 Technology Utilized for Digital Circuit Elements 84
References 85
CHAPTER 6
Radio-Frequency Filters 87
6.1 Introduction 87
6.2 Review of Basic Concepts and Fundamentals 87
6.3 Technology Options 89
6.4 LPFs Formed with Cascaded Microstrips 90
6.5 Microwave BPFs 92
6.6 Suspended Substrate Stripline Filters 96
6.7 Inline Microstrip Filter Structures 97
6.8 Filters Using Defected Ground Plane Technology 98
6.9 Dielectric Resonators and Filters Implementing Them 98
6.10 SIW-Based BPFs 100
6.11 Millimeter-Wave BPFs 101
6.12 Tunable BPFs 102
References 102
CHAPTER 7
Antennas 105
7.1 Introduction 105
7.2 Antenna Fundamentals 106
7.2.1 Near-Field and Far-Field Conditions 107
7.2.2 Radiation Patterns and Beamwidth 108
7.2.3 Directivity 109
7.2.4 Radiation Efficiency 109
7.2.5 Aperture Efficiency 110
7.2.6 Effective Area 111
7.2.7 Gain 111
7.2.8 Equivalent Isotropic Radiated Power 112
7.2.9 Friis’ Equation 112
7.2.10 Impedance Matching 113
7.2.11 Polarization 113
7.2.12 Antenna Noise Temperature 114
7.2.13 Gain-Temperature Ratio 115
7.3 Dish Reflector Antennas 116
7.4 Flat-Panel or Patch Antennas 117

x Contents
7.5 Analog, Digital, and Hybrid Beamforming 118
7.6 Active Electronically-Scanned Arrays 119
References 121
CHAPTER 8
Small-Signal RF Amplifiers 123
8.1 Review of Amplifier Fundamentals 123
8.2 Basic RF Amplifiers 125
8.2.1 Practical RF Amplifier Realization 125
8.2.2 Interstage or Inner Matching Networks 126
8.3 The Vital Issue of Stability 127
8.4 Fundamental Receiver Characteristics Leading to the Need for AGC 129
8.4.1 Toward an Effective AGC Circuit Design 129
8.5 High-Gain RF Amplifiers 131
8.6 Broadband Amplifiers 134
8.6.1 Basic Requirements 134
8.6.2 Balanced Amplifiers 135
8.6.3 Distributed Amplifiers 136
References 138
CHAPTER 9
Noise and LNAs 141
9.2 Noise Factor, Noise Figure, and Equivalent Noise Temperature 142
9.3 Noise Figure for an Attenuating Element 144
9.4 Minimum Detectable Signal 145
9.5 Noise in Transistors 146
9.5.1 Thermal Noise, Particularly Thermal Diffusion Noise 147
9.5.2 Shot Noise 148
9.5.3 Flicker Noise 148
9.5.4 Phase Noise 149
9.5.5 Variation of Noise Figure with Frequency 150
9.6 Overall Noise Figure for Cascaded Blocks 151
9.7 Noise-Matching and Narrowband LNA Design 156
References 159
CHAPTER 10
RF Power Amplifiers 161
10.2 Some Basic Aspects of RFPAs 161
10.3 Transistor Choices, Hybrid Circuits, and MMICs 162
10.4 Power Levels, Power Gains, and Efficiency 163
10.4.1 Internal Transistor Output Characteristics 163
10.4.2 RFPA Output-Input Power Transfer Characteristics 164
10.4.3 Amplifier Efficiency 164

Contents xi
10.5 Compression and Peak-to-Average Power Ratio 166
10.5.1 Compression and a Summary of Main Parameters 166
10.5.2 Peak-to-Average Power Ratio 167
10.6 Error Vector Magnitude 167
10.7 Classifications of Power Amplifiers 168
10.7.1 Class A Amplifiers 168
10.7.2 Class B and AB Amplifiers 170
10.7.3 Class C Amplifiers 171
10.8 Harmonically Matched Power Amplifiers 171
10.8.1 Switched-Mode RFPAs 171
10.8.2 Class F Power Amplifiers 175
10.9 The Doherty Power Amplifier Configuration 178
10.10 The Envelope-Tracking Amplifier 180
10.11 High Power Push-Pull Amplifiers 181
10.12 Other Practical RFPA Circuits 181
10.12.1 Ka-Band PA MMIC Examples 182
10.13 The Distortion Issue and Linearization Techniques 183
10.13.1 Linearity and Intermodulation Distortion 183
10.13.2 Linearization Techniques 185
10.14 Some Final Overall Comments Regarding RFPAs 186
References 187
CHAPTER 11
RF-Oriented ADCs and DACs 189
11.2 ADCs 189
11.2.1 Quantization and Sampling 189
11.2.2 Sampling in Practical ADCs 191
11.2.3 Effective Number of Bits 191
11.2.4 Quantization Error and Quantization Noise 193
11.2.5 Quantization Static Error and Sampling Distortion 194
11.2.6 Sampling Jitter 195
11.2.7 Aliasing and Antialiasing 197
11.2.8 Adjacent Channel Power Ratio 199
11.3 ADC Architectures 200
11.3.1 The Flash ADC Architecture 200
11.3.2 The Folding ADC Architecture 201
11.3.3 Pipelined ADC Architecture 201
11.3.4 Time-Interleaved ADCs 202
11.4 Digital-to-Analog Converters 204
11.4.1 Basic Structure and Functionality of a DAC 204
11.4.2 DAC Resolution, Speed, and Figures of Merit 204
11.4.3 Some Practical Aspects of High-Speed DACs 207
References 207

xii Contents
CHAPTER 12
Radio Frequency Sources 209
12.1 Some Fundamental Aspects of RF Oscillators 209
12.2 Quartz Crystal Oscillators 210
12.2.1 The Quartz Crystal 210
12.2.2 Quartz Crystal-Based Oscillators 211
12.3 Oscillators Controlled by Dielectric Resonators 212
12.4 VCOs 214
12.5 Importance and Impact of Phase Noise 215
12.6 Frequency Multipliers 220
12.7 Frequency Dividers 221
12.8 Phase-Locked-Loop-Based Frequency Synthesizers 222
12.8.1 Basic Configuration 222
12.8.2 The Fractional-N Frequency Synthesiser 222
References 224
CHAPTER 13
Frequency-Band Conversion 225
13.2 Fundamentals of Mixers 226
13.2.1 Basic Features 226
13.2.2 Image Frequency 227
13.3 Diode-Based Mixers 228
13.3.1 The Single-Ended Diode Mixer 228
13.3.2 The Double-Diode Mixer 230
13.3.3 The Image-Reject Mixer 231
13.3.4 Upconverters 232
13.4 Transistor-Based Mixers 233
13.4.1 The Single-Ended FET Mixer 233
13.4.2 Differential FET Mixer 234
13.4.3 CMOS-Based Mixers 235
13.4.4 Mixer Implementing a Cascode Circuit 236
13.4.5 The Gilbert Cell Mixer 236
References 239
CHAPTER 14
Modulation Techniques and Technologies 241
14.2 Amplitude Modulation 242
14.3 Frequency Modulation 245
14.4 Digital Modulation 247
14.4.1 Specific Aspects Relating to Digitally Modulated Systems 247
14.4.2 ASK, OOK, and FSK 250
14.4.3 BPSK and QPSK 251
14.4.4 M-PSK, QAM, and APSK 255

Contents xiii
14.4.5 Spectral Efficiency of the Various Digital Systems 257
14.4.6 Probability of Bit Error or Bit Error Rates 257
14.4.7 Closed-Form Expressions for the Complementary Error Function 259
14.4.8 BER Data Compared 259
14.4.9 Spread-Spectrum Modulation 260
14.4.10 Orthogonal Frequency Division Multiple Access 262
14.5 Transceivers 262
14.5.1 Basic Concept of a Transceiver 262
14.5.2 Software-Defined Radio 263
14.5.3 Full-Duplex Radios 263
14.5.4 Transceiver Modules for Short-Range Radio 263
References 264
Appendix A
Logarithmic Units 265
Appendix B
S-Parameters and X-Parameters 269
B.1 Scattering (S)-Parameters 269
B.2 X-Parameters 270
References 272
Acronyms and Abbreviations 273
About the Author 277
Index 279

xv
Acknowledgments
In writing any book, one requires inspiration, passion, perseverance, time, knowl-
edge (including ‘where to find things’), and, above all these aspects, the invaluable
support of others. Easily my mainstay has come from my wife Patricia Adene, with-
out whose support and patience this book would never have been completed.
There are several other people whom I want to sincerely thank. Among them
are:
•
• Nick Riley, who was my manager at the University of Hull, where I provided
part-time lectures on RF/microwave technology for the communications sys-
tems M.Sc. course. Nick initiated the idea I might write this book.
•
• Don Black and Dave Taunton, whom I taught and who then graduated from
La Trobe University, Melbourne, Australia, with B. Comm. (Eng.) degrees.
Don and Dave were the first people to suggest to me that I should write a
book.
•
• Malcolm Edwards and Andrew Wallace of NI AWR, who were greatly help-
ful in terms of providing access to EDA examples.
•
• Steve Edwards, a good friend who so ably and professionally prepared ap-
proximately 90% of all the drawings in this book.
•
• Brandon Browne, who, at the age of 16, led me to the desmos program and
hence the generation of representative amplitude and frequency modulation
waveforms.
•
• My reviewer, without whom many calculation results in this book would
have been inaccurate and much important material would have been missed.
My reviewer has also excellently served to encourage me during the some-
times quite stressful activities toward the completion of this work.
•
• Oren Hagai, CEO of Interlligent, for his support in the area of signal
converters
•
• Finally, Steve Manton, who expertly rechecked the math.

1
C H A P T E R 1
Twenty-First Century RF Systems and
Electronics
1.1 Introduction
For well over a century, radio frequency (RF) technology has been understood in
sufficient detail for the design of basic communications systems. The application of
RF to radar began in the 1930s and accelerated during World War II, driven by the
pressing needs of the major war effort.
In both instances, communications and radar, electronic vacuum tubes (or
valves��
) dominated the scene regarding almost all requirements relating to active de-
vices. What changed the entire electronics scene forever were the pivotal inventions
of the transistor in 1947 and the integrated circuit (IC) in 1958. Key developments
in microwave integrated circuits during the 1950s (right up to the present era)
ensured an ever-advancing solid state era for RF technology. Twenty-first century
RF systems exhibit the following trends: they are increasingly digital, increasingly
software-based, and almost entirely solid state.
The overall physical dimensions of critical components and devices involved in
communications systems and radars embrace an extremely wide range from mas-
sive communications towers (and phased-array radars) all the way down to the
nanometer-scale semiconductor devices involved in the electronics (see Figure 1.1).
Between these extremes, there exist many types of modules and subsystems that
perform specific signal-processing functions, most of which are described in detail
within this book.
Monolithic microwave integrated circuits (MMICs) typically embody several
transistors on the same chip, some tens through several hundred in the case of rela-
tively complex silicon realizations.
If all the transistors (mainly digital ICs and MMICs) involved all the subsys-
tems involved in the Hamburg Tower were counted up the total would amount to
many billions. And the great majority will be taken up by the highly transistor-
intensive digital ICs.
A technique called space-division multiple access (SDMA) is used extensively
as part of the infrastructure for mobile (cellular) networks. This arrangement is il-
lustrated conceptually in Figure 1.2.

2 ��
Twenty-First Century RF Systems and Electronics
The narrow electronically steerable beam picks out the required user for the
(very short) periods of time required in each instance. The entire system comprises
a highly dynamic and extensively integrated network-of-networks.
Figure 1.1 Physical scales of RF communications technologies (in descending orders of magnitude).
(a) A photograph of the Hamburg Tower in Germany. (Horizon House Publications are thanked for
permission to use this image, a photograph originally taken by Kristof Hamann, in [1].) (b) A MMIC
chip capable of 10W of output microwave power. (Transcom, Inc., are thanked for their permission
to reproduce this image.) (c) A small snowflake shown approximately to scale.

1.2 Abbreviations Relating to Symbols Used in this Book 3
A major trend in mobile communications networks involves the development
and application of small cells and what is termed densification. This will mean far
denser subnetworks than hitherto conceived, mainly around urban regions, global-
ly. It also means quite a dramatic shift toward millimeter-wave links and associated
technology in order to obtain the much higher bit rates (typically several gigabits
per second) that are greatly desired.
The remaining sections of this chapter start with presenting a summary of the
symbols used through the book. Following this there are brief descriptions of cur-
rent and prospectively important antennas and systems. Finally some significant
subsystems are described before proceeding all the way down to the devices and
components level.
1.2 Abbreviations Relating to Symbols Used in this Book
Most of the component and circuit symbols used throughout this book are fairly
standard and a summary is presented in Table 1.1.
The only device not included in Table 1.1 is the bipolar junction transistor,
shown in Figure 1.3.
1.3 Antennas
Transmitting and receiving antennas are often the clearest external evidence of an
RF system (for example, the total number of microwave and millimeter-wave dish
Figure 1.2 Part of a cellular network indicating a steerable beam for SDMA. (In reality, the beam
does not shrink with distance as shown here; actually it expands. The focused shrinkage just indicates
the concentration of the single target user.)

4 ��
Table 1.1 Common RF Components with Descriptions and Symbols
Component Description Symbol
Absorber Material (generally ferrite-loaded) that
absorbs electromagnetic energy
Antenna Radiates electromagnetic energy into
free space
Attenuator Resistive element that adds loss
Balun Transformer that converts a balanced
signal (two signals with no fixed
ground) to an unbalanced signal (clear
common ground)
Bias tee Three-port network that combines DC
with RF or takes a signal and splits it
into DC and RF components
Capacitor Basic element that blocks DC and pass-
es AC and that can also store charge
Circulator Three- (or more) port network that
restricts the flow of electromagnetic
energy to one direction
Coupler Four-port network that splits the input
to two equal or unequal amplitude
outputs and that has an isolation port
Diode Basic element that only passes current
in one direction (the direction the
triangle points)
Diplexer Three-port network that splits into
two ports with different frequency
responses
Duplexer Allows a transmitter and receiver to
share a single antenna
Equalizer Flattens a response (such as gain) over
frequency
Filter Changes the amplitude of a signal
based on the frequency response (band
pass filter shown)
Inductor Basic element that blocks AC and
passes DC and stores magnetic flux
Isolator Two-port network that restricts the
flow of electromagnetic energy to one
direction
Limiter Prevents output power from exceeding
a threshold
Low-noise am-
plifier (LNA)
Amplifier optimized for high gain and
low noise generation
Mixer (down-
converter)
Multiplies an input signal (RF) by a
fixed frequency (LO) to downconvert
to an intermediate frequency (IF)

1.3 Antennas 5
reflector antennas that are fitted on the Hamburg Tower shown in Figure 1.1[a]
approaches 100).
But dish and other large-scale antennas are only a part of the radiating elements
story. In situations where substantial antenna gain is not a priority, it is possible to
design and implement planar antennas and an example is shown in Figure 1.4 [2].
In this structure, metamaterials and composite right- or left-handed transmis-
sion lines (CRLH-TLs) are implemented. A good reference regarding the principles
and applications of metamaterials has been provided by Brookner [3].
Flat-panel antenna arrays are increasingly being implemented into the multiple
input/multiple output (MIMO) systems that are being put forward as potential
candidates for several new types of communications systems, notably the 5G. A
technique known as beamforming is necessary to shape and direct the beams as-
sociated with the flat-panel arrays and either analog or digital beamforming can
be applied. Hybrid beamforming combines the analog and digital approaches. The
article by Amitava Ghosh [4] provides an excellent overview of the technology.
Table 1.2 (continued)
Component Description Symbol
Power
amplifier
Amplifier optimized for high output
power
Power
combiner
Multiport network that combines mul-
tiple input ports into a single output
port with increased amplitude
Power splitter Multiport network that splits a single
input into multiple output ports with
reduced amplitude
Resistor Basic element that attenuates voltage
Switch Basic element that directs a signal from
one path to another
Thermistor Resistor with predictable temperature
response
Transistor Voltage-controlled resistor and basic
element in an amplifier
Varactor Voltage or mechanically tunable
capacitor
Source: [2]
Figure 1.3 Schematic structure (a) and circuit symbol (b) for an NPN BJT.

