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Practical Radio Engineering and Telemetry for
Industry

Titles in the series
Practical Cleanrooms: Technologies and Facilities (David Conway)
Practical Data Acquisition for Instrumentation and Control Systems (John Park,
Steve Mackay)
Practical Data Communications for Instrumentation and Control (Steve Mackay,
Edwin Wright, John Park)
Practical Digital Signal Processing for Engineers and Technicians (Edmund Lai)
Practical Electrical Network Automation and Communication Systems (Cobus
Strauss)
Practical Embedded Controllers (John Park)
Practical Fiber Optics (David Bailey, Edwin Wright)
Practical Industrial Data Networks: Design, Installation and Troubleshooting (Steve
Mackay, Edwin Wright, John Park, Deon Reynders)
Practical Industrial Safety, Risk Assessment and Shutdown Systems for
Instrumentation and Control (Dave Macdonald)
Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems (Gordon
Clarke, Deon Reynders)
Practical Radio Engineering and Telemetry for Industry (David Bailey)
Practical SCADA for Industry (David Bailey, Edwin Wright)
Practical TCP/IP and Ethernet Networking (Deon Reynders, Edwin Wright)
Practical Variable Speed Drives and Power Electronics (Malcolm Barnes)

Practical Radio Engineering and
Telemetry for Industry
David Bailey BE (Comms) BAILEY AND ASSOCIATES, PERTH,
AUSTRALIA
OXFORD AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK
PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

Newnes
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
200 Wheeler Road, Burlington, MA 01803
First published 2003
Copyright  2003, IDC Technologies. All rights reserved
No part of this publication may be reproduced in any material form (including
photocopying or storing in any medium by electronic means and whether
or not transiently or incidentally to some other use of this publication) without
the written permission of the copyright holder except in accordance with the
provisions of the Copyright, Designs and Patents Act 1988 or under the terms of
a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road,
London, England W1T 4LP. Applications for the copyright holder's written
permission to reproduce any part of this publication should be addressed
to the publisher
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN 07506 58037
Typeset and Edited by Vivek Mehra, Mumbai, India
Printed and bound in Great Britain
For information on all Newnes publications, visit
our website at www.newnespress.com

Contents
Preface xi
1 Radio technology 1
1.1 Introduction 1
1.2 Fundamentals of radio operation 2
1.2.1 Components of a radio link 4
1.3 The radio spectrum and frequency allocation 6
1.3.1 General 6
1.3.2 Single and two frequency systems 7
1.4 Gain, level, attenuation and propagation 8
1.4.1 Gain and loss 9
1.4.2 Level 10
1.4.3 Attenuation 12
1.4.4 Propagation 12
1.4.5 Line of sight propagation path attenuation 16
1.4.6 Reflections 17
1.5 Criteria for selection of frequency bands 18
1.5.1 HF 18
1.5.2 Very high frequency (VHF) 20
1.5.3 UHF 22
1.5.4 Frequency selection 24
1.5.5 Summary 27
1.6 Modulation and demodulation 28
1.6.1 Carrier wave modulation 28
1.6.2 Sidebands and bandwidth 29
1.6.3 Amplitude modulation(AM) 31
1.6.4 Frequency modulation (FM) 34
1.6.5 Phase modulation (PM) 37
1.6.6 Pre-emphasis and de-emphasis 37
1.6.7 Comparison of modulation schemes 39
1.7 Amplifier 40
1.8 Power amplifier (PA) 40
1.9 Oscillator 40
1.10 Filters 42
1.10.1 Filter characteristics 43
1.10.2 Types of filters 45
1.11 Transmitters 50
1.11.1 AM transmitters 50
1.11.2 PM and FM transmitters 51
1.12 Receivers 52
1.12.1 AM receiver 52
1.12.2 FM and PM receivers 53

vi Contents
1.13 Antennas 54
1.13.1 Introduction 54
1.13.2 Theory of operation 54
1.13.3 Types of antennas 57
1.13.4 Antenna installation 59
1.13.5 Stacked arrays 60
1.14 Cabling 61
1.14.1 Leaky coaxial cable 62
1.15 Intermodulation and how to prevent it, using duplexers, multicouplers, circulators,
isolators, splitters and pre-amplifiers 62
1.15.1 Introduction 62
1.15.2 Intermodulation 62
1.15.3 Circulators and isolators 64
1.15.4 Multicoupler and cavity filters 65
1.15.5 Duplexer 67
1.15.6 Splitters 69
1.15.7 Receiver pre-amplifiers 70
1.15.8 Typical configuration 71
1.16 Implementing a radio link 71
1.16.1 Path profile 71
1.16.2 RF path loss calculations 74
1.16.3 Transmitter power/receiver sensitivity 75
1.16.4 Signal to noise ratio and SINAD 76
1.16.5 Fade margin 77
1.16.6 Summary of calculations 78
1.16.7 Miscellaneous considerations 78
1.17 Types and brands of radio equipment 79
1.17.1 Types 79
1.17.2 Brands 81
1.18 Data transmission over analog radio 82
1.18.1 Modulation techniques 82
1.18.2 Transmission limitations 83
1.19 Regulatory licensing requirements for radio frequencies 85
1.20 Duplication and self-testing 85
1.20.1 Duplication 85
1.20.2 Standby transmitters 87
1.20.3 Self-testing and remote diagnostics 87
1.21 Miscellaneous terminology 87
1.21.1 VSWR and return loss 87
1.21.2 Desensitization and blocking 92
1.21.3 Continuous tone coded sub-audible squelch (CTCSS) 92
1.21.4 Selective calling 94
1.22 Additional features and facilities for analog radio 95
1.22.1 Trunking radio 97
1.23 Digital modulation radio 98
1.23.1 Digital private mobile radio 98
1.24 Digital radio specifically for telemetry applications 100
1.25 Digital wireless communications 101