6 ��
1.4 The Challenge of Frequency Bands and Wavelengths
The unwary engineer might just possibly be forgiven for assuming anyone, any en-
gineer, can simply choose some likely practical-looking frequency band and design
and manufacture the required circuit.
However, this approach would, if unfettered, lead to total spectral anarchy and
instead a disciplined approach has been instituted, principally via the International
Telecommunication Union (ITU). Table 1.2 exhibits the main (RF) frequency bands
designated by the ITU.
There are several further frequency bands accommodating signals below 30
kHz but the RF bands are all cited in Table 1.2.
Microwave bands mainly encompass ultrahigh frequency and superhigh fre-
quency. Millimeter-wave bands (strictly speaking) are within extrahigh frequency.
However, it is notable that extrahigh frequency covers a massive 270-GHz range
of frequencies. Millimeter-wave bands extend to well over 100 GHz, after which
the term submillimeter becomes more widely used (i.e., corresponding to free-space
wavelengths below 1 mm).
The term radio frequency applies in general to electromagnetic signals operat-
ing at frequencies ranging from around 30 kHz through to over 100 GHz. The
fundamental unit of frequency is the hertz (Hz), which means that 1 kHz equals
103 Hz and 1 GHz equals 109 Hz.
Carrier frequencies ranging from 0.5 GHz to 6 GHz are very important for
terrestrial systems (including cellular mobile), while 12.4 to 18 GHz, known as
Ku-band, remains in substantial use for many satellite communications systems.
Figure 1.4 Four-element wideband patch antenna array. (Based on a photograph in [5].)
Table 1.2 Main RF Communications Frequency Bands
Band Name
Low
Frequency
Medium
Frequncy
High
Frequency
Very-High
Frequency
Ultrahigh
Frequency
Superhigh
Frequency
Extrahigh
Frequency
Frequency
Range
30–300 0.3–3 3–30 30–300 0.3–3 3–30 30–300
Units kHz MHz MHz MHz GHz GHz GHz

1.4 The Challenge of Frequency Bands and Wavelengths 7
Meanwhile, millimeter-wave (in practice around 26 GHz and above) is increasingly
used for satellite systems and also prospectively for terrestrial communications’ 5G
links.
Historically originating in radar systems (with rectangular waveguide technol-
ogy) but in general use in most microwave and millimeter-wave systems, there are
the specific two groups of letter-designated bands shown in Table 1.3.
Unfortunately, each sequence of bands shown in Table 1.3 shares two letters:
I and L—although the actual frequency bands differ markedly. As usual, careful
questioning is needed in specific situations to decide upon precisely which frequen-
cies are intended. The NATO bands are rarely if ever encountered outside of certain
military (especially European) scenarios.
It follows, for example, that when a system or a part of a system is described as
operating within the X-band it could be utilizing the entire 8.2 to 12.4-GHz band
or a portion thereof (e.g., 9 to 10 GHz).
The terms microwave and millimeter wave are commonly used and it is im-
portant to clarify these terms as closely as possible. In general, microwave refers to
signals ranging from around 500 MHz (middle of the ultrahigh frequency) to 26
GHz, which is almost the top of the K-band. In practice, millimeter waves extend
from 26 GHz to over 100 GHz. Very often, it is stated that millimeter waves start
at 30 GHz, although this is really a theoretical approximation and in practice 26
GHz is commonly understood. This is important, for example, in view of the first
officially approved millimeter-wave 5G frequency band, which is in the 26–29.5-
GHz range (indicating a 3.5-GHz operating bandwidth).
Higher-frequency millimeter-wave bands such as V-band and E-band are also
important. The definitions are:
•
• V-band: 57 to 64 GHz;
•
• Lower E-band: 71 to 76 GHz;
•
• Upper E-band: 81 to 86 GHz.
Note that the V-band is 7 GHz wide and is principally focused on the often-
used 60-GHz near-center frequency. There is a gap of 7 GHz between the upper
extreme of the V-band and the beginning of the lower E-band. The lower and upper
E-bands both have bandwidths of 5 GHz. The gap between the lower and upper
E-bands is reserved for the nominally 77 GHz automotive (ACC) radars.
Table 1.3 Standard Frequency Band (Letter) Designations Compared (from 1
GHz upwards to 100 GHz)
Waveguide
Bands
1.12 2.26 3.95 8.2 12.4 18.0 26.5 36 46 56 100
Designation
Letter
L S C X Ku K Ka Q V W —
NATO Bands 1 2 3 4 6 8 10 20 40 60 100
Designation
Letter
D E F G H I J K L M —

8 ��
Also note that V-band is being defined in the usual way associated with milli-
meter-wave link applications, that is, not the relatively broad IEEE definition that
extends from 40 to 75 GHz.
It is vital to be able to convert from frequency to wavelength (and vice versa).
The standard general expression is
( )
= m
v
f
λ (1.1)
here λ is the wavelength (m), v is the velocity of the wave (m/s), and f is its frequency
(Hz). For electromagnetic waves in free space or air, v is very close indeed to 3 ×
108 m/s. This 3 factor accounts for the extensive use of 3 in the frequency bands
cited in Table 1.2, because this makes for relatively rapid calculations of associated
wavelengths.
At high frequencies, notably through superhigh frequency and extrahigh fre-
quency, working in mm.GHz units is more practical and appropriate so that (1.1)
becomes
( )
=
300
mm
( )
f GHz
λ (1.2)
where f is substituted directly in gigahertz. A signal at 30 GHz, for example, has
a free-space wavelength of 10 mm. Radio waves traveling in dielectrics have their
wavelengths reduced by the square root of the dielectric’s permittivity εr. So the
wavelength in this case is given by the following modified version of (1.2), that is:
( )
=
300
mm
r
f
λ
ε
(1.3)
This means that if, for example, a 30-GHz signal is traveling entirely with-
in a dielectric that has a permittivity of 2.3 (e.g., circuit board) the wavelength
is reduced to 6.59 mm. Where RF design is required involving signals on circuit
boards, ceramic substrates, or semi-insulating semiconductors, the general concept
regarding wavelength calculation is vital. This book includes many examples of
this situation.
For microstrip or any other quasi-TEM transmission, which is very important
in microwave or millimeter-wave design, some of the electromagnetic fields extend
into the air as well as the substrate. In these circumstances, (1.3) still applies, albeit
with the relative permittivity replaced with a quantity called the effective permittiv-
ity, εeff. Equation (1.3) then becomes
( )
=
300
mm
eff
f
λ
ε (1.4)

1.5 Software-Defined Radio and Cognitive Radio 9
There is much more information on this important topic in Chapter 3.
1.5 Software-Defined Radio and Cognitive Radio
A detailed systems-level block of a software-defined radio (SDR) is shown in Figure
1.5.
The first (overall) observation to be made is the fact that most of the hardware
is digital, including software processing. Only the block to the far left-side (flexible
RF hardware) is RF. Most of the circuit functions are described elsewhere in this
book (notably Chapters 5 and 11), but several other aspects need clarification:
•
• Flexible RF hardware refers to RF circuits that come under the control of the
processing software.
•
• CORBA (Common Object Request Broker Architecture) is software that
provides dynamic (strategic) decisions within a network.
•
• Virtual radio machine (alternatively, radio virtual machine) provides the
SDR with portability and platform reconfigurability.
Most SDRs operate at frequencies below around 6 GHz and are therefore read-
ily implemented mainly using silicon complementary metal-oxide semiconductor
(CMOS) ICs. However, in some instances, Gallium arsenide (GaAs) or Gallium
nitride (GaN) chip technology may be required at the final front-end to provide
greater efficiency and/or higher RF output power. SDR technology is used in the
high-volume example of cell-phones (i.e., mobile phones).
In the late 1990s the concept of SDR was taken a radical step further with the
early development of cognitive radio. Mitola’s dissertation from 2000 [6] is widely
regarded as the birth of cognitive radio as such, with Bostian [7] providing much
detailed information on this subject.
Figure 1.5 Block diagram of a typical software-defined radio (SDR).

10 ��
One definition is that a cognitive radio (CR) is a radio system that has been
programmed and dynamically configured to select the best RF channels locally
available. A CR automatically detects the channels that are available within the RF
spectrum and subsequently alters its internal parameters in order to accommodate
the maximum possible number of live communications channels at the particular
location. The process can be viewed as a type of dynamic spectrum management.
Originally CR was considered merely as an extension of software-defined ra-
dio, but it soon became evident that CR is much more than that.
Spectrum sensing for CR (notably across TV bands) is a very important re-
search subject and major issues include the design of sufficiently high-quality devic-
es and algorithms to enable the exchange of spectrum sensing data between nodes
in the networks. It is abundantly clear that basic energy detectors are nowhere near
sufficient for the task of accurately detecting the presence of signals at CR network
nodes. Therefore more sophisticated spectrum sensing techniques are necessary. It
is also clear that (as might be anticipated intuitively) increasing the number of co-
operating sensing nodes progressively decreases the probability of false detection.
One approach that shows considerable promise is to implement a technique
known as orthogonal frequency-division multiple access (OFDMA) so as to adap-
tively fill available RF bands.
1.6 The Challenge of Noise
In the broadest sense, noise comprises unwanted and apparently random perturba-
tions that can potentially cause damage to audio or video content or cause errors
in digital signals. Electrically, noise power covers a very wide spectrum and has
several specific origins such as thermal, flicker (1/f) or phase noise. All these types
of noise are important in characterizing and designing circuits and systems. Phase
noise, in particular, is an important parameter in oscillators and many amplifier
configurations.
Any receiver will have many specification characteristics and one of these is the
minimum detectable signal (MDS). A concept of MDS is provided in the spectrum
shown in Figure 1.6, which shows the MDS almost buried in noise.
Practically every component in an electronic circuit, active or passive, contrib-
utes to the noise and further details are provided in Chapters 2 and 3. Even basic
resistors produce thermal noise and every transistor generates still more of this
unwanted noise. Phase noise, especially close-in to the carrier, represents a very im-
portant quantity in this respect, notably in frequency sources but also in amplifiers
designed for sensitive requirements. For a fundamental crystal-stabilized oscilla-
tor, the phase noise even as close as 100 Hz from the carrier (typically 100 or 140
MHz) can be as low as −144 dBc. However, the phase noise for most other types of
frequency source is more like −70 dBc, even at frequencies much further removed
from the carrier (e.g., 10 or even 100 kHz away).
Signal-to-noise ratio and also noise figure represent important specifications
for amplifiers and receiver subsystems. Noise figure always increases with frequen-
cy and the first stage in any receiver is always the most sensitive. Alternatively,
another quantity termed equivalent noise temperature can be used, particularly

1.7 RF Receivers 11
in sensitive systems but also helpfully toward the analysis of noise in almost any
system.
Much more detail is provided in Chapters 9 and 12.
1.7 RF Receivers
Until the twenty-first century, RF receivers were largely analog-based and took on
the overall configuration shown in Figure 1.7.
Two bandpass filters are implemented in this receiver: the initial BPF and the
image-reject filter. Oscillators are dealt with in Chapter 12 and mixers are covered
in Chapter 13.
However, there are many systems that operate at RF and microwave frequen-
cies up around 6 GHz and these types of systems increasingly demand extensive all-
digital signal processing. The advancing frequency capabilities of analog-to-digital
converters (ADCs) mean that after the LNA the next active circuit block an input
signal to a receiver reaches is the ADC (ADCs are covered in detail in Chapter 11
and relatively briefly in Section 1.9).
The requirement for the initial BPF still very much exists, except that now the
specification of that filter is much more stringent. In particular, a high degree of
Figure 1.7 Traditional RF receiver architecture.
Figure 1.6 RF Signal almost buried in the noise.

12 ��
frequency agility is demanded (to cope with rapidly changing input channels) and
a tuned BPF must now be designed. The initial sections of such a receiver chain are
With the receiver architecture shown in Figure 1.8, the first two stages are
similar to those shown in Figure 1.7, except (very significantly) the initial BPF must
now be tunable. The second BPF is designed to substantially reduce any unwanted
spurious components that may arise from the digital circuits. The ADC generates
the digital bit streams required for digital signal processing. To the right-hand side
of this system all signals are digital bit streams.
Software-defined radio (SDR) and cognitive radios, cited in Section 1.5, repre-
sent increasingly important examples of these classes of systems.
Channel multiplexers can readily be realized by inserting different BPFs into
the arms of signal dividers, such as the Wilkinson power dividers described in
Chapter 5. Each BPF is individually designed to pass the band of frequencies as-
sociated with a specific channel.
1.8 RF Filters
Frequency filters, notably bandpass filters (BPF) but also lowpass filters (LPF), are
critical functional blocks in all RF systems. In particular, a BPF is almost always
required between the antenna and the first low-noise amplifier (LNA) in a receiver.
Traditionally, this initial BPF has fixed parameters: center frequency, channel
bandwidth, attenuation levels, and so on. This type of BPF will remain important,
especially as operating frequencies are shifting upwards to include millimeter-wave
bands.
The practical realization of any filter varies greatly, but many examples adopt
planar technologies because these are consistent with both hybrid and MMIC/
RFIC circuit approaches (see Section 1.2). Both lumped-element and transmission
line technologies can be implemented and Figure 1.9 illustrates these options.
Figure 1.9(a) is representative of a ��
lumped-element LPF. This layout could ap-
ply directly to a real (RF) LPF circuit, or it could be the low-frequency prototype
that will ultimately lead to the design of the full RF LPF.
For general rule-of-thumb guidance where Figure 1.9(a) is actually an RF
lumped-element LPF, the values of the components will define the frequency
response:
•
• Through microwave frequency bands, the component values will be pF of
capacitance and nH of inductance.
Figure 1.8 Digitally oriented RF receiver architecture.

1.8 RF Filters 13
•
• Into the lower millimeter-wave bands, the component values will be more like
fF of capacitance and maybe still some nH of inductance (possibly sub-nH).
However, the Q-factor (defined by the power losses) of the inductors in partic-
ular will usually limit the applicability of lumped components and instead lengths
of transmission line are then adopted. The transmission lines are usually microstrip
the circuit of Figure 1.9(a) first into Figure 1.9(b) (where the transmission line rep-
resents the inductor) and finally to Figure 1.9(c) where the inductor and the two
capacitors are replaced by appropriately designed microstrip sections.
The microstrip transmission line approach is taken a stage further to design
bandpass filters and the resulting structures typically take on the configuration
Many practical hybrid and monolithic (MMIC) circuits can be designed imple-
menting this type of BPF.
It is important to appreciate that all filters inherently suffer from the following
defects in their spectral characteristics:
Figure 1.10 Typical bandpass filter realized using a cascade of half-wavelength coupled microstrip
resonators (six-resonator BPF in an angled layout).
Figure 1.9 (a–c) Progression of a basic LPF lumped-component p-section through to a microstrip
realization.

14 ��
1. Finite insertion loss through the passbands (typically a fraction of a decibel);
2. Finite skirt insertion loss slopes (i.e., it is physically impossible to obtain
sudden [infinitesimal] changes from passband to stopband).
1.9 ADCs and DACs
An analog-to-digital converter (ADC) accepts an analog (real-world) signal as input
and this input is processed (i.e., converted) into a corresponding digital output sig-
nal. Symbolically an ADC is represented in any RF system as shown in Figure 1.11.
In many cases, the digital output comprises a 2’s complement binary number
that closely represents the analog input, although there are other possible digi-
tal representations. The sampling, quantization, and coding are especially critical
electronic operations. There are many basic types of ADC, but only some of these
are applicable to RF systems. Good examples of such ADCs include flash, folding,
pipelined, and time-interleaved configurations. Details are provided in Chapter 11.
Following the digital signal processing it is necessary to convert the signal back
to analog format and this function is performed by a digital-to-analog converter.
However, the raw output from a DAC is usually only a rough representation of
the original analog signal, as indicated in Figure 1.12, and a lowpass reconstruction
filter (LPF) is necessary after this output.
There are many specification points associated with DACs but for present pur-
poses only one is considered: resolution.
A DAC’s resolution is a strong function of the number of bits N involved in
the digital input code. The resolution value expresses the number of different states
possible for the DAC, and determines the minimum value of the step in voltage that
the circuit can resolve.
More details on this together with several other specification points are pre-
sented in Chapter 11.
1.10 Oscillators, Mixers, and Frequency Converters
The broad scope considering radio frequency sources includes the following types
of circuits and subsystems:
•
• Oscillators of various types, especially oscillators based upon quartz crystals;
•
• Dielectric resonator-based oscillators;
•
• Voltage-controlled oscillators (VCOs);
Figure 1.11 An ADC shown symbolically.

1.10 Oscillators, Mixers, and Frequency Converters 15
•
• Frequency multipliers;
•
• Frequency dividers;
•
• Frequency synthesizers (notably phase-locked-loop-based).
Many direct sources (also many high-stability master oscillators) comprise
quartz-crystal-stabilized oscillators. Over recent years, various alternative technol-
ogies such as microelectronic mechanical systems (MEMS) have contended for ap-
plication as frequency stabilizing elements, but concurrent developments in quartz
crystals have predominated.
For many frequency sources, it is necessary to be able to electronically control
the final output frequency and the basic concept is illustrated in Figure 1.13.
In practice, the tuned element is subject to extremely fast frequency variation
by means of a varactor diode. The capacitance of such a diode alters as a function
of the applied voltage and although this variation is highly nonlinear, linearization
techniques can be applied. Varactor diodes are described in detail in Chapter 2.
Mixers are mainly required to deliver an output known as the intermediate
frequency (IF) for further signal processing. There are many types of mixers (de-
scribed in detail in Chapter 13), but the important example of the Gilbert cell is
covered briefly here. The Gilbert cell mixer is also known as a four-quadrant mul-
tiplier because it mixes two signals by effectively multiplying them.
Silicon technology is most commonly encountered but silicon germanium
(SiGe) bipolar complementary metal-oxide semiconductor (BiCMOS) and GaAs
high electron mobility transistor (HEMT) approaches have also been demonstrated
[5, 6].
This type of double-balanced mixer exhibits conversion gain and a superior
noise performance compared with other mixer configurations (particularly when
Figure 1.12 Basic schematic diagram of a DAC.
Figure 1.13 Concept of a tunable (variable-frequency) oscillator.

16 ��
SiGe heterojunction bipolar transistors [HBTs] are implemented). Being transistor-
intensive, this circuit is also highly suited to MMIC/RFIC realization.
Extensive further details regarding mixers, including the Gilbert cell, are pro-
vided in Chapter 13. ��
Occasionally, it is necessary to divide down a particular fre-
quency and various options are available for this purpose. A good example of the
requirement is within a phase-locked (PLL)-based frequency synthesizer, which is
now briefly described here because these are of particular importance in commu-
nications systems.
The basic configuration is shown in Figure 1.15.
Key features of this synthesizer include:
1. The reference oscillator is usually a quartz crystal-based circuit providing
very high stability and the lowest possible phase noise (see Section 12.3).
2. The phase detector will almost certainly comprise the balanced detector
(two-diode) configuration, described in Chapter 13.
3. The amplifier can comprise an operational type as cited elsewhere in this
book (e.g., in Chapter 11).
4. Lowpass (loop) filters are described in Chapter 6.
5. VCOs and frequency dividers are described in Sections 12.5 and 12.7,
respectively.
The major key to the operation of this PLL is the phase detector where the
phases of the reference oscillator and the divided output frequency are compared.
Bearing this critical feature in mind, major aspects in the operation of this loop are
described in Chapter 12.
The fractional-N subsystem represents a very important variation of this basic
synthesizer and full details of this are also given in Chapter 12.
Figure 1.14 Basic structure of the Gilbert cell mixer, implemented using BJTs but omitting the
required input/output baluns [8]. (© Artech House, 2016.)

1.11 Semiconductor Device Requirements 17
1.11 Semiconductor Device Requirements
Exactly what is meant by the term semiconductors? To an industrialist, semicon-
ductors is a term embracing discrete devices and integrated circuits (diodes, tran-
sistors, and ICs), whereas to a professor of electronics, semiconductors mean the
basic materials from which diodes, transistors, and ICs are manufactured. Either
way, a basic understanding of the semiconductor materials and of the devices (the
diodes, transistors, and ICs) are both essential because these underlie the detailed
RF circuits and systems.
It is being assumed that the reader of this book is already well aware of di-
odes, transistors, and ICs as these apply within lower-frequency systems or within
computers and digital control configurations. The purpose here is to focus on the
special requirements regarding RF semiconductors.
1.12 Semiconductor Manufacturing
This is a highly specialized subject in its own right; therefore, only a brief descrip-
tion is provided here. A very good detailed coverage is supplied in [2, 9].
However, it is not particularly important for RF communications technologists
to be familiar in great detail with semiconductor manufacturing.
The approach depends on the exact nature of the semiconductor products to
be manufactured, although all processes have one thing in common: they all begin
with a high-purity, single-crystal boule of the intrinsic semiconductor (i.e., silicon,
gallium arsenide, gallium nitride, or other semiconductor material). In the case of
silicon, this single-crystal boule is often around 15 cm in diameter. However, for
GaAs or GaN, the boule diameters are generally much smaller, less than 10 cm.
The issue of boule diameter is a serious one because this diameter determines the
approximate number of die that can be made on the wafer (the top portion of the
boule). In turn, this leads to the production yield of good die, which has immediate
economic implications.
Photolithography, successive selective diffusion (of dopant materials), and se-
lective metallization are all essential steps toward the manufacture of any RF semi-
conductor device. For MMICs (or RFICs), it is necessary to add the realization of
vias connecting between various nodes on the chip.
Figure 1.15 Basic subsystem of a PLL-based frequency synthesizer.

18 ��
The manufacturing of SiGe BiCMOS MMICs is particularly specialized be-
cause this requires many more mask stages than the other technologies. As a result,
this specific technology is only economically viable for high-volume applications.
1.13 Diodes and Transistors
The diode is the most basic RF semiconductor device, and it remains of great im-
portance in many circuits.
Schottky-barrier diodes (often just called Schottkys) embody a metal anode
that directly joins a semiconductor cathode. The physical structure and commonly
used circuit symbol are shown in Figure 1.16.
In Figure 1.16 an N-doped semiconductor is shown. This is the most common
form because the mobility of electrons (hence, N) is always much higher than that
of holes. This, along with many other aspects, is fully described in Chapter 2.
Various other types of RF diodes are available, notably PIN diodes and varac-
tor diodes. These, including noise characteristics, are described in Chapter 2.
Basic aspects regarding bipolar junction transistors were described earlier (no-
tably Figure 1.3 in which C refers to the device’s collector, B refers to its base, and
E refers to the emitter). An NPN transistor is selected because most RF transistors
adopt this structure, dominated by the relatively high mobility N-doped semicon-
ductor. These general types of devices are available as some particularly sophisti-
cated structures, such as the GaAs HBT shown in Figure 1.17.
In contrast, the relatively straightforward field-effect transmitter (FET)-type
transistors involved in CMOS transistors are also very important.
The basic CMOS logic inverter represents an important example of how two
contrasting types of MOS transistor are interconnected to form a fundamental type
of circuit configuration, shown in Figure 1.18.
It is important to observe the small circle symbol on the (gate) input to the up-
per transistor, which means that this p-type metal-oxide semiconductor (PMOS)
transistor’s gate directly connects to an N-well region. In contrast, the lower n-type
metal-oxide semi-conductor (NMOS) transistor’s gate directly connects to the P-
type substrate. Further details are beyond the scope of this book. When both tran-
sistors have minimum feature dimensions down into the submicron levels (increas-
ingly nanometers), these types of circuits can be designed to process low-power
microwave and millimeter-wave signals.
RF CMOS and its derivatives now represent a mainstream RF technology that
can be adopted for the relatively low-power portions of MMICs/RFICs. In order to
increase the operating speed (hence also frequency), it is also possible to add one
(sometimes two) bipolar transistors to a CMOS circuit stage.
Figure 1.16 Schottky-barrier diode: (a) schematic of structure, and (b) circuit symbol.

1.14 Hybrid Circuits and MMICs 19
GaAs HEMTs, GaAs pseudomophic HEMTs (pHEMTs), and GaN HEMTs
are also vital transistor types that are designed into several types of microwave and
millimeter-wave circuits.
1.14 Hybrid Circuits and MMICs
The basic requirement for any solid state amplifier is the internal transistor, and
these fundamental semiconductor devices are described in Chapter 2 of this book.
While the main thrust of technology choice is toward MMIC/RFIC realizations,
discrete transistors are required where:
•
• There is a need for relatively high output power, generally upwards of several
tens of watts (CW) or kW (pulsed);
Figure 1.17 Cross section through an NPN gallium aluminum arsenide (GaAℓAs)/GaAs HBT (the
actual size has all dimensions in nanometers; the largest dimension can approach 100 nm).
Figure 1.18 Basic CMOS logic inverter circuit.

20 ��
•
• Scenarios where the RF output power may be relatively low (generally below
a few tens of watts), but custom or low production rate designs are the order
of the day.
As an example, a 100-W RF power amplifier (RFPA) operating around 2 GHz
typically implements one or more discrete GaN HEMTs and such a design would
almost certainly take the hybrid circuit route, most likely on a polymer-based cir-
cuit board with excellent heat-sinking.
In contrast, an RFPA required to provide a 10-W output at moderate frequen-
cies would very likely be designed in MMIC format—provided the production rate
is at least several thousand pieces. The transistor process in this case will depend
mainly on the signal frequencies involved: typically GaN HEMTs for lower micro-
wave frequencies although more likely GaAs pHEMTs for designs around or above
26 GHz.
However, the strong trend is toward silicon transistor processes for the lower-
power scenarios. All these types of processes are described in Chapter 2.
As hinted above, the selection of the transistor process depends critically on the
operating frequency. This feature is a consequence of the internal and parasitic re-
active elements associated with every transistor, which again are described in Chap-
ter 2. These reactive effects strongly influence the RF output power and power gain
as functions of frequency for any transistor together with RFPAs implementing
these devices. In general, both available output and power gain tend to decrease
with increasing frequency.
An example of a MMIC-based RFPA is shown in Figure 1.19.
Lange couplers can be seen at both the input and output sides of this chip. The
Lange coupler is described in detail in Chapter 4. Where the circuit is integrated (as
in this example), all the bond connections within the Lange couplers are formed
using air bridges.
1.15 The Challenge of RF Power Amplification
Every basic power amplifier will always exhibit signal distortion, whatever the tech-
nology. This distortion is a fundamental issue that has exercised RFPA designers
Figure 1.19 E-band MMIC implementing Lange couplers at both input and output ends. (Courtesy
of Plextek RFI.)

1.16 Electronic Design Automation 21
over the years. The distortion arises mainly because of the inherent nonlinearity in
the internal transistor current-voltage characteristics, which are particularly pro-
nounced under large-signal (PA) conditions. An example of the resulting power
amplifier input-output power transfer characteristic is shown in Figure 1.20.
It can be seen from Figure 1.20 that a somewhat more linear behavior is ob-
tained by backing off the input so the output power is also automatically backed
off, but clearly there is still a substantial amount of distortion.
Predistortion techniques are usually applied to compensate for this distortion,
notably digital predistortion (DPD).
Another important parameter associated with power amplifiers is the overall
efficiency and this requirement has yet another strong bearing on the choice of
amplifier configuration. Details regarding efficiency, DPD, and further aspects of
RFPAs are provided in Chapter 10.
1.16 Electronic Design Automation
Electronic design automation (EDA) is a vital, ongoing necessity for electronic cir-
cuit and system design and simulation. Several companies have embraced the spe-
cial requirements of RF-EDA, and in this section brief summaries are provided of
each vendor’s main offerings. First, here is some general information regarding
EDA software tools.
The main aim of any EDA package is to enable the full design of a circuit or
subsystem and then to simulate its performance. Toward this aim, every EDA pack-
age requires the following minimum precisely detailed inputs:
Figure 1.20 General output-input power transfer curve for an RF power amplifier.

22 ��
•
• The LC overall final circuit or subsystem specifications (e.g., center fre-
quency, bandwidth, noise figure, power levels);
•
• The basic technology to be used, especially whether hybrid of monolithic
(i.e., MMIC/RFIC, immediately this will decide as to whether millimeter or
micrometer dimensions are appropriate);
•
• Following on from the last input, details concerning the substrate, notably
thickness and permittivity;
•
• The LC specifications regarding active devices, mainly transistors: discrete
devices for hybrid circuits, processes where MMICs are concerned (e.g.
CMOS, GaAs pHEMT, GaN HEMT).
In most instances, the software will already contain extensive libraries of data
on various typical components and processes. Design routines will be available for
transmission lines (coplanar waveguide or microstrip), for specific types of passive
circuit structures (e.g., baluns, Lange couplers, or Wilkinson dividers) and also for
lumped components such as capacitors, inductors and resistors.
All the required data must be inputted. Outputs will usually include the final
circuit layout together with graphical plots of performance, typically to a base of
frequency (i.e., simulation results).
References
[1] Khanna, A., “mmWaves Hit the Highway,” Microwave Journal, August 2017, pp. 22–42.
[2] Kingsley, N., and J. R. Guerci, Radar RF Circuit Design, Norwood, MA: Artech House,
2016.
[3] Brookner, E., “Metamaterial Advances for Radar and Communications,” Microwave Jour-
nal, November 2016, pp. 22–42.
[4] Ghosh, A., “The 5G mmWave Radio Revolution,” Microwave Journal, September 2016,
pp. 22–36.
[5] Kovitz, J. M., J. H. Choi, and Y. Rahmat-Samii, “Supporting Wide-Band Circular Polariza-
tion,” IEEE Microwave Magazine, July/August 2017, pp. 91–104.
[6] ��
Mitola, J., “Cognitive Radio: An Integrated Agent Architecture for Software Defined Ra-
dio,” Ph. D. dissertation, Dept. Tech. Royal Inst. Tech., Sweden, 2000.
[7] Bostian, C. W., N. J. Kaminski, and A. S. Fayez, Cognitive Radio Engineering, Edison, NJ:
SciTECH Publishing/IET, 2016.
[8] Camarchia, V., R. Quaglia and M. Pirola, Electronics for Microwave Backhaul, Norwood,
MA: Artech House 2016.
[9] Edwards, T., and M. Steer, Foundations for Microstrip Circuit Design, 4th ed., New York:
John Wiley & Sons, 2016.