Contents vii
2 Line of sight microwave systems 104
2.1 Introduction 104
2.2 Background 105
2.2.1 Overhead lines 105
2.2.2 Underground copper cables 105
2.2.3 Radio 105
2.2.4 Fiber optic cable 106
2.3 Point-to-point radio systems 106
2.4 Point-to-multipoint 108
2.4.1 Theory of operation 110
2.5 A typical radio terminal 111
2.5.1 The transmitter 111
2.5.2 The receiver 112
2.6 Modulation methods 112
2.7 Standards 114
2.8 Multiplex equipment and data rates 114
2.8.1 Analog multiplex systems 114
2.8.2 Digital multiplex 115
2.8.3 Voice compression 118
2.8.4 Synchronous digital hierarchy 119
2.8.5 The 2 Mbps world 119
2.9 Antennas and multicouplers 121
2.9.1 The use of reflectors 123
2.9.2 The use of passive repeaters 124
2.9.3 Duplexers and multicouplers 125
2.10 Coaxial cables and waveguides 125
2.10.1 Avoiding the use of waveguides? 126
2.11 Power supplies 126
2.12 Path loss 127
2.12.1 Free space attenuation 127
2.12.2 Rain attenuation 127
2.12.3 Fade margin 128
2.13 A simple path calculation 128
2.14 Multipath propagation and diversity operating 129
2.14.1 Multipath propagation 129
2.15 Ducting and overshoot 134
2.15.1 Ducting 134
2.15.2 Overshoot 135
2.16 Duplication of equipment 136
2.16.1 Cold standby 136
2.16.2 Hot standby 137
2.16.3 Parallel operation 137
2.16.4 Diversity 138
2.17 Duplication of routes 139

viii Contents
3 Satellite systems 140
3.2 Classes of satellite services and their relevant organizations 141
3.2.1 International services 141
3.2.2 Regional systems 144
3.2.3 Domestic 145
3.2.4 Low earth orbit (LEO) satellite 145
3.2.5 Global Positioning System 147
3.3 Frequency band allocation for satellites 147
3.3.1 The satellite bands 149
3.4 Satellite systems and equipment 151
3.4.1 Configuration 151
3.4.2 Multiplexing 153
3.4.3 Modulation techniques 157
3.5 Satellite equipment 157
3.5.1 Uplinks 158
3.5.2 Satellite transponder 158
3.5.3 Downlinks 159
3.6 Antennas 160
3.7 Link equation 160
3.8 Footprint 161
4 Reliability and availability 163
4.2 Reliability 164
4.2.1 Definition of reliability 164
4.2.2 Manufacturing 165
4.2.3 Operation 166
4.2.4 Maintenance 166
4.3 Availability 166
4.3.1 Radio and microwave 166
4.3.2 Landlines 169
4.3.3 Satellites 170
4.4 SCADA system reliability (or failure) rates 171
4.5 Complete system testing 171
4.6 Improving reliability 171
4.7 Reliability calculations 172
4.7.1 Failure rate 172
4.7.2 Mean time between failure 173
4.7.3 Availability 173
4.7.4 Comments on calculations 174
4.8 Qualification of the processes 174
5 Infrastructure requirements for master sites and RTUs 175
5.1 Location selection 175