23
C H A P T E R 2
RF Semiconductors
2.1 Introduction
Exactly what is meant by the term semiconductors? To an industrialist, semicon-
ductor is a term embracing discrete devices and integrated circuits (diodes, transis-
tors, and integrated circuits [ICs]), whereas to a professor of electronics the word
semiconductor means the basic materials from which diodes, transistors, and ICs
are manufactured. Either way, a basic understanding of the semiconductor mate-
rials and the devices (the diodes, transistors, and ICs) is essential, and this is the
purpose of this chapter.
It is being assumed the reader of this book is already well aware of diodes, tran-
sistors, and ICs as these apply within lower-frequency systems or within computers
and digital control systems. Several texts covering fundamentals of semiconductors
are readily available, Sze and Ng [1] and Shur [2], for example.
The purpose here is to focus on the special requirements regarding RF semicon-
ductors. This chapter begins by studying key aspects of semiconductor materials
and then majors on a detailed examination of diodes, transistors, and ICs for RF
applications. Many useful aspects are also provided in [1–3].
2.2 Semiconductor Materials
It is very well known that the great majority of electronic systems are based around
silicon. This fundamental semiconductor material forms the basis for almost all the
ICs designed into systems ranging from computers through to iPads, mobile phones
(cell phones), and a wide variety of other devices. However, although silicon is
increasingly important in RF communications systems, it is certainly not the only
semiconductor material required in this context.
To explore why this is the case, it is necessary to review some fundamental
aspects and characteristics associated with an important range of semiconduc-
tor materials. As well as silicon, several other types of semiconductor materials
are considered, notably gallium arsenide (GaAs), gallium nitride (GaN), indium
phosphide (InP), and silicon germanium (SiGe) all of which are termed compared
semicondutors.

24 ��
RF Semiconductors
There are many important aspects of semiconductors and the concepts of band-
gap and drift velocity are particularly significant.
2.2.1 Bandgap
In any solid material, including semiconductors, if N valence electrons (all having
the same energy) are combined to form bonds, then N possible energy levels will be
the result. Exactly half of these energy levels will be decreased in energy while the
remaining half will exhibit increased energy.
However, a statistical situation will exist whereby each half cannot simply con-
tain exactly identical energy levels and instead in practice there is always a statisti-
cal distribution of electron energy level occupancies. For semiconductors, this dis-
tribution follows the Fermi-Dirac function, which is exponential and leads to the
important exponential current-voltage relationships that characterize most diodes
and transistors. Further details explaining this are beyond the scope of this text,
but it is useful to appreciate the concept of a bandgap diagram. A simple, basic
example is presented in Figure 2.1.
In Figure 2.1 EF is the Fermi level (middle of the bandgap = Ec − Ev).
It requires a specific amount of energy to cause an electron to become released
from the valence band, cross the bandgap, and hence be available for conduction.
The energy difference between the top of the valence band and the bottom of the
conduction band is called the bandgap and bandgaps for a range of important
semiconductor materials are quoted in Table 2.1.
Bandgaps (also mobility, considered later) are amongst the important criteria
leading to the choice of semiconductor material that may be used as the basis for
any semiconductor component. Also, very importantly, additional materials can be
introduced (such as aluminium) so as to alter the bandgap according to require-
ments. This is known as bandgap engineering.
Semiconductor materials are generally classed as being either:
•
• Narrow bandgap semiconductors: all except GaN are within this category
(see Table 2.1);
Figure 2.1 Basic concept of the energy band structure in a solid material.

•
• Wide bandgap (WBG) semiconductors. GaN is the prime example, although
there are some other WBG materials.
Regarding field strength units of kV/cm, bear in mind a distance of, say, 0.5
mm may be the case with practical semiconductor devices, and 10kV/cm translates
to 1 kV per mm (i.e., 500V across a 0.5-mm distance).
Semiconductor devices, particularly transistors, comprising narrow bandgap
semiconductor devices imply relatively low DC supply voltages for circuits com-
posed of these, whereas wide bandgap (WBG) semiconductors tend to mean tran-
sistors embodying these require relatively high DC supply voltages. Most transis-
tors based on GaN semiconductor require DC supply voltages of the order of some
tens of volts, whereas many circuits using GaAs or silicon transistors will operate
using DC supply voltages as low as 3V.
2.2.2 Drift Velocity
In any semiconductor material the electrons or holes will accelerate under the at-
traction of an applied electric field. The speed of the electrons is termed the drift
velocity and the ratio of this drift velocity to the applied electric field is termed the
mobility. This mobility, symbol µ, is a very important parameter for selecting semi-
conductors that could be candidates for RF applications. The term drift velocity is
somewhat misleading in that it may suggest the electrons travel slowly and even
randomly, whereas they travel at high speed.
For real devices, various parts of the diodes or transistors are doped N-type
(electron-intensive) or P-type (hole-intensive) by the introduction of various doping
materials into the intrinsic (high-purity) semiconductor.
For reasons that are beyond the scope of this book the mobility of electrons
µe is always much higher than that of holes µh, which means electrons will travel
much faster than holes under the same electric field. This is the reason why most
RF or microwave diodes and transistors are designed so that the critical electron-
transport sections are N-type rather than P-type. Electron mobilities for a range of
important semiconductor materials are quoted in Table 2.2.
Table 2.1 Energy Bandgaps for Various Selected
Semiconductor Materials
Semiconductor Material
Energy Bandgap (eV)
at Room Temperature
~300K
GaAs 1.42*
GaN 3.36**
InP 1.35
Silicon (crystalline) (Si) 1.12
Silicon-Germanium (SiGe) 0.67 to 1.11***
Notes: *1.5 eV at a field strength of 4 kV/cm then down to 1.0 eV
at higher field strengths; **Decreases to 2.9 eV at a field strength of
150 kV/cm and reduces further to 1.5 eV at higher field strengths;
***The extremes of SiGe can either be very like germanium or much
more like silicon, dependent on the alloy choice, hence the extremes
of possible bandgaps.

26 ��
RF Semiconductors
From the data in Table 2.2, it is clear that there is no outright winner in terms
of mobility. InP is rarely used in commercial RF semiconductor devices on account
of important issues such as dielectric loss and wafer cost, which prohibit its use
except in the most demanding applications.
GaAs benefits from a notably high electron mobility and over several decades
this semiconductor material has been the top choice with RF and microwave engi-
neers. For many years since the 1970s, practically anything microwave meant the
use of GaAs for most diodes, transistors and integrated circuit chips. GaAs devices
such as GaAs pHEMT transistors (and ICs implementing these) remain impor-
tant for practical twenty-first-century RF circuits. Further details are provided later
here.
2.2.3 Resistors Made from Semiconductors
ICs designed for digital applications (processors, gate arrays, memories) are gen-
erally transistor-intensive and do not require any passive components such as ca-
pacitors, inductors, transmission lines, or resistors. However, ICs designed for RF/
microwave applications are radically different and it is vital for such passive com-
ponents to be designed. Chapter 3 deals with these requirements in some detail but
at this point, armed with a knowledge concerning mobilities, in particular, it is ap-
propriate to examine the design of a semiconductor-based resistor.
The first question to pose and to answer is how to calculate the conductivity of
a semiconductor material. The following equation provides this:
=
e e e
n q
σ µ (2.1)
in which σe, ne, and µe are, respectively, the conductivity, electron concentration,
and mobility of the N-type semiconductor material and q is the charge on an elec-
tron (1.601 × 10−19 C).
Finally, the resistance R of the rectangular strip is calculated using the classic
formula:
=
e
R
A
σ

(2.2)
where < is the length of the strip and A is its cross sectional area.
Table 2.2 Electron Mobilities for Various Selected N-type
Doped Semiconductor Materials
Semiconductor Material
Approximate Electron Mobility µe
(cm2V–1s–1 at Room Temperature
GaAs 8,000*
GaN 990 to 2,000**
InP 5,400 (max.)
Silicon (Si) 1,400
Silicon germanium (SiGe) 1,800
Notes: *Can be as low as 2,500 cm2V–1s–1 at high electron concentra-
tions. **Can be as high as 10,000 cm2V–1s–1 at moderate electron
concentrations.

As an example, calculate the length required of a rectangular strip resistor,
value 200Ω, to be integrated within a SiGe-based RF IC. There are 1015 electrons
per cubic centimeter in the conduction band and the resistor measures 300 µm ×
150 µm in the cross section.
For the solution, first calculate σe using (2.1) and with mobility from Table 2.2:
σe = 1015 × 1.601 × 10−19 × 1.8 × 103
where µe for SiGe was obtained using Table 2.1. It must be checked that all the
units are consistent, leading to the final units: S/cm.
This calculates to σe = 0.288 S/cm.
Next rearrange (2.2) to express the length of the resistor strip < and substitute
all the quantities, giving:
{ }
− −
=
= × × × × ×
2 2
0.288 3 10 1.5 10 200 units:cm
eAR
σ


hence, < = 2.594 × 10–2 cm (i.e., < = 259.4 µm).
The length of this resistor on the SiGe substrate is ~259 µm.
2.2.4 Electron Speed and Transit Time
Typical electron velocities within a transistor are in the order of 105 m per second.
The typical linear dimensions over which the electronics will transit are in the order
of a 1 µm.
It is therefore very straightforward to determine the order of magnitude of
the transit time of electrons traveling within the active region of a transistor. This
transit time τ is:
{ }
−
×
=
6
5
1 10 units: seconds
10
τ
∴= τ = 10–11 seconds – or 10 ps
This is a fairly typical time interval, applying to microwave electronics.
It is however vital to appreciate that other parameters associated with transis-
tors and related types of devices restrict the behavior and limit the device’s time-
domain characteristics and frequency response.
2.2.5 Some Further Important Properties of Semiconductors
In a later section of this chapter monolithic microwave integrated circuits (MMICs)
or radio frequency integrated circuits (RFICs) are covered in some detail. For these
technologies, the substrates are the semiconductor wafer materials and therefore
relevant material properties beyond those described above are of importance. Table
2.3 provides some data of this nature.
All the parameters quoted in the above tables are important in understanding
semiconductor devices. In the case of Table 2.3, the properties listed are vital input

28 ��
RF Semiconductors
values for the design of passive components and interconnections on semiconduc-
tor substrates (MMICs, RFICs). Details are provided in Chapter 3.
2.2.6 Semiconductor Manufacturing
This is a highly specialized subject in its own right; therefore, only a brief descrip-
tion is provided here. A very good detailed coverage is available in [4].
However, it is not particularly important for RF communications technologists
to be familiar in detail with semiconductor manufacturing. The approach depends
on the exact nature of the semiconductor products to be manufactured, although
all processes have one thing in common: they all begin with a high-purity, single-
crystal boule of the intrinsic semiconductor (i.e., silicon, gallium arsenide, gallium
nitride, or other semiconductor material). In the case of silicon, this single-crystal
boule is often around 15 cm in diameter. However, for GaAs or GaN, the boule
diameters are generally much smaller, <10 cm. The issue of boule diameter is a
serious one because this diameter determines the approximate number of die that
can be made on the wafer (the top portion of the boule). In turn, this leads to the
production yield of good die, which has immediate economic implications.
Photolithography, successive selective diffusion (of dopant materials), and se-
lective metallization are all essential steps toward the manufacture of any RF semi-
conductor device. For MMICs (or RFICs), add the realization of vias connecting
between various nodes on the chip.
The manufacturing of SiGe BiCMOS MMICs is particularly specialized be-
cause this requires many more mask stages than the other technologies. As a result,
this specific technology is only economically viable for high-volume applications.
SiGe BiCMOS is discussed in some detail in Section 2.4.4.
2.3 Semiconductor Diodes (RF-Oriented)
2.3.1 Some Semiconductor Junction Diode Fundamentals
Any two-terminal semiconductor junction device is termed a semiconductor diode.
The simple basic structure and its common circuit symbol are shown in Figure 2.2.
Table 2.3 Properties of Various Selected Semiconductor Materials Relevant to Their Use as IC
Substrates
Semiconductor
Substrate
Relative
Permittivity
(εr)
Dielectric Loss
Tangent (tan δ
at 10 GHz)
Approximate
Thickness
h(mm)
Surface State
Roughness
(µm)
Dielectric
Strength
(kV/cm)
Thermal
Conductivity
(W cm–1K–1)
GaAs 12.85 6 0.5 0.025 350 30
GaN 8.9 — 0.5 0.025 4,000 140
InP 12.4 5 0.6 0.025 350 40
Si (high resis-
tivity silion
[resistivity >2
kΩ.cm])
11.9 ≈1 0.36 <0.001 300 120
SiGe ≈13 — — — 220 84

When a positive DC voltage is applied to the anode (left side in Figure 2.2[a]),
a forward current IF flows from P to N (follow the anode arrow in Figure 2.2[b]).
This current varies exponentially as the voltage across the junction increases, as
indicated in (2.3):
( )
1
V
F SAT
I I eα
= − (2.3)
where ISAT is the reverse-bias saturation current, V is the (DC) forward voltage, and
the coefficient α is expressed as follows:
q
mkT
α = (2.4)
in which q is the charge on an electron (1.601 × 10−19 C), k is Boltzmann’s constant
(1.38 × 10–23 J.K–1), T is the absolute temperature (K), and m, the ideality factor,
is a number between 1 and 2 dependent on the structure of the diode (notably the
semiconductor material used).
Equation (2.3) is often referred to as the Shockley Ideal Diode Law.
The general semiconductor diode I/V characteristic is shown in Figure 2.3.
There are two important features in this overall characteristic:
1. The first quadrant, which is generally known as the forward-biased
characteristic;
2. The third quadrant, which is generally known as the reverse-biased portion
of the overall characteristic.
Depending on the detailed structure of the diode, for most RF diodes, the for-
ward-biased region is characterized by DC current levels increasing through mA
Figure 2.2 Semiconductor junction diode: (a) schematic of structure and (b) circuit symbol.
Figure 2.3 The general semiconductor junction diode I/V characteristic.

30 ��
RF Semiconductors
values, while corresponding DC voltages increase through hundreds of mV values,
up the maximum voltage VF,max of around 0.7V or 0.8V.
Contrastingly, for most RF diodes the reverse-biased region is characterized
by the DC current level saturating at a maximum value ISAT in the order of µA or
even nA. Across this region DC voltages can easily reach several tens of volts before
voltage breakdown occurs. This voltage breakdown value is often used in diodes
specially designed to deliberately limit DC voltages (Zener diodes). However, this
area is outside the scope of this book.
Schottky-barrier diodes (often just called Schottkys) embody a metal anode
that directly joins a semiconductor cathode. The physical structure and commonly
used circuit symbol are shown in Figure 2.4.
In Figure 1.16 an N-doped semiconductor is shown. This is the most common
form because the mobility of electrons (N) is always much higher than that of holes
as discussed earlier in this chapter. The shape of the general Schottky diode I/V
characteristic is very similar to that of the junction diode, as shown in Figure 2.3.
However, the value of VF,max tends to be somewhat lower, around 0.6V to 0.7V.
The energy band characteristics of a Schottky diode can be developed from
Figure 2.1, resulting in the diagram of Figure 2.4. In Figure 2.4, the Schottky bar-
rier height, φB, is the difference between the interfacial conduction band edge Ec
and the Fermi level EF. It is notable that Ec decreases within the semiconductor,
resulting in an asymmetric energy level situation. The dimension z is the distance
progression through the structure from the metal anode to the cathode at the op-
posite end of the semiconductor.
2.3.2 P-I-N Diodes
By introducing an intrinsic layer of (most usually) silicon between the P and N
materials of a junction diode, a switching (or sometimes attenuating) device is ob-
tained. The intrinsic silicon has to be very slightly N-type or P-type because 100%
intrinsic semiconductors are not realizable in practice.
PIN diodes are implemented in medium-to-high power systems, which make
these devices of substantial importance. However, in lower-power communications
systems, PIN diodes are only occasionally used. The great majority of communica-
tions systems operate using relatively low-power signal levels.
Figure 2.4 Energy-band diagram for a Schottky-barrier diode.

2.3.3 Varactor Diodes
On a DC I/V basis, the reverse-bias characteristic of a P-N junction is unremark-
able, with simply the shape indicated in the third quadrant of Figure 2.3.
However, this situation is far from the end of the story regarding P-N junc-
tion diodes because under reverse bias the junction exhibits capacitance, and this
capacitance CT varies strongly with the DC applied voltage. The basic and general
varactor capacitance equation is:
( )
D
q
T
K R
k
C
V V
=
+ (2.5)
where kD is a constant dependent on the diode structure; VK is the knee voltage for
the diode; VR is the magnitude of the applied reverse voltage across the diode; and q
is a parameter dependent on the type of junction. q = 0.5 for alloy junctions forming
these types of diodes or 0.333 for diffused junctions.
This type of semiconductor diode is variously known as a varactor diode, or a
varicap or a voltage-variable capacitance diode. The most commonly encountered
term in the RF or microwave context is varactor diode and this terminology will be
used consistently here.
From (2.5), it can be seen that the junction capacitance varies in a highly non-
linear fashion as a function of the applied reverse voltage and one example of the
type of resulting curve is shown in Figure 2.5. This result is unfortunate from the
viewpoint of the major application of varactor diodes, namely, voltage-controlled
oscillators (Chapter 12) because a linear C(V) behavior would have been ideal.
Many techniques have been adopted in an effort to linearize the final C(V) function.
A cross section through a typical varactor diode is shown in Figure 2.6.
To provide some idea regarding the behavior of this type of diode, consider the
following sequence of events:
•
• As the reverse bias voltage is decreased, so the depletion layer narrows. This
reduces the thickness of the depletion layer, which, in turn, increases the
capacitance.
•
• As the reverse bias voltage is increased, so the depletion layer widens and the
capacitance decreases.
Figure 2.5 An example of a capacitance/voltage curve applying to a varactor diode.

32 ��
RF Semiconductors
•
• This behavior is highly nonlinear as indicated by (2.5).
The precise nature of the variation is determined by the doping densities and
the size and geometry of the diode construction.
Most varactor diodes embody a doping profile that is highly abrupt (i.e., the
P-N junction is extremely thin in comparison with the remaining dimensions). On
each side of the junction, the doping concentration is maintained as constant as
practically possible and these types of varactors are simply known as abrupt varac-
tors. Due to the abrupt junction, the C-V characteristic follows an inverse square
law.
There are also hyperabrupt varactors for which an inverse square law C-V
characteristic also applies, although only over a portion of the C-V curve. An im-
portant consequence of this is that over a narrow range there is a linear frequency
variation when the varactor is used in a VCO. However, the lowered Q-factor
means that hyperabrupt varactor diodes can only be used up to fairly low micro-
wave frequencies.
Circuit symbols for a varactor diode can either be displayed as Figure 2.7(a) or
more simply (although much less meaningfully) by Figure 2.7(b).
A small-signal equivalent circuit for a varactor diode is shown in Figure 2.8.
In this equivalent circuit, Rs is a series resistance representing losses in the elec-
trodes and RV represents the losses associated with the depletion layer.
Varactor diodes are most often fabricated in silicon but sometimes in GaAs,
which generally leads to higher frequency operation. Q-factors range from 1,200
to (exceptionally) 8,000, although these are usually measured at 50 MHz and will
decrease (i.e., deteriorate) with increasing frequency.
2.3.4 Noise in RF Diodes
In common with all electronic devices, all types of RF diode generate noise as well
as processing the desired signal. For example, both resistances indicated in Figure
2.7 will generate thermal noise, the mean-squared current being given by:
Figure 2.6 Cross section through a varactor diode. This would typically be of the order of a few
millimeters in diameter and some hundreds of micrometers high.

2.4 Transistors 33
2 4
R
kTB
i
R
= (2.6)
where all the parameters have been defined previously.
Also, the shot noise and the flicker noise generated by the perturbations in the
P-N junction diode (also the Schottky diode) are expressed by:
2
2 d
d d f
I
i qBI K
f
= + (2.7)
where most of the parameters have been defined previously; also Id is the DC cur-
rent and f is the spot frequency. Kf is strictly a frequency dependent whose value
tends to stay between 0.6 and 1.0.
Extra noise is delivered by varactor diodes (notably random variations in the
capacitance) and this can seriously affect the phase-noise in VCOs. This is covered
extensively in Chapter 12.
2.4 Transistors
2.4.1 Introductory Remarks
It is well known that the transistor was invented in 1947 by Shockley, Bardeen, and
Brattain at Bell Labs. That landmark invention gave rise to an explosion in transis-
tor development, leading to today’s technologies, without which most of the world
could barely function.
Many would understandably ask: What is a transistor? The answer to that ba-
sic question leads to a truly remarkable story. Until the invention of the integrated
circuit (Kilby, Noyce) all transistors were discrete devices, and many twenty-first-
century examples are indeed discrete transistors.
Figure 2.7 Circuit symbol options for a varactor diode.
Figure 2.8 A small-signal equivalent circuit for a varactor diode.

Exploring the Variety of Random
Documents with Different Content

"I did. I watched and waited yonder. And then?" asked the Turk.
"Go and kill him, that's all! Go and slay the man!" von Hildemaller
told him, turning upon his emissary just as friendly a smile as ever
he had turned upon Joe Douglas. "There is no need to discuss the
matter further, for you know the man and you have the method. Go
then! When it is done come back to me and you shall be rewarded."
Who would have thought the worthy von Hildemaller capable of such
words, or of giving such a dastardly order? Indeed, at the very
moment when he was condemning the gallant Major to death by the
hand of this Turkish assassin, the stout German looked so utterly
genial, so entirely friendly and harmless, that none could possibly
have suspected the real gist of his orders. Yet, as we have inferred
already, behind those smiling, merry eyes, which looked so frankly
and so honestly at people, there was a clever scheming brain, and
behind those lips which were never stern, and seemed ever to be
parted amiably, was a tongue given to much lying. Let us add, too,
the fact that that brain was capable of inventing acts which would
have shamed an Englishman, and of producing orders even more
dastardly than that which had already been given. Indeed, there was
no limit to the crimes which von Hildemaller could perpetrate, more
particularly if they were for the ultimate benefit of his own country.
With the smooth, smiling, genial face almost of a child, he was at
heart a wretch, a cruel, scheming, cunning creature, an
unscrupulous agent, capable of planning any atrocity. When that was
said, we have von Hildemaller's full character, and we have merely to
add that, like many of his kidney, when the planning was done,
when the schemes for assassination and murder were arranged, the
power for evil of this German suddenly subsided. He could scheme,
but he lacked the courage to carry out his enterprise. His was the
crafty brain which arranged the deed but contrived to get another to
carry it out for him. Thanks to a Government which supplied him
with ample funds, he could command in this country a host of
ruffians. Pooh! The assassination of a British officer was quite a
small matter, to be arranged on the spur of the moment, and to cost

not so much as a second thought, and no great sum of gold when all
was considered.
Von Hildemaller snapped his fingers and mopped his face again as
the Turk sped away from him; then, lighting a German cigar, and
puffing at it till he got it going to his satisfaction, he strolled—
waddled rather—through the Bazaar, and on to his own quarters.
"Quite a nice sort of fellow, that Douglas Pasha!" he was telling
himself as he went. "For a Briton, quite a respectable individual!
Conceited? Yes! But then, that's a fault of the nation; but honest,
clear-headed, I think, friendly and—yes—certainly—simple!"
"Simple!" did he say? If the worthy German, waddling through the
Bazaar, could have seen Major Douglas at that moment, he might
have had cause to reflect a little, and to change his opinion. For,
though the gallant Major may have made pretence at simplicity
when meeting the German, though he may have given the
impression of being shallow, of being thoughtless, and of possessing
not so much as an atom of cunning, yet Douglas Pasha had not
travelled through Mesopotamia, had not met hosts of Germans, had
not studied the history of Germany and her people, without learning
many lessons. It was a habit of this gallant officer to study
unconsciously the character of every individual with whom he came
in contact, and thus it happened that the worthy von Hildemaller
had, as it were, come under the microscopic examination of this
British officer.
"Very charming, ahem! I am sure. A most excellent fellow to meet in
a café, say on the Grand Boulevard in Paris, or in the Unter den
Linden in Berlin. A generous host, a loud-speaking, merry fellow, but
insincere, unscrupulous—like his people—out for something big,
something to benefit his own country; to be carefully watched, and
distrusted, and yet to be met in the most friendly manner possible."
That was the Major's summing-up of the excellent and cunning von
Hildemaller; and now, as he took the nearest cut back to his own

apartments in the city of Bagdad, apartments which he had occupied
on more than one occasion, there was something in his face which,
if the German could have seen it, would have warned him that
Douglas Pasha was hardly so simple as he anticipated.
"Unfortunate meeting that German," Joe Douglas was telling himself
as he hurried along. "Of course he knows just as well as I do that
war has been declared between Great Britain and Germany, and that
Turkey is likely to come into the conflict. That being the case, he and
I are hardly likely to remain on speaking terms after this; indeed,
he'll look upon me as a dangerous enemy, just as I look upon him.
Shouldn't wonder if his hirelings are already watching me, and—yes
—there are tales of the worthy Herr von Hildemaller which aren't too
pleasant."
Rapping sharply on the door of his lodgings, he was admitted by an
Armenian servant, and at once strode into his sitting-room. Throwing
himself into a cane-seated chair and lighting a cigarette, he then
rapped sharply on the table.
"Pack up," he ordered; "we leave in five minutes. Wait! What's that?"
Someone was rapping on the floor below them, someone who called
in low tones for admission. Instantly Joe Douglas sprang to his feet,
and, pulling the chair away, and the table, dragged a piece of
Turkish carpet on one side, disclosing a narrow trap-door.
"Enter!" he called, and helped the person below who had demanded
admission to raise the opening.
And slowly, as he did so, there emerged from a dark hole below, by
means of a roughly-made ladder, the big, bony, angular form of that
same hook-nosed Jew with whom he had haggled in the Bazaar not
half an hour before.
"H-h-'sh! Listen, Excellency!" The man stood half in and half out of
the opening, one warning talon held upward, his beady eyes fixed

on Douglas Pasha, his lips trembling. "That man! That German
hound! That scoundrel!"
The gallant Major was the very last individual to show alarm. In fact,
fuss and worry were things he hated intensely, and his nonchalance
on all occasions was something which long ago had attracted the
admiration of his comrades. He still smoked on, and, throwing
himself into his chair, and flinging his legs on the table, he smiled at
the Jew and bade him proceed with the story.
"Yes, the German, von Hildemaller!" he said. "A most excellent
gentleman! And you said beware, my friend, did you not? But surely
——"
He gave vent to a laugh, an ironical laugh, which grated on the ears
of those listening, and which warned them that, though the German
may have considered this British officer to be childishly simple, he
was yet well aware of the danger which surrounded him.
"Listen, Excellency!" said the Jew, emerging now completely from
the chamber beneath the room in which Joe Douglas was seated. "I
watched the scene from my stall. Long ago I warned Your Excellency
that this German had no love for you, that his hirelings were
watching you and dogging your steps, and that some day he would
do you a mischief. Now the day has arrived! Even as you hurried
away from that accidental meeting with him, I saw him call to one
whom I know to be nothing but an assassin—a wretch—whose knife
is at the bidding of anyone who can pay him money—one who
should long ago have been hanged in the market-place. Leaving my
stall, I followed this rascal, and saw him call to others. Even now
they are arming, and, as dusk falls—which will be within an hour
perhaps—they will break a way into this dwelling and carry out the
purpose of this German."
Joe Douglas whistled, a merry whistle, and smiled in the most
friendly fashion at the Jew. He even got up from his chair, still
smoking, and patted him reassuringly on the shoulder.