Contents ix
5.2 Site works and access 176
5.3 Antenna support structures 176
5.4 Lightning protection 178
5.4.1 Levels of lightning protection 181
5.4.2 Separating equipment and lightning 182
5.4.3 Dissipating the lightning elsewhere 182
5.4.4 Dissipation of high voltages or currents 183
5.4.5 Conclusion 183
5.5 Equipment shelters and temperature management 184
5.5.1 Temperature management 184
5.5.2 Other aspects of building design 185
5.6 Power supplies 186
5.6.1 dc supply and batteries 186
5.6.2 Mains power supplies 188
5.6.3 Non-essential, essential and uninterruptible supplies 188
5.6.4 Mains power station 189
5.6.5 Solar power supplies 190
5.6.6 Wind generators 191
5.6.7 Diesel generators 192
5.6.8 Filtering dc supplies 194
5.7 Distribution (dc) 195
5.8 Monitoring site alarms 197
5.9 Voice and data cabling – distribution systems 199
5.10 Equipment racks 202
5.11 Interference in microwave and radio systems 203
5.11.1 High voltage power lines 203
5.11.2 Electromagnetic interference 203
5.11.3 Radar interference 204
5.11.4 Foreign system interference 204
5.11.5 Harmonic and intermodulation interference 204
5.12 Service channel 204
6 Integrating telemetry systems into existing radio systems 205
6.1 General 205
6.2 Appropriate radio systems 206
6.3 Traffic loading 207
6.4 Implementing a system 209
6.5 Trunking radio 211
7 Miscellaneous telemetry systems 212
7.2 Ocean data telemetry application 212
7.2.1 Electromechanical cable 213
7.2.2 Acoustic modem 214
7.2.3 Inductive modem 216
7.3 Physiological telemetry application 218

x Contents
7.4 Tag identification system using modulated UHF backscatter 220
8 Practical system examples 224
8.1 A dockside communications system for LNG tankers 224
8.2 Remote oceanographic sensor system 226
Appendix A Glossary of terms 231
Appendix B Path loss calculation formulae 254
Bibliography 272
Index 273

6XKLGIK
This book covers the fundamentals of telemetry and radio communications, describes their application
and equips you with the skills to analyze, specify and debug telemetry and radio communications
systems.
The structure of the book is as follows.
Chapter1 8GJOUZKINTURUM_. This chapter goes through the fundamentals of radio theory
and then introduces all the elements of a complete radio system. Finally a coherent methodology will
be provided to systematically design, install, and test a successful radio system for use in telemetry
systems.
Chapter2 2OTKULYOMNZSOIXU]GKY_YZKSY. In this chapter the theory of radio link
transmission and the components which make up a microwave system are described.
Chapter3 9GZKRROZKY_YZKSY. This chapter will examine the types of satellite services that
are available, the different service providers and their satellites, the satellite frequency bandplan, the
fundamentals of satellite systems, their operation and then, finally, look at the various satellite services
available along with their relevance to telemetry.
Chapter 8KROGHOROZ_GTJGGORGHOROZ_. This chapter looks at reliability with relevance to
an overall telemetry installation and then will look at availability, with respect to the various types of
telemetry communication links that have been discussed.
Chapter5 /TLXGYZX[IZ[XKXKW[OXKSKTZYLUXSGYZKXYOZKYGTJ8:;Y. This
chapter identifies the different issues such as location selection, lightning protection, equipment
shelters, power supplies and voice and data cabling involved in setting up a master radio telemetry
site.
Chapter6 /TZKMXGZOTMZKRKSKZX_Y_YZKSYOTZUK^OYZOTMXGJOUY_YZKSY. This
chapter describes what is involved in implementing telemetry systems into existing radio systems
ranging from high to low integrity categories.
Chapter 7 3OYIKRRGTKU[Y ZKRKSKZX_ Y_YZKSY. Three different telemetry systems
which differ from standard telemetry systems are discussed here.
Chapter8 6XGIZOIGRY_YZKSK^GSVRKY. This chapter reviews two typical applications
of radio communications for a dockside operation and a remote oceanographic sensor system.

Radio technology
1.1 Introduction
A significant number of telemetry systems use radio as a communications medium. It is
often chosen as a communications medium in preference to using cables (landlines) for a
number of reasons:
• Telemetry systems are often operated over large distances, and the costs of
installing cables can far exceed the costs of installing radio equipment
• If landlines are to be rented from a telephone company, it is often found that
the costs of buying radio equipment are amortized in several years while
rental costs continue (this can depend on whether the lines are switched or
dedicated)
• Depending on the type of equipment being installed and the distance involved,
radio can generally be installed faster than other communication mediums
• Radio equipment is very portable and can be easily moved to new locations as
when plants are relocated. If a plant or equipment goes out of service, the
radio equipment can be reused in other locations with little or no modification
• The user can own the radio communication links, which allows him to
transmit any information in any format that he requires (assuming that all
physical and regulatory constraints are taken into consideration)
• Reasonably high data rates are available
• The radio unit can be used as a backup to landline circuits where high
integrity communication circuits are required
Radio can be broken down into two general classifications:
• Equipment that operates below 1 GHz, and
• Equipment that operates above 1 GHz

2 Practical Radio Engineering and Telemetry for Industry
The latter is generally referred to as microwave radio and will be discussed in detail, in
Chapter 2. The former will be referred to as just radio in this book. This chapter is
devoted to developing an understanding, of the concepts of radio and how these can be
practically applied to the implementation of a telemetry system.
The science of radio communications, and how it can be successfully implemented into
a practical solution, in an industrial environment, is often considered a difficult and
esoteric task that is best left to the experts. Many readers of this book would know of a
radio system installation that does not work well or suffers from intermittent problems.
This should not be the case if a system is engineered correctly! A logical methodology
can be used in designing radio links for telemetry.
This chapter will first go through the fundamentals of radio theory and then introduce
all the elements of a complete radio system. Finally, a coherent methodology will be
provided to systematically design, install, and test a successful radio system for use in
telemetry systems.
1.2 Fundamentals of radio operation
A single unshielded conductor that carries an alternating current (ac) and has a certain
amount of internal resistance will radiate energy in the form of electromagnetic waves.
The frequency of the radiated waves is equal to the oscillation rate of the alternating
current. For example, an alternating current that has electrons moving backwards and
forwards along a conductor at a speed of 100 oscillations per second will radiate
electromagnetic waves at a frequency of 100 hertz (Hz).
Figure 1.1
Production of an electromagnetic wave
Hertz is the term used to describe the oscillation of electrons, in oscillations per second,
or the number of complete cycles of an electromagnetic wave, in cycles per second.
Cycles/second and hertz are used interchangeably.
The following abbreviations are used to describe frequency:
• kHz kilohertz = 1 × 103
Hz
• MHz Megahertz = 1 × 106
Hz
• GHz Gigahertz = 1 × 109
Hz

Radio technology 3
Figure 1.2
One cycle
One cycle is one wavelength. The effectiveness, of the element as a radiator (antenna)
of electromagnetic waves, is dependant upon its degree of resonance at the oscillating
frequency. (Resonance occurs when the energy injected into a load is absorbed by the
load because the particles that the load is constructed of oscillate at the same frequency as
the injected energy.)
Electromagnetic waves are purportedly very small particles called photons (much
smaller than an electron) that travel in a close sinusoidal pattern through space. There are
two branches of complex physics that study the nature of electromagnetism, particle
physics, and wave mechanics. Each theory studies the phenomenon from a different
perspective, sometimes with quite different results. This book will avoid any involve-
ment in the long convoluted mathematics of these studies, adhering to the more practical
realities of working with radio.
Electromagnetic waves can be represented by an electric field and a magnetic field. The
electric field is produced by the moving electrons in the conductor. The electric field
itself appears to produce an equivalent magnetic field perpendicular to the electric field.
Figure 1.3 illustrates the components of an electromagnetic wave.
Figure 1.3
Construction of an electromagnetic wave
With the antenna of a radio, the electric field of an electromagnetic wave is parallel to
the antenna elements; that is, it is parallel to the moving electrons. The relevance of this is
discussed further in section 1.13.
When electromagnetic waves are traveling through space and pass through a conducting
element, if the electrical field is parallel to the conductor, the wave will cause electrons in
the conductor to move up and down in sympathy with the incoming sinusoidal wave. This
conductor would be the receiving antenna, which feeds the retrieved signals through to
the receiver. The receiver would detect the movement of electrons and process it as the
incoming signal.
The mediums of air and vacuum are quite often referred to as free space.
Electromagnetic waves travel at the speed of light (3×108
meters/sec) through a
vacuum. The more dense the medium through which the waves are required to travel, the

more they slow down. The speed of electromagnetic waves in air is almost the same as
through a vacuum.
For determining the wavelength of a certain frequency the following formula is used:
Where:
C = speed of electromagnetic waves in meters per second
λ = wavelength of signal in meters
f = frequency of signal in hertz.
For example, if we have a radio system operating at 900 MHz then the wavelength of
this signal is 33 cm.
Electromagnetic waves that have a short wavelength (high frequency) tend to travel in a
straight line and are quickly absorbed or reflected by solids. Long wavelength (low
frequency) electromagnetic waves tend to be more affected by atmospheric conditions
and travel a more curved path (that follows the curvature of the earth). They are also more
able to penetrate solids. In this chapter, short wavelengths will be regarded as frequencies
from 335 MHz up to approximately 960 MHz and long wavelengths being frequencies of
1 MHz up to approximately 225 MHz.
Electromagnetic waves, in the radio frequency and microwave frequency bands
(10 kHz to 60 GHz), are often just referred to as RF signals (radio frequency signals).
1.2.1 Components of a radio link
A radio link consists of the following components:
• Antennas
• Transmitters
• Receivers
• Antenna support structures
• Cabling
• Interface equipment
Figure 1.4 illustrates how these elements are connected together to form a complete
radio link.
Figure 1.4
Fundamental elements of a radio link
f
C λ
=