"My friend," he said, "I thank you from the bottom of my heart for
this warning; not this time alone, but on many occasions, have you
proved a real friend to me, and may it be many a day before I forget
your loyalty. But, as it happened, I guessed the intentions of our
worthy friend von Hildemaller. Already I have given orders to pack
up all my belongings, and soon, in a little while indeed, we shall be
out of this place, leaving it to the hired assassins of the German."
There was bustle in that little house in the ten minutes which
followed, all hands being engaged in packing the Major's belongings.
Then, having completed the work to his satisfaction, the Jew and the
Armenian servant of Douglas Pasha dragged his trunks through the
opening down into the cellar beneath. Long before that, Joe Douglas
had transformed himself into an absolute replica of the Jew who had
come to warn him, and, indeed, looked the part to perfection. Then,
casting a hurried glance round, and throwing the light from an
electric torch into every corner—for already the dusk was falling, and
the house opposite darkened that in which he had been living—he
slid through the opening in the floor, and gently lowered the trap-
door after him, having just before that dragged the table across it.
Then the three made their way to the far edge of the cellar, and,
ascending some steps, entered a narrow alley. There, at the bidding
of the Major, his two companions went off to their left, while Joe
Douglas made ready to venture into the open.
"You will go to the old quarters," he told them in a whisper, "while I
see what is happening in the street yonder. To-night, as the moon
rises, you will have a conveyance ready for me, and to-morrow we
shall be well out in the desert."
But a minute before, Douglas Pasha, in spite of the rags with which
he was now covered, was without doubt the tall British officer who
had made his way into the heart of the city of Bagdad; but now, as
the need to act up to his disguise arrived, he became transformed in
a manner which was really remarkable. Leaning on a long, stout
stick, his head and shoulders bent, and his legs tottering, he
stumbled from the alley into the open street, and shuffled and

clattered his way along past the door of his own dwelling. It was
there that he almost collided, in the dusk, with three Turkish rascals,
one of whom was preparing to break the door in with a crowbar. Yet
the Jew took no notice of them, but stumbled past, muttering into
the cloak which covered his head, talking to himself, and pulling his
rags round him. A little farther on, less than a hundred yards,
perhaps, he caught sight of a rotund and perspiring figure in a
sunken doorway—a figure which was faintly illuminated by an oil
lamp hanging in a passage opposite. It was the figure of von
Hildemaller, who had crept to this spot to watch the doings of his
hired assassins. Again it was characteristic of the Major that he
halted in front of the man, careless of the consequences.
"Money! Money to buy food and lodging," he whined, holding out a
shuddering, shaking hand, while his whole frame swayed and
tottered. "Money, Excellency, to keep body and soul within me!"
"Money! Bah!" The German struck at him with the light cane he was
carrying, and threw a glance of hatred and contempt after the
tottering figure of the Jew as he retreated.
Then with wide-open ears he listened as the door of the house along
the street was burst open, and waited breathlessly for news from his
assassin. It was with a storm of rage and disappointment that he
learned that the place was empty, that Douglas Pasha was gone, and
that the scheme for ending his energies in Mesopotamia had been
defeated.
Yet the cunning of this German was not always to meet with such ill
success, for though Douglas Pasha contrived to escape from Bagdad
that night, and made his way into the desert, there came a day
when von Hildemaller traced him. Also there came a day when
Douglas Pasha—a prisoner then, and none too well treated—
contrived to get a message out of the Turkish fortress in which he
was incarcerated. Even as Geoff Keith, and Philip, and Commander
Houston braced themselves for a stiff engagement with the Turks
aboard the steam-launch which had been pursuing them, that

message was speeding down the Tigris towards the British forces. It
was a request for help, but with no definite statement of the position
where Douglas Pasha was imprisoned. And there were miles of
desert country to traverse, and hundreds of enemies to pass, ere the
messenger could bear his missive to our Head-quarters. It was a
toss-up, indeed, as to whether the news of the Major's plight would
ever reach his own people; just as it was a toss-up whether Geoff
and his comrades would ever contrive to beat off the Turks who
were about to assail them.

CHAPTER VIII
The Motor-boat in Action
There was a deathly silence about the reed-clad island which
separated the motor-boat, with its British crew, which was stealing
along one side of it, and the wide-stretching marshes on the farther
side, where the Turkish launch forged her way slowly, steering for
the far end of the island. There was just the gentle purr of the petrol
motor aboard the British boat as it slowly turned over—that and the
occasional click of a rifle-lock, as one of the men saw to his weapon.
From the far side, however, there came voices on occasion,
smothered every now and again by the burr and hiss of steam as it
escaped from the safety-valve above the boiler. Geoff looked over
the side and peered into the water; then he took a boathook and
thrust it downward till it struck the bottom of the swamp close
beside them. An instant later he had plucked the Commander by the
sleeve, and was whispering to him.
"Look, sir," he said; "not much more than two-feet-six of water; you
can see the mark on this boathook; and it's hard ground down below
—listen!" He sent the boathook down through the water again till
the end struck heavily on the bottom, and sent forth a dull, ringing
sound.
As for the Commander, he drew the inevitable pipe from between his
lips and looked inquisitively at Geoff and then at the boathook.
"Yes?" he asked. "What then?"
"Might be useful," Geoff ventured. "A couple of men dropped
overboard could take cover at the edge of the island in amongst the
reeds, and might help us immensely."

Commander Houston smiled an indulgent smile at him, and gripped
him by the shoulder.
"Well done, Keith!" he said in that sharp, commanding tone of his.
"Take a man with you, and get a rifle. Quick with it! for those Turks
will be clear of the island within a few minutes. Here, Smith! You're
one of my best shots. Overboard with you!"
There were spare rifles lying in the open cabin of the motor-boat,
and beside them clips of cartridges. Geoff instantly seized one of the
weapons, and filled a pocket with ammunition; then he dropped
overboard, while the man who had been called joined him within
half a minute with a grin of expectation, while on the faces of his
comrades there was a look almost of envy.
"Come!" said Geoff, wading through the water and finding the
ground at the bottom as he had expected—hard, and giving firm
foothold.
Indeed, it would appear that the wide swamps they were now
traversing, and which seemed to be composed of practically
stagnant water, were stirred and swept now and again by eddies
from the main stream. Perhaps in those violent gales, which every
now and again sweep across Mesopotamia, the waters from the
Euphrates are driven into the marsh lands, and, instead of flowing
slowly and almost imperceptibly across them, filtering through them,
as it were, they rush and sweep through every channel, heaping
islands of mud here and there where there happen to be eddies, and
carrying on vast accumulations of ooze and slime to other quarters.
No doubt, too, in dry seasons, when the Shatt-el-Arab has fallen
considerably, and the depth of the water in the main stream is much
reduced, the waste of water lying at such a time across these marsh
lands drains away, leaving a glistening, sandy desert. In any case,
there was good going at this spot, and Geoff and his comrade made
the most of it.

Wading up beside the island, they advanced, within a couple of
minutes, some yards towards the upper end, to which the Turkish
launch was fast approaching.
"In here," said Geoff, seeing an opening between some reeds where
the bank jutted out a little and formed an angle or depression. "Now
cut some of the reeds away with your knife, so as to give you a
good field of fire and clear vision."
"Make ready!" they heard the Commander call to them gently, just
after they had got into position, and, turning to look at the motor-
boat, they saw that she had moved farther out from the island, and
was now lying end-on, her bows presented to the spot where the
enemy was to be expected.
Almost at the same instant, the shriek of a steam siren came from
the far distance—from that big Turkish steamer which had so
unexpectedly opposed the advance of this British party on the River
Euphrates, and, following it, an answering shriek, more piercing in
its intensity, from the steam-launch drifting but a few yards away
from them. Then her bows appeared, to be followed in a little while
by her funnel, and then by the whole length of her. There was foam
at her stern, while smoke was blowing out from the top of her
funnel, for she was under way again, and, indeed, was steering a
course towards another island which dotted the marshes in the
distance. Perched on a raised portion of the deck, just in front of her
funnel, was a Turkish officer, shouting loud commands; while on the
deck for'ard of him were gathered some twenty or more soldiers, all
eager and expectant; yet, as it happened, their gaze was fixed on
the distant island, and not upon the water beyond that from behind
which they were just emerging. Thus it followed that more than a
minute passed before one of them noticed the motor-boat stealing
gently, bow on, towards them. The man started and shouted, lifting
his rifle high over his head.
"Look!" he shouted, so suddenly, and in such a voice of alarm, that
the officer was startled. Swinging round, he too saw the motor-boat,

and himself took up the shout with a vengeance.
"The enemy! Swing the ship round! Fire into them!" he bellowed.
"Steady lads!" cried Commander Houston, standing erect in his
cabin. "Marsden, stop her! Now, boys, let 'em have it!"
A volley burst from the weapons of the sailors in the motor-boat,
and several of the Turks fell from the steam-launch and splashed
into the water. By that time bullets were sweeping about the head of
the Commander, while not a few struck the sides of the motor-boat
or the surface of the water near at hand, throwing up spray which
swept over the heads of those who manned her. But not a man
flinched; while Commander Houston, snatching his pipe from
between his teeth, roared encouragement at the sailors.
"Let 'em have it!" he cried. "Now, Keith," he bellowed, swinging
round to our hero, "put in your bullets as fast as you are able. Ah!
That has dropped their officer. Just keep your eye on the man at the
wheel, and the man who's running the engine, for we can't afford to
allow that boat to get away from us."
His teeth had gritted on the stem of his pipe a few seconds earlier,
and, unseen by his men, the Commander clapped a hand to one
shoulder. Perhaps it was a minute later that he wiped blood from his
lips with his handkerchief, and then, like the old "sea-dog" he was,
thrust his pipe back into his mouth and went on smoking, still
careless of the bullets humming about him, his eyes fixed all the
while upon the enemy.
As for Geoff and the man with him, they were able to make excellent
shooting from the point of advantage where they had taken cover.
Seeing the Turkish officer level his revolver at the Commander, and
pull his trigger—a shot which caused the Commander to act as
already narrated—Geoff levelled his own piece on him, and gently
pressed the trigger, sending the Turkish officer in amongst his
soldiers. Then Smith, the watchful sailor beside him, grim and silent
and stern now, picked off the man at the wheel of the steam-launch,

while Geoff transferred his attention to the Turk whose head bobbed
up and down above the engine.
"GEOFF LEVELLED HIS OWN PIECE ON HIM"
Perhaps two minutes had passed since the first exchange of shots,
two busy minutes, during which more than half of the crew of the
Turkish launch had been killed or wounded, while as yet, but for a

slight wound here and there, not one of the British sailors had been
damaged. And now a figure suddenly took the place of the Turkish
officer.
"An under officer," shouted the Commander, "look out for him!"
"He is giving orders for the steam-launch to get under way again,"
cried Geoff—for at the first discharge the engine aboard the enemy
vessel had been stopped. "Come along, Smith, we'll wade out to her
and stop any sort of movement."
Floundering out from behind the cover he had selected, and with his
rifle held well above the water, Geoff led the way direct to the
enemy vessel, while a well-timed shot from the motor-boat sent the
under-officer in amongst his fallen comrades. Then the engine
aboard Commander Houston's little vessel began to thud, while the
water behind her was churned, and as the screw got into operation
she darted forward towards the steam-launch, the rifles of her crew
spitting bullets still at the Turks who remained in evidence. Then, at
a shout from the Commander, the fusillade ceased absolutely,
though the motor-boat still pushed on towards the enemy.
"Cease fire!" bellowed the Commander; "they have surrendered; see
that man holding his hands up towards us."
Taken by surprise as the Turks were, and broken indeed by the first
volley, it was not extraordinary that this little British force had at the
very commencement the best of the argument. The raking volley
which they had poured into the enemy had thrown them into instant
confusion, while the shots which Geoff and the man Smith, who
went with him, had fired, had contributed not a little to the success
of the operation; and now, with her deck covered with wounded or
dead, the launch surrendered; a soldier, a huge, well-grown Turk,
standing there amongst his comrades, with both arms held over
head, and calling to the British to spare them. By then Geoff was
within a few yards of the launch, and, staggering on, clambered
aboard her. A glance into the open engine-room showed him a man

cowering there, the one whose head he had seen bobbing above the
side of the vessel a few moments earlier.
"Come out!" he commanded briskly. "No, you won't be shot, and
don't fear it, for you've been captured by British sailors. Smith, get
hold of that wheel. Now let every man who has escaped injury 'fall
in' on the deck, so that you may be counted."
A hail reached him a moment later from the motor-boat, and,
turning for a second, and so taking his eyes from the Turks now
mustering on the deck quite close to him, he saw Philip waving
frantically to him; but of the Commander there was not a sign, for
indeed that gallant individual was reclining in the depths of his
cabin.
"Geoff, ahoy!" he heard. "I'm coming up close to you. Commander
Houston's wounded."
"Stop!" Geoff shouted back at him. "Back your boat in behind the
island, where I'll join you. Smith, can you see any sign of that
Turkish boat we met in the river?"
There was half a minute's pause before he received an answer, and
then the fine fellow he had posted at the wheel called gently to him.
"Not a sign, sir," he said; "those islands yonder, through which we
came on our way here, hide the channel of the river. She's out of
sight, and can't see us either, though there's no doubt that she's
within fairly close distance."
"Which means that she will have heard the firing. Hum!" thought
Geoff, as he swept his eyes round the waste of waters and
wondered what would happen. Then he called to the Turk who had
been manning the launch engine.
"Get down to your engine again," he commanded, "and give her a
little steam. Smith, swing her round behind the island. We'll lie up
there with the motor-boat for a while, and see to the Commander,
and repair damages."

The minutes which followed were busy ones indeed, for, as may be
imagined, there was much to be done after such a brisk little
encounter. Swinging the launch round, while the Turk gave the
engine steam, Smith steered her in till she was quite close to the
island; then the motor-boat came alongside her, and the two vessels
were moored there, the crew of the British vessel taking ropes
ashore, and their own and the launch's anchor.
"I'm not a sailor," Geoff told the men aboard the motor-boat, when
at last they were secured to the island, "so I'll leave it to the senior
amongst you to look to your damages. You've got some shot-holes
about your hull, I'm sure, for I heard the bullets strike, and I can see
water spurting in in more than one direction. Just post four men up
on to the deck of the launch to look after our prisoners, and let one
man make his way through the reeds of the island to the far side to
keep watch for the arrival of more enemies. Now, Philip, give a hand
and let us look to the Commander."
Leaping down into the cabin, they found Commander Houston lying
full length upon the floor, his face wonderfully changed from that to
which they had become accustomed. Instead of displaying a ruddy
countenance, and cheeks which glowed with health and vigour, there
was now a deathly pallor upon the merry face of their friend, which
seemed to have shrunken and grown smaller. But if the gallant sailor
had suffered an injury, as indeed he had without a doubt, and if he
were placed hors de combat by it, there was yet no loss of spirit, no
lack of joviality; indeed the same happy smile wreathed the pallid
face of this most gallant fellow, while he was still actually making a
pretence of smoking.
"A nice brisk little affair; eh, boys?" he said weakly, in tones which
evidently astonished and disgusted himself, for he apologized for
them. "Don't take any notice of my voice," he told them; "it's
nothing, believe me; merely a shot through my chest, for which I
have to thank that Turkish Commander. A mere trifle, I assure you,"
he went on, and then coughed violently, while blood dribbled from
the corner of his mouth.

He shut his eyes, and, in the midst of calling to them again, fell
backwards heavily, leaving both Geoff and Philip dismayed at his
appearance. Springing forward, Phil lifted his head and supported
the Commander against his knee, while Geoff rapidly undid his tunic,
and, seeing clearly from the stain upon it where the wound must be,
tore the shirt open. But what to do further was the question with
him, for, though our hero may have had some experience already of
travelling, and had undoubtedly seen rather more of foreign places
than is the lot of most young fellows, yet he was singularly ignorant
of wounds, had seen few indeed, and had practically no training in
minor surgery. But amongst the crew there was one who was quite
an experienced old sailor, who, had he cared to tell his tale, no
doubt could have yarned to them of many a naval scrap in out-of-
the-way places. It was the Cox who joined them now—a short,
broad-shouldered, rather wizened fellow, with a cheerful smile
always on his face, and with that brisk, respectful, helpful way about
him so common to his counterpart, the non-commissioned officer, in
the army.
"You just hold on to him like that," he told Philip, who was
supporting the Commander's head and shoulders. "No," he added in
a warning voice, "no, I wouldn't let him lie down flat, sir, if I was
you, 'cause, you see, sir, he's hit through the lung, and he's bleeding
internally. If you just think for a moment, sir, you'll see that that sort
of thing is likely to drown a man, to swamp his lungs, as it were, and
the more you can sit him up for a while the better. Hi, Marsden," he
called, "let's have that surgical pannier!"
If Geoff and his chum were entirely ignorant of wounds beyond what
knowledge was required to place a first field dressing in position—
and that was a task which every officer and man learned as a matter
of course—the Cox was, on the other hand, quite a respectable
surgeon. While Philip held the Commander's heavy frame up, the
broad-shouldered little sailor cut away his tunic and shirt, and,
having exposed the wound both at the front and at the back—for the
bullet had passed right through the body—he swiftly dabbed each

wound with his brush loaded with iodine, and then clapped on a
dressing.
"Next thing is to bandage him up so as to leave the other side of his
chest free to move, and keep this side just as still as possible," he
told Geoff; "that will give the damaged arteries and veins a chance
to heal and stop bleeding. Beg pardon, sir, but if you'd hold the box
of dressings I can help myself easier."
With dexterous hands—hands which were as gentle as might be, in
spite of this sailor's rough calling—the Cox rapidly secured the
dressings with a roller bandage. Meanwhile, at a call from Geoff, the
cabin cushions had been laid on the boards at the bottom of the
cabin, and on this improvised bed the Commander was now laid, his
head well propped up with cushions.
"And we'll just roll him over on to his damaged side, like that," the
Cox told them. "That means that, as he breathes, that side won't
move, and can't move overmuch, while the other one will be doing
all the work for him. He is opening his eyes, I do declare! Why!—--"
Two penetrating and rather fierce optics were fixed on the trio in the
cabin at that moment, while the Commander struggled to move.
Then the eyes opened quite widely, the lips curved, and in a second
or two he was smiling serenely.
"So the Cox is practising on me all that I've taught him, eh?" he
asked, and Geoff noted with relief that the voice was stronger and
steadier. "I knew it would come to that some day; I kind of guessed
it. Well, Cox, what's the verdict? What's the diagnosis? Is it a cure
this time, or has that Turkish officer put in a shot likely to deprive
His Britannic Majesty of a somewhat valuable officer? Ahem!"
The gentle cough he gave brought another driblet of blood to the
corner of his lips, and caused Geoff to kneel down beside the
Commander and expostulate with him.
"Really, sir," he said, "you must keep quiet and stop talking. You——"

The eyes of the old sea-dog who had seized so greatly upon the
fancy of Geoff and his chum, opened widely again, and that same
expansive, warm-hearted grin was turned upon them.
"Oh! oh!" he exclaimed; and, there was no doubt about it now, his
voice was growing steadily stronger. "So our young officers wait until
their senior is knocked out, and then start bullying and ordering! Oh!
So that's the game, is it, Keith? You are beginning to show up in
your true colours! Believe me, my lads, I'm not nearly so bad as you
imagine, and, 'pon my word, in a little while I shall be fit to get up
and start smoking."
Then he laughed, or, to speak the truth, cackled, for the effort of
real laughter was beyond him, while he glanced quizzically at Geoff
as that young officer coloured furiously. Yet, though he knew that
the Commander was making fun of him, he was delighted at his
progress, and a moment later was joining in the merriment.
"Come now," said the Commander, a little later, "tell me all about the
thing. You had just knocked that Turkish officer out, and a huge Turk
was lifting his hands in token of surrender. I don't seem to
remember anything after that; I must have tumbled backwards into
this cabin. And now that you have laid me on the floor, there's no
seeing anything but the sky above me. Where are we? Where's the
Turkish launch? What happened? And, of course, we captured the
beggars!"
Very quickly Geoff told him precisely what had resulted from their
attack upon the Turkish launch, and how they had captured the
vessel, and what remained of her crew.
"We are lying to, behind the island, at this moment, sir," he added,
"for by doing so we are hidden from the enemy. I thought it best to
repair damages."
"Yes, yes! Human and material," smiled the Commander, who was
ever on the look-out for some little joke. "But wait! I may not be the
only one wounded. What's the report from my fellows?"

Philip had already obtained it, and at once communicated the facts
to Commander Houston.
"One man hit through the fleshy part of his arm, and only slightly
incapacitated; another has lost the tip of one finger. That's all the
human part about it, sir," he said, with a grin. "As for the material:
there are half a dozen holes bored through your motor-launch, and I
believe the Cox has already made a cure by means of filching
material from the box containing surgical dressings."
"Good! We have come through that little business splendidly," said
the Commander. "And now, what next?" he asked, fixing his eyes on
Geoff and then swinging them round to Philip. "What next? You have
captured the launch——"
"We!" expostulated Geoff. "You were in command, sir, don't forget
that! And by the time you fell their resistance was almost finished."
"Then 'we'—we have captured the launch, and that, you will
remember, was a point I laid stress on. Then?" asked Commander
Houston, peering into Geoff's face. "Did it occur to you, young Keith,
that——"
Geoff smiled at the wounded Commander, and seated himself
opposite to him.
"I think the same idea occurred to me, sir," he said, "and perhaps
somewhere about the same moment. You see, the Turks aboard that
steamer, the fellows who fired that gun at us, know now well
enough that the British have sent a motor-boat up the River
Euphrates, and a motor-boat is a thing they will be hunting for. But a
steam-launch, one of their very own, manned by a Turkish officer
and Turkish soldiers, would have a chance to pass up the river right
under their noses. In command of a boat like that, a fellow might
find out a great deal more than if still aboard this motor-boat. So I
thought that if we were lucky enough to capture the launch we
might send off a party on her."

"Showing that wise heads think in the same direction," the
Commander laughed a second later, though his eyes were twinkling
with excitement. "Confound this wound! But for that, I can tell you, I
should have commanded this second expedition. The scheme is just
one that is likely to succeed, and, as you say, Keith, has better
chances than we should have, now that the Turks have dropped
upon us. Being wounded myself, of course, I shall have to give way
to another, and it looks to me as though our friend the Cox would
have to command this little expedition."
You could have knocked Philip and Geoff down with the proverbial
feather. Their faces, which had been smiling before and lit up with
enthusiasm, suddenly lengthened, while they regarded the
Commander with something akin to horror, if not positive anger.
"But," exploded Philip, "I—you—we——"
Commander Houston laughed again, laughed till he choked and
coughed, and until Geoff begged of him to take things quietly.
"I—you—we——" he said at last, mimicking Philip. "Well, well! I'll
tease you no further. Of course, Keith will take charge of this little
affair; and seeing that you, Denman, are, as it were, under his direct
command, why, of course, he'll take you with him. For me, though I
like to take things in the right way, and not make a fuss, I realize
well enough that that Turkish officer has knocked me out completely.
Don't worry!" he went on. "I'm hit hard, I know, but it takes a
precious deal to kill a man of my stamina; and, to tell you the truth,
though I feel weak and rather knocked out for the moment, I'm very
far from dying. But marsh lands and swamps, such as we lie in, are
not good for wounds; and that being the case, and since I should be
a hindrance to the whole party, I shall 'bout ship and steam down to
the Shatt-el-Arab. We know the route now, we shall have little to
fear once we are away from this neighbourhood, and we can travel
with a diminished crew. Keith, my boy, set about investigating the
contents of our capture."

Leaving the Commander in the cabin, and taking the precaution to
haul a piece of sailcloth over the opening above so as to shelter him
from the direct rays of the sun—which were now pouring down upon
the marshes—Geoff and Philip stepped aboard the captured launch,
and made a thorough survey of her, discovering a quantity of rifles
and ammunition, besides a supply of dates and coffee. In a cabin aft
of the engine-room there were some tinned provisions, which no
doubt had belonged to the officer. For the rest, there was sufficient
fuel aboard to take the vessel a considerable distance, and, in fact,
little was required to make her fit for service.
"We could go off on her right away," Geoff told his chum, his voice
exultant, "for there is food enough on board to feed you, and me,
and the crew we shall require to man her. As to water, we can get
that from the boiler at any time, and so need have little fear of fever.
I vote we ask the Commander to allow us a certain supply of
provisions and ammunition for the men we take with us. As to the
number of the latter, of course, he will decide upon it; but the
sooner we select our men the better, for they must discard their
present clothing and dress up in the uniforms of the Turkish
soldiers."
When they came to the point of selecting the half-dozen men that
the Commander decided to allot them, Geoff found that he was face
to face with an unexpected difficulty. For, calling the sailors about
him on the deck of the Turkish launch—as he wished to leave the
Commander quietly resting—he had barely opened his mouth
sufficiently to explain what was about to happen, and to call for
volunteers, when every man of the party stepped forward. More
than that, there was an insinuating smile on the faces of all, without
exception, the sort of smile a man indulges in when he wishes to ask
a favour. It was a kind of dilemma which an older man than Geoff,
and one far more experienced, would have dealt with at once,
though not without difficulty; but Geoff, we admit the fact, was
utterly confounded.

"But," he stuttered, "I—don't you know—I—well, that is, I only want
six of you, so what's the good of all of you volunteering?"
"That's just it, sir," the Cox explained. "There's not a single man jack
here who don't want to be one of the party. Beg pardon, sir," he
added, a moment later, seeing that Geoff was puzzled and
perplexed, "if you was to leave it to us we'd soon fix the business.
We'd draw lots, and then not a single one of the men could feel that
he was out of favour. The lucky ones would be envied, that's all, and
the rest of 'em would go back with the Commander as pleasant as
possible."
Within a few minutes, as a matter of fact, the whole matter had
been amicably settled; and thereafter Geoff and Philip were busily
engaged in dressing the men they were to take with them, securing
for that purpose the clothing of Turks who had fallen during the
conflict. Then, about an hour before dusk fell, they set off from the
place where they had been lying behind the island, the Turkish
engineer still manning his engine, while one of their own men was at
the wheel. Philip was right for'ard, quite a fierce-looking Turk in his
dirty khaki uniform and fez head-covering. As for Geoff, he sat on
the little platform just in front of the funnel, and no one taking even
the closest look at him would have suspected him of being a British
officer. A moment before, he had gripped the Commander's hand
and had received a cheery send-off from him. Then smoke gushed
from the funnel, the Turkish engineer pulled gently at his throttle,
and the screw of the steam-launch began to churn the water. Signals
were exchanged between those seeming Turkish soldiers on the
deck of the launch and the British sailors still remaining on board the
motor-vessel. Then the launch gained the far end of the island, and,
swinging round it, disappeared, the last glance which Geoff cast over
his shoulder showing him a number of disconsolate individuals
watching their departure, while, seated aft on the motor-vessel,
were the nine or ten prisoners whom they had captured. Stealing
silently across a wide stretch of swamp, and answering cheerily a
signal flung out from the bigger Turkish steamer somewhere away

on the river, the launch was headed to the left until she gained a
group of islands.
"In here, Excellency," said the native, who, naturally enough, formed
one of the party. "There's a channel amongst those islands which I
have followed, and which will take us up within half a mile of the
river stream, yet hidden from it. Let the man drive the boat faster
while there is nothing here to impede us."
As darkness fell that night, the launch was tearing along through the
stagnant water, flinging a bow wave on to the islands which cropped
up, now to the right and now to the left of her. Sometimes her
steersman was forced to make her swerve somewhat violently, to
avoid an obstruction consisting of ooze and mud and covered with
thick-growing reeds, but for the most part her course was directly
forward, and parallel to the river. At length, as darkness fell, the
engines were stopped, and the boat was brought to a halt between
two islands. There the anchor was dropped, and the little force made
ready to spend the night and to prepare for an eventful to-morrow.

CHAPTER IX
A Cutting-out Expedition
"What's that? Listen! I heard something!"
Geoff cocked his head up over the side of the cabin in which he and
Philip had been partaking of their evening meal, and turned his face
towards the River Euphrates, across the waste of ooze and mud and
water which separated their captured launch from it—a waste hidden
by the darkness, and yet illuminated ever so faintly by a crescent of
the moon, which was already floating above them, while stars
peppered the sky in every direction, and helped to make things
visible. Across the waste of water, dulled by the whisper of the
evening breeze as it rustled through the reeds and osiers, a sound
had come to Geoff's ears, a sound which caused him to enjoin
silence upon all aboard the steam-launch. Then, as he listened,
there came to his ears, at first faintly only, but growing steadily yet
gradually louder, the plug, plug of the paddles of a river steamer.
"The Turk who had the cheek to fire that shot at us!" exclaimed
Philip. "Listen to him! He's going up the river, and I dare say he's
wondering what's happened to his launch, and whether he'll find
that rather nice and comfortable little vessel waiting for him up-
stream. Eh, Geoff?"
"Listen! The paddles are going slower, and it sounds to me as if the
steamer was going to pull up for the night. You must remember that
the Euphrates isn't the sort of river that one cares to steam up at
any kind of pace during the hours of darkness, for by all accounts it's
stuffed full of sand-banks and muddy islands, which are always
changing, 'specially after rains and storms. There's a voice," Geoff

went on; "that's someone giving an order! And now the paddles
have ceased altogether."
"Plunk! There goes her anchor. She's come to a roost without a
doubt!" ejaculated Philip. "That's rummy, ain't it? Our Turkish friends
will be settling down for their evening meal—or whatever sort of
thing they have—within sound of us, and, I'll lay my hat, without
suspecting that their precious steam-launch is within easy reach of
them."
Geoff stretched out a hand in the semi-darkness and gripped his
chum by the shoulder.
"Splendid!" he said.
"Eh?" asked the other, a little bewildered. "What's splendid? Having
the Turks so close to us? 'Not 'arf', as 'Tommy' is fond of saying.
Why, we shall have to lie as quiet as mice here, and the next thing
you'll be doing will be to order us to cease smoking, for fear the light
of our pipes should be seen aboard the steamer. Most inconsiderate
of that Turk, I call it! For he might at least have stopped down the
river, or gone a little higher, so that we might have passed a peaceful
night, and made ready for all sorts of things to-morrow. 'Splendid!'
Hum! Sorry I can't agree with you, my dear fellow."
If he could only have guessed what was in Geoff's mind at the
moment, and could have seen that young fellow quite clearly, Philip
might easily have given expression to quite different opinions. For, to
be precise, our young hero, dressed in the uniform of a Turkish
officer, and with a Turkish fez perched on his head, was as near the
actual thing as could well be imagined. In daylight, in the city of
Bagdad, and, for that matter, in any other city, he might very well
have passed muster; while the fact that he was able to speak the
language fluently—as fluently as any native—made his disguise all
the better; and now, with some idea in his head to which Philip was
a stranger, there occurred to Geoff the thought that the coming of

this steamer to such close quarters presented a splendid opportunity.
He shook his chum savagely, so as to silence him.
"You don't let a fellow finish!" he exclaimed. "But it's splendid, really
splendid, that that steamer should have dropped her anchor within
easy reach of us."
"And why, pray?" asked Philip, rather inclined to banter with his
senior officer.
"Why, being so near makes it all the easier for a fellow to get aboard
her."
"A—bo—ard her!"
Philip opened his mouth wide, and his eyes too, though that didn't
help him to see his chum any the better.
"Well—but—surely—you don't mean to——Well, I'm hanged!" he
exclaimed. "And—of course—of course it's splendid, as you say—a
splendid opportunity. But you'll never think of going alone, eh,
Geoff?" he asked, with a pleading note in his voice. "Supposing a
Turkish sentry caught hold of you? Supposing you got 'lagged'
immediately you were on board, what then? I——"
"You would be required aboard this launch to take command of the
expedition," Geoff told him curtly. "But let's be serious, Phil. We're
out to learn all we can of the Turks, and, as you know, it's been
reported that the enemy are gathering somewhere up the River
Euphrates, behind or in this long stretch of marsh land. We might
push up the river in the early morning and discover them. We might
barge into the very midst of them, and find ourselves surrounded,
with no chance of getting away and carrying our information to
Head-quarters. But what we want to know is known aboard that
steamer. The officer in command is nearly sure to be of superior
rank, and in any case he must know where the Turks are
assembling."