Radio technology 5
Antenna
This is the device used to radiate or detect the electromagnetic waves. There are many
different designs of antennas available. Each one radiates the signal (electromagnetic
waves) in a different manner. The type of antenna used depends on the application and on
the area of coverage required.
Transmitter
This is the device that converts the voice or data signal into a modified (modulated)
higher frequency signal and feeds it to the antenna, where it is radiated into the free space
as an electromagnetic wave, at radio frequencies.
Receiver
This is the device that converts the radio frequency signals (fed to it from the antenna
detecting the electromagnetic waves from free space) back into voice or data signals.
Antenna support structure
An antenna support structure is used to mount antennas in order to provide a height
advantage, which generally provides increased transmission distance and coverage. It
may vary in construction from a three-meter wooden pole to a thousand-meter steel
structure.
A structure, which has guy wires to support it, is generally referred to as a mast. A
structure, which is freestanding, is generally referred to as a tower.
Figure 1.5
Illustration of mast and tower
Cabling
There are three main types of cabling used in connecting radio systems:
• Coaxial cable for all radio frequency connections
• Twisted pair cables for voice, data, and supervisory connections
• Power cables
Interface equipment
This allows connection of voice and data into transmitters and receivers, from external
sources. It also controls the flow of information, timing of operation on the system, and
control and monitoring of the transmitter and receiver.

1.3 The radio spectrum and frequency allocation
1.3.1 General
There are very strict regulations that govern the use of various parts of the radio
frequency spectrum. Specific sections of the radio frequency spectrum have been
allocated for public use. All frequencies are allocated to users by a government regulatory
body. Figure 1.6 illustrates the typical sections of the radio spectrum allocated for public
use around the world. Each section is referred to as a band.
Figure 1.6
The radio spectrum for public use
Certain sections of these bands will have been allocated specifically for telemetry
systems.
In some countries, a deregulated telecommunications environment has allowed sections
of the spectrum to be sold to large private organizations that manage it and in turn, sell to
(smaller) individual users.

Radio technology 7
An application is made to a government body, or independent groups that hold larger
chunks of the spectrum for reselling, to obtain a frequency license as no transmission is
allowed on any frequency unless a license is obtained.
1.3.2 Single and two frequency systems
The governing body will allocate one or two frequencies, per channel, to the user. In the
radio bands, each frequency will generally be 12½ or 25 kHz wide.
In radio terms, this represents the difference between operating a simplex or duplex
system. Note that simplex and duplex have slightly different meanings in radio than in
data communication.
1.3.2.1 Single frequency allocation
If a user is allocated a single frequency, then the system is said to be a simplex system.
There are two modes of simplex radio transmissions:
Single direction simplex
Here, information in the form of single frequency radio waves will travel in one direction
only, from a transmitter to a receiver.
Figure 1.7
Single direction simplex
Two direction simplex
Here, the single frequency is used to transmit information in two directions but only in
one direction at a time.
Figure 1.8
Two direction simplex
1.3.2.2 Two frequency allocation
Here two frequencies are allocated, approximately 5 MHz apart, depending on the
operating band. This type of system is referred to as a duplex system. There are two
modes of duplex radio transmission:

Half duplex
Here, one frequency is used for transmission in one direction and the other frequency is
for transmission in the other direction, but transmission is only in one direction at one
time.
Figure 1.9
Half duplex
This mode is most commonly used for mobile vehicle radio systems, where a radio
station is used to repeat transmissions from one mobile to all other mobiles on the
frequencies (i.e. talk through repeaters).
Full duplex
Here, one frequency is used for transmission in one direction and the other frequency is
for transmission in the other direction, with both transmissions occurring simultaneously.
Figure 1.10
Full duplex
1.4 Gain, level, attenuation and propagation
To fully understand the nature of radio communications, it is important to have an
understanding of the fundamental parameters used to describe and measure the
performance of a radio system. This section will provide details of important
measurements and performance parameters associated with radio communication.

Radio technology 9
1.4.1 Gain and loss
As an electronic signal passes through a circuit or system, the strength of that signal
(referred to as its level), will vary. The strength of the signal is measured as the voltage,
or current levels of the electrons, at a particular point in the circuit or system. Figure 1.11
below illustrates a device where levels can be measured at three points (A, B and C).
Figure 1.11
Circuit gain and loss
In this case, measurements are made between points A, B or C and earth, and are made
in volts, millivolts, or microvolts.
If the voltage level at B is greater than at A then Circuit 1 has provided GAIN to the
signal. Gain is a measurement of the level at the output of a device compared to the level
at the input at the device.
Therefore:
A
B
1
V
V
GAIN =
If the voltage at B was 10 volts and the voltage at A was 5 volts, circuit 1 would be
providing a GAIN factor of 2. If the voltage at C is less than the voltage at B, then circuit
2 has introduced a LOSS. For example, if the voltage at C is measured as 5 volts, then
circuit 2 has introduced a loss factor of 2
2
5
10
C
B
2
V
V
LOSS =
=
=
The accepted convention is to always express levels as GAIN and not loss.
Therefore, in the example the gain factor would be ½.
2
1
10
5
B
C
2
V
V
GAIN =
=
=
In radio systems, the measure of the strength of an electromagnetic signal, received by
an antenna is expressed as a voltage level, measured within the receiver.
For the radio system, shown in Figure 1.12, the same principles apply. The gain
measurement can be applied between any two points in a complete communications
system.

Figure 1.12
System gain for a radio link
1.4.2 Level
The majority of engineering measurements performed on radio systems are carried out as
a measurement of power levels. The equations for power are:
R
V
P
VI
P
2
=
=
P = I2
R
Where:
P = power (in watts)
V = voltage (in volts)
I = current (in amperes)
R = load resistance
The measurement of level with respect to power originated when Alexander Graham
Bell invented a unit of measure for sound levels. This unit became known as the ‘bel’.
One tenth of a bel is called a decibel.
The human ear hears sound in a logarithmic manner. Therefore, a level of 100 watts to
the human ear would sound twice as loud as a level of 10 watts (not 10 times).
A one-decibel increase in sound is approximately the smallest increase in sound level,
detectable by the human ear.
This unit of measure is now used as the basis for measuring relative power levels in
radio, voice, and data networks. For the radio network in Figure 1.12 the gain of the
system becomes:
bels
GAIN
A
B
10
P
P
Log 





=
decibels
10
A
B
10
P
P
Log 





=
Note that this is a relative measurement. The resulting value is a measure of the power
level at point B with reference to the power level at point A (power at B relative to power
at A). The resultant is NOT an absolute value.
For example, if for the system shown in Figure 1.12 there is an input signal at point A
of 1 watt and an output signal at point B of 10 watts, the system gain is:
Written as 10 dB.

Radio technology 11
decibels
10
1
10
1
10
10
GAIN 10
)
(
Log
=
×
=






=
When working with radio equipment, measurements can be made at single points in a
system with reference to 1 watt or 1 milliwatt (instead of with reference to a level at
another point). The equation then becomes:
dBM
:
1
10
LEVEL 10 





=
P
Log
(With reference to 1 watt)
or
dBM
:
10
10
LEVEL 3
10 





= −
P
Log
(With reference to 1 milliwatt)
If measurements are required to be carried out in, volts or amperes then replacing power
with:
R
V 2
or I2
R
dB
.
.
10
GAIN
B
2
A
A
2
B
10 







=
R
V
R
V
Log
When RA = RB
dB
20
GAIN
A
B
10 







=
V
V
Log
dB
10
GAIN
A
2
A
B
2
B
10 





=
R
I
R
I
Log
or if RA = RB
dB
20
GAIN
A
B
10 





=
I
I
Log
For most radio systems, RA will equal RB and the second formula can normally be used.

Voltages are also sometimes given in decibel forms, where they are measured with
respect to 1 volt or 1 microvolt i.e. dBV or dBµV respectively.
1.4.3 Attenuation
The term attenuation is used to express loss in a circuit. Negative gain is therefore
considered to be attenuation.
Figure 1.13
Attenuation
With reference to Figure 1.13, the output signal is 5 dBm less than the input signal, and
therefore the system has an attenuation of 5 dB (NOT 5 dBm). Attenuation, like gain, is
simply a relative measure of output level compared to input level.
1.4.4 Propagation
The methods and parameters involved in transferring electromagnetic waves from one
point to another point some distance away, and the way in which these electromagnetic
waves are affected in their traveled path by the environment, are embraced by the study
of electromagnetic wave propagation. Because there are so many environmental factors
that influence the propagation of electromagnetic waves, a high degree of uncertainty
exists in determining the reliability of a signal. When designing a radio system, the
engineering and prediction of a radio path performance, is quite often the most difficult
and involved aspect of radio design.
There are a number of modes of propagation of radio waves across the planet. The
mode that takes place and the losses associated with the mode are affected by the
following factors:
• Frequency used
• Terrain type
• Time of year
• Weather conditions
• Moisture and salt content of the terrain
• Distance of propagation
• Antenna heights
• Antenna types and polarization
The various modes of propagation are described below:

Radio technology 13
1.4.4.1 Surface wave
Here waves travel across the surface of the earth to the destination. The wave appears to
hug the earth’s surface as it moves across it and will provide communication below the
horizon.
Figure 1.14
Surface waves
1.4.4.2 Ionospheric reflection and scatter
Here radio waves reflect off the ionosphere (a layer of the atmosphere where the air
molecules have been ionized by the sun and free electrons exist) back to the earth’s
surface. Generally, multiple reflections of a single radio wave take place causing, what is
referred to as, SCATTER. Here communication well below the horizon is possible.
Figure 1.15
Ionospheric reflection and scatter
1.4.4.3 Ionospheric refraction
Here, due to the sudden changes in the characteristics of the atmosphere where the lower
atmosphere meets the ionosphere, the radio waves will bend in an arc back to the earth’s
surface. Again, communication below the horizon is possible.