"And so," argued Phil, as he bit at a cigarette, "and so, my boy,
you've designs on the steamer. 'Pon my word! I wish I was able to
speak the lingo. Languages are things I've always hated; but I can
see what advantages they give to a fellow, what fun they bring him,
and—ahem!—what chances of promotion. So you'll go aboard? Wish
the dickens I could come with you."
"I shall go aboard and find out the whereabouts of this officer."
"And then you'll listen to his conversation through the keyhole if
need be," said Philip, whose buoyant spirits always made him seize
upon the smallest opportunity of being facetious. "Keyhole, eh?
Wonder if Turks have 'em? Anyway, you'll contrive to find a spot
from which you can hear the old bounder; and then, of course, the
business will be to make him converse upon the subject upon which
you are most interested. That's a teaser, eh? How will you do it?
Supposing he's immersed in an argument about the war, and about
the rights and wrongs of the Turks and the Germans; or supposing
he's only telling his under-officer—for I suppose there is such an
individual—all about his home life, his wife and his children, his
house and his garden. Supposing, in fact, he won't get on to your
line of argument, and won't babble about the Turks and their
concentration in the marshes."
Hum! It certainly was a teaser, and the situation as Philip drew it had
not occurred to Geoff before. That it was possible to reach the
steamer in the tiny dinghy carried aboard the launch, and to clamber
unseen aboard her, he did not doubt; that he might, by skill and
cheek, contrive thereafter to get within sight and sound of the
Commander, he thought was within the bounds of possibility; but to
make that Commander talk, to make him give the information which
Geoff sought, was an entirely different matter altogether.
"By George!" he exclaimed; "that would be awkward."
"It would," Philip told him in tones of irony. "You're aboard the
steamer, you've—not actually, but let us say metaphorically—sat

down in the cabin occupied by this old bounder, and then he won't
talk, you can't make him talk; he's glum, we'll say; he's agitated
about the loss of the steam-launch; he can't make up his mind what
all that firing meant, and where his twenty-odd soldiers and the two
officers who commanded them have got to. In fact, he's in the
dickens of a stew, in a beastly temper, smoking a cigar, and won't
say 'nothink'."
"Oh, shut up!" Geoff told him angrily.
"Like the Turkish captain, in fact," Philip laughed. "But, seriously, just
as you said a moment ago, seriously, what's to be done? You know
the old adage: 'You can take a horse to the water, but no amount of
kicks or coaxing will make him drink'; well, this old Turk may be just
like that obstinate old horse. He's there, aboard his steamer, and
nothing will make him talk, not even——"
"Stop!" commanded Geoff abruptly. "'Nothing will make him talk,'
you say? Won't it? I mean to get information out of the old beggar—
for I presume he is old—but don't forget that neither of us have
seen him yet, so he may be young and active. All the same, I am
going aboard now, and, of course, if I don't come back within
reasonable time you will have cause to believe that I have been
captured. Then the command of the expedition devolves upon you,
and it is for you to carry out the work entrusted to us. Just launch
that dinghy, quietly, my lads," he called over the front of the cabin,
"and see that there's a paddle in her."
Geoff began to grope in the cabin of the steam-launch, till his hand
presently lit upon the pannier containing dressings, which had been
handed over to them by the gallant Commander, whom they had left
wounded aboard the motor-boat.
"You may want it, lads," he had told them. "There is never any
saying when you may come up against the Turks, and, having had
one brisk little engagement with them, you may have another, and,
of course, may very well have some of the crew wounded. Of

course, I hope that that won't be the case, but you never know your
luck. For that reason we'll divide up the dressings, I taking sufficient
for my own purposes while you take enough for yours."
"Got it!" exclaimed Geoff, as his hand lit upon the pannier. "Now for
a pad of cotton-wool and a couple of bandages."
"Eh!" asked Philip curiously; "'Couple of bandages,' 'cotton-wool'—
you're going aboard a steamer, now what in the name of the dickens
is that for?"
Geoff didn't tell him to mind his own business, for he was far too
polite a young fellow to give such an answer, neither did he speak to
his inquisitive chum gruffly even; instead, he maintained silence,
whilst he carefully picked out the bandages and the pad of cotton-
wool. Then Phil suddenly gripped him by the shoulder.
"I've got it!" he exclaimed.
"Got what?" asked Geoff curtly.
"Got it, of course," came the answer; "the bandages and the pad of
cotton-wool; the idea, my dear boy, the very smart and brilliant
brain-wave that's come to you. You're going to——"
"What?"
"What! Why of course the brain wave," Philip told him hotly. "I've
guessed your idea; you're going to get aboard that steamer, and just
because that old bounder of a Turk——"
"What old bounder of a Turk? The Captain?" asked Geoff. "He isn't
old. At least, how do we know that he's old? He may be young,
middle-aged, bald-headed and toothless."
The two of them were getting quite angry, and for a moment or two
it looked as though the wordy warfare in which they were beginning
to be engaged would develop into quite a battle. Then Geoff giggled

—an excited little giggle—while Phil joined his chum heartily, and
brought one hand down with a thump on the broad of his back.
"Jingo!" he exclaimed. "You're right, of course we don't know
whether the old bounder is young or old, or even toothless; but we
do know that there's a captain or an officer in charge of that
steamer, and, what's more, we know, what you want and didn't tell
me, that we're going to capture him."
"We're going to!" exclaimed Geoff. "I thought I'd already said, as the
officer commanding this expedition——"
"Ahem!" coughed Philip. "Certainly, sir, you did say that," he said in
his most demure manner. "But the job, if you'll allow me to say so, is
rather a big one—in short, and in fact, it's a 'tough nut' you propose
to crack, and in cracking it you're just as likely to come to grief
yourself, and possibly to have your head cracked. Indeed, as your
immediate junior, as one anxious for the success of this most
important expedition, it becomes my duty to point out that failure on
your part, failure because you have gone into the matter without
sufficient forces at your command, will lead inevitably to the ghastly
failure of the whole expedition. Once the alarm is given, once there
is no longer the chance of a surprise, in fact, once the Turks are on
the qui vive, and know what we are up to, the game's up, and we've
lost! Nice to have to return to the camp on the Shatt-el-Arab, and
tell 'em that we've been a hideous failure!"
He was piling it on with a vengeance, was Philip, but then he was an
artful, if light-hearted and jovial fellow, and here he had a most
distinct object in view. He plucked Geoff eagerly by the sleeve.
"Rotten, that!" he told him. "Just fancy what the fellows would say!
They'd not forget to tell us all about it, and make nasty remarks
about chaps with swollen heads who'd gone up the river on their
own, thinking to do a heap, and returning without carrying out their
object, or even nearly completing it. See?" he asked Geoff, with
decided emphasis, and repeated his demand as a movement of his

chum seemed to denote some signs of giving way. "Just think it over,
Geoff! You go aboard the steamer and creep along the deck till you
come to the Captain's cabin. Don't forget that you want the bounder
to talk about the Turks and their position, and just remember what I
said when I suggested that he'd talk on any and every subject rather
than that. Well, aboard the steamer you can't make him answer your
questions, or launch out into an explanation of the Turkish plans of
campaign; so you decide to kidnap him, and have the idea of
plugging his mouth with that cotton-wool, and winding a bandage
about his head. Very pretty! Awfully nice if the thing works! But will
it? Supposing he shouts before you plug his toothless mouth—he
was toothless I think we agreed—supposing he's not alone, what
then? You're done! Your plan's defeated. You might just as well have
stayed aboard this launch and rested. But——"
"But if Phil—the eager Phil—happened to be close at hand, ready to
brain the other fellow. Ah!" exclaimed Geoff, and for the life of him
he couldn't help laughing at the excitement and the eager pleading
of his chum.
It made him laugh when he remembered how adroitly and how
expertly Philip had worked round the question, had pointed out so
very clearly the chances of failure, and then had come in at the end
with the greatest arguments for his own inclusion in the adventure.
Arguments which Geoff himself could not deny; for a friend at hand,
a stanch friend, might very well turn the scales in his favour, and,
after all, what a prize the Captain of that steamer would be, if they
could only lay their hands on him.
"Better far than the chief I bagged at the very beginning of the
campaign," he told himself, though he spoke aloud.
"Agreed!" said Philip. "I don't, of course, want to say that that wasn't
quite a nice little business, but then, this is really 'It', or will be if we
bring it off. So I come, don't I?"
"You do. Your revolver's loaded, eh?"

"And ready," Phil said, "and the dinghy is alongside."
"Then come on."
Leaving the oldest sailor in charge of the launch, with instructions to
lie in that position till morning came, and then to look about for
them, and to return down the Euphrates in the event of not
discovering their officers, Geoff and Philip crept gingerly into the
dinghy, which had been brought close alongside, having been
launched from the deck of the little steamer where it was usually
carried.
"Push off," said Geoff, "and keep your ear open for a hail, for it'll be
no easy job to find you in the darkness."
"Aye, aye, sir," replied the man, "good luck to you."
Geoff dipped his paddle in the water, and thrust hard with it, while
Philip, seated in the stern, used a paddle as a rudder. Stealing along
the narrow channel in which the steam launch lay, they soon
rounded the end of one of the islands which formed it, and halted
there for a while to allow their eyes to grow accustomed to the
darkness. Then they turned sharp left, facing the direction in which
the River Euphrates lay, and stole onward across the waste of
waters, threading their way between muddy banks where the slime
and ooze clung, and often diving under perfect archways of reeds,
where the islands were close together. Once or twice they had to
return on their tracks, finding their way obstructed, and on one
occasion they bumped gently into an island, and stuck fast for a
while, till Geoff came aft—thus tipping the bows of the dinghy
upward and so loosening her. It was perhaps half an hour later that
they felt, rather than saw, that they had gained the main stream, the
wide expanse of smooth, almost motionless water, where eddies
from the river sometimes stirred the surface, and where the flow,
moderately rapid in the centre, was so retarded as to be almost
imperceptible.

"Straight across," whispered Geoff, "there are the lights of the
steamer just up-river of us, so we'll cut across to the centre, where I
reckon her to be lying, and then steal up behind her. Gently with
your paddle, Phil, for a splash might attract the attention of a sentry
and bring rifle-fire upon us."
Another ten minutes passed, during which they plunged their
paddles gently though firmly into the stream, and forced the little
boat steadily upward, and during that time the dull, dimly visible hull
of the vessel lying out in mid-stream gradually grew bigger and
bigger. At length they were right under her stern, and found that,
though low-built in the centre, and indeed generally, she was yet
well above their heads, so much so that the dinghy lay close to the
rudder and practically under the stern of the vessel. It was just then
that the end of a trailing rope struck Phil gently across the face, and,
groping for it, he had soon seized upon it firmly.
"Half a mo'!" he told Geoff. "What's this? A rope, a rope to make our
boat fast to. Now I call that particularly accommodating of this old
party we've come to visit."
"What, eh?"
Geoff chuckled. It did him good to hear Philip's innocent banter, and
showed him also at the same time what an excellent fellow he had
to assist him.
"Make fast," he whispered. "Give a good haul on it first, though, and
if it's stout enough I'll make use of it to get aboard, though I
imagine by getting on your back I could easily reach the rail, and so
the deck of the steamer."
A minute later they had secured the dinghy to the rope, and the
wise Philip made fast the other end of it to a bolt-hole in the rudder,
thus keeping their little boat right under the stern of the steamer,
where she would remain unseen. Then Geoff gripped the rope which
had been dangling over the rail, and, putting all his weight on it to
test it, swung himself out of the dinghy and clambered up till he

could grasp the rail above. One strong heave and his face was above
its level, and he was able to look along the deck of the steamer.
Then very slowly he clambered upward, and slid on to the deck,
where he crouched under the rail to watch and listen.
Hark! There were voices somewhere. There was a light shining on
the deck on either side, through what appeared to be the skylight of
a cabin, while the voices, no doubt, came from that direction. But it
was not that alone which Geoff had heard, it was something else—
the gentle slap, slap of feet on the deck, the soft footfall of a man
shod with sandals perhaps, or more likely entirely unshod, perhaps a
barefooted sentry pacing the deck to and fro, turning when he had
accomplished a dozen paces. Geoff peered into the darkness, hoping
to see the man, but failed, though the sounds were still quite
audible. Then he stole forward till quite close to the cabin's skylight,
where he hid behind a mast in a dark corner between it and the
bulkhead of the cabin. Yes, the sounds made by that sentry—for if
not a sentry what else could he be?—were clearly audible, while the
figure now came into view, feebly outlined it is true yet quite
sufficient for Geoff's purpose. There was a Turk, perhaps a Turkish
sailor, striding to and fro some twenty yards farther forward, turning
about each time he reached the rail, striding this way and that like
an automaton—as if indeed he were a clockwork figure.
"Rather too near to be pleasant," thought Geoff, "and the bother of
it is that he makes it difficult for a fellow to peer into the cabin. Ah!
one of these sky-lights is lifted. It's been a hot day, and I've no
doubt it's stuffy down in the cabin. That's really very considerate of
our friend, the Captain, as Phil would say. Yes, voices—Turkish voices
—let's see what's happening."
He went on all fours, and stole along beside the cabin's skylight till
he came to the panel which was lifted. There was an opening,
perhaps some six inches in width, through which the light was
streaming, and also the voices of two men, at least, down in the
cabin. But six inches is hardly sufficient space to admit a head, and
Geoff at once increased the size of the opening by lifting the panel.

"Stop, there! Enough! It's cool enough below!" he heard someone
exclaim an instant later. "Idiot, leave the thing as it is now, and wait
next time till you are told to make an alteration."
By then Geoff was flat on the deck, listening to the voice so near to
him, and watching that sentry, that automaton, as he moved to and
fro; watching him and hoping that he would take no notice. Indeed,
he need hardly have worried himself, for the man did not even deign
to turn his head, but strolled on across his beat, his rifle now visible
as it thrust upward above his shoulder. For the life of him Geoff
could not help chuckling again, and repeating the words which Philip
had used but a few minutes earlier.
"A most accommodating sentry," he said. "If only he'll continue to
march to and fro without looking this way it'll give me a chance of
peeping into the cabin. Here goes! Oh! Three of 'em, eh! All officers,
and, by George, the chief of 'em is bald-headed, or I'm a
Dutchman!"
How Philip would have laughed had he been beside his chum and
recollected their conversation aboard the steam-launch, for as Geoff
peered down into the cabin, his head screened to a certain extent
from the view of those below by the supports of the skylight, and by
the swinging oil-lamp which illuminated the interior, his eyes fell
upon three individuals—three Turkish officers—one of whom sat
back in a chair in the most dilettante attitude, smoking a cigarette; a
young man without doubt, handsome as the Turks go, but decidedly
effeminate. Near him was another officer, rather older, with a
handsomely curled moustache, who leaned both elbows on the cabin
table and seemed to be already nodding. And opposite the two,
lounging full length on a divan, was a stout broad-shouldered Pasha,
a senior Turkish officer, whose fez now reposed on the floor,
exposing a head which shone and glistened in the rays of the lamp-
light. As to his being toothless, that was another matter, though the
memory of what had passed between himself and Philip, once again
caused Geoff to give vent to a silent chuckle.

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