Figure 1.16
Ionospheric refraction
1.4.4.4 Tropospheric scatter
Here, radio waves reflect off a layer of the atmosphere called the TROPOSPHERE, back
to earth. The troposphere is the layer of the atmosphere where temperature decreases with
height, and this is where most clouds are normally formed. All weather, as a general term,
originates in the troposphere. Because of the many particles in the troposphere, multiple
reflections of a single radio wave occur. This is tropospheric scatter.
Figure 1.17
Tropospheric scatter
1.4.4.5 Line of sight
Here radio waves travel in an approximate straight line to the destination. There is slight
bending of radio waves due to refraction of the earth’s atmosphere. Although this is only
slight, it does allow the radio wave to travel a little over the straight line to the horizon.
The previous four modes of propagation were able to travel significantly over the straight
line to the horizon. Light rays seen by the human eye will behave in a similar manner to
RF waves. Hence this mode of propagation is referred to as ‘line of sight’ propagation.

Radio technology 15
Figure 1.18
Line of sight
1.4.4.6 Diffraction
Radio waves, in a similar manner to light, are able to bend around corners or obstructions.
This phenomenon is referred to as diffraction. The degree of diffraction depends on the
frequency used and the terrain over which the radio wave is passing. This allows below
horizon communications for a line of sight system but introduces significant signal
attenuation.
Figure 1.19
Diffraction
1.4.4.7 Ducting
In some parts of the world where large arid landmasses meet the ocean, large areas of
temperature inversion can occur in the lower part of the troposphere. This sets up a duct
through which radio waves will reflect and refract, for many hundreds of kilometers.

Figure 1.20
Ducting
The majority of telemetry systems are designed to operate in the line of sight mode, and
to a lesser extent, in the diffraction and surface wave mode. But the discussion of other
propagation modes, as stated above, provide the reader with an understanding as to how
interference can be caused by distant users operating on the same frequencies.
1.4.5 Line of sight propagation path attenuation
Under conditions of free space propagation between two antennas that are in line of sight
and where the signal is completely unaffected by other environmental factors, the
attenuation of the radio wave can be calculated by the following formula:
A = 32.5 +20Log10F +20Log10D
Where:
A = Attenuation in dB
F = Frequency in MHz
D = Distance in km
There are many complex empirical mathematical models that have been developed to
describe the different modes of propagation, (discussed in the last section), over different
terrains.

Radio technology 17
Figure 1.21
Graph of free space attenuation for commonly used radio frequencies
1.4.6 Reflections
When setting up telemetry links in the radio frequency band, one of the main causes of
degradation of the RF signal will be reflection from the surrounding environment.
Figure 1.22
Reflected signals
The source of multiple reflections may be the earth, hills, billboards, cars, buildings,
airplanes, lakes, rivers (water masses), etc. Reflections from different sources would
arrive at the receiving antenna at different times. Because of the different distances each
wave may have to travel, they may arrive in or out of phase with the direct signal, which
will cause a degree of addition or cancellation of the direct signal respectively. The

degree of cancellation will depend on the strength of the reflected signal, which is
dependant on the type of surface that reflected it, and the difference in path lengths
between the reflected and direct signals, i.e. the phase difference.
1.5 Criteria for selection of frequency bands
Each band of the frequency spectrum behaves differently when used in different physical
environments, and therefore, different bands are chosen for specific applications. The
choice of correct frequency, for a particular application, is essential in guaranteeing
effective and reliable operation of a telemetry system.
The behavioral characteristics of each band will now be considered.
1.5.1 HF
HF radio waves travel around the surface of the earth in two modes. The first is the
surface wave or ground wave propagation, as discussed in section 1.4.4. Here the wave
front hugs the ground as it follows a curved path across the surface of the earth. The
distance, to which the wave can travel and still be at a useable level, depends on the type
of terrain and on the conductivity of the earth’s surface. The more electrically conductive
the surface, the further the ground wave will travel across that surface. For example, the
best ground wave propagation occurs over the sea.
Most commercial HF ground wave systems are designed to operate in the frequency
band of 1.8 to 3.5 MHz.
Depending on the equipment configuration, distances of approximately 250–500 km are
possible over sea, for low power (50–100 watts) transmitters, and approximately
700–1000 km for high power (up to 1000 watts) transmitters. Over relatively flat land,
distances of approximately 100–150 km are possible for low power transmitters and
approximately 200–300 km for high power transmitters.
Figure 1.23
Ground wave attenuation for average rolling terrain

Radio technology 19
Figure 1.24
Ground wave attenuation for salt water terrain
One important use of the ground wave phenomenon is for communicating with
submarines. Here frequencies of 10–30 kHz are used to communicate over sea, spanning
distances of many thousands of kilometers. (At these frequencies, there is certain
penetration of the sea.)
The second mode of HF propagation is referred to as sky wave communication. This is
the Ionospheric mode of propagation, referred to in section 1.4.4. The ionospheric layers
are where the electromagnetic waves are affected by the masses of free electrons floating
around, because of the ionization of air molecules in these outer layers. There are several
layers to the Ionosphere. The first is the E layer, which is approximately 110 km above
the earth’s surface. Then there are two F layers referred to as F1 and F2 at distances of
approximately 230 km and 320 km, respectively. These distances vary depending on the
time of day and the two F layers combine at night to form one layer, at approximately
280 km.
An HF radio wave travels from the transmitter to the ionosphere where it then reflects
and/or refracts back to the earth’s surface. Depending on the type of the earth’s terrain,
the signal may reflect back up to the ionosphere again. This process may continue until
the wave has traveled half way around the earth.

Figure 1.25
Sky wave propagation
The distance, which is covered by a single hop, is referred to as the skip distance. Most
HF communication occurs via reflections off the F layers. The maximum distance from a
single hop off the F layer is approximately 4000 km. Electromagnetic waves, between the
frequencies of 1.8 and 3.5 MHz, are not reflected by the ionosphere and will travel
through into space. Therefore, most sky wave communication takes place in the fre-
quency bands above 4 MHz (4–30). The critical frequency of 3.5 MHz is referred to as
the absorption limiting frequency (ALF).
For this reason also, ground wave communication is normally carried out between 1.8
and 3.5 MHz. This ensures that a ground wave and a sky wave, from the same
transmitter, do not arrive at the same point and cause phase cancellation.
Because of the changing nature of the atmosphere, HF radio transmission is not a very
reliable medium of communication. If HF radio is to be used for telemetry purposes then
normally only ground wave communication is used, since it provides a signal of higher
continuous availability than a sky wave.
The HF radio band is very noisy and because of this, only very slow baud rates can be
used (normally 300 or 600 baud).
There is a unique science to predicting and using sky wave communications. Sky wave
prediction charts are available each month, showing which frequencies will work and at
what time of day these frequencies will operate successfully.
1.5.2 Very high frequency (VHF)
The VHF band covers frequencies from approximately 30 MHz through to 225 MHz.
This band is then broken down into three sub-bands as indicated in section 1.3. Each of
these sub-bands has slightly different behavioral characteristics.
1.5.2.1 Low band VHF (31—
59 MHz)
This band is not commonly used for telemetry system applications. It is considered a high
noise band and in particular, is very susceptible to man-made switching and engine-
induced noise (though less susceptible than the HF band).
The radio equipment available for use in this band is relatively bulky compared to the
equipment used in higher bands. Users generally prefer to set up telemetry systems in

Radio technology 21
higher VHF bands. The higher noise environment of the low band region, limits data
speeds (relative to other bands), and raises the minimum receive signal level requirement.
Because these are not popular bands of operation, very few radio manufacturers
produce equipment that operates in this band. Being tied to one or two manufacturers will
obviously limit the ability to obtain competitive prices for equipment.
There are, however, some advantages in using low band VHF. The lower the VHF
frequency used, the more capable the signal is of penetrating solids. For this reason, low
band VHF systems are often used in heavily forested areas.
Another advantage is that the lower the VHF frequency used, the better the surface
wave propagation, and this is also beneficial in heavily forested areas (although not as
effective as HF frequencies). Using line of sight propagation modes, at higher
frequencies, for telemetry systems in heavily forested areas has proven to be very
unreliable (from the author’s own experience). Therefore, the use of low band frequencies
is a common solution. In addition, the wet-conditions often encountered in forest areas,
provide good soil conductivity and, at low band VHF frequencies, increase the effective
antenna height (by providing a good ground plane), and decrease the attenuation of the
surface wave. These conditions are significantly less effective for higher VHF bands.
The decision to use low band VHF frequencies in a telemetry system must therefore be
carefully considered on the basis of environmental conditions, equipment availability,
required data rates and acceptable noise levels.
1.5.2.2 Mid band VHF (60—
100 MHz)
Mid band VHF frequencies, are more extensively used than low band VHF frequencies.
These frequencies are in a lower noise environment than low band VHF frequencies, and
are less affected by switching and engine noise. Therefore, to a limited extent, higher data
rates with improved signal availability are possible. Note though, that the noise
environment is not as good as that at higher VHF or UHF frequencies.
Although there is considerably more equipment available from manufacturers in this
band, it is still limited when compared to the higher frequency bands. Care must be taken
when planning a telemetry system in this band to ensure that an adequate range of
equipment is available from different manufacturers and that it is locally supported and
maintained.
Mid band VHF frequencies, are also relatively good at penetrating solids and are used
in some moderately dense forest environments. Wet soil conditions provide no noticeable
benefit when operating in this or higher bands. At these higher frequencies, changes in
effective antenna height and surface wave attenuation, become negligible.
Because these lower frequencies have good diffraction properties, this frequency band
is often used where remote terminal units need to be accessed over hilly terrain, covering
large distances. This effect is often utilized in bore-field applications, where it is often not
justified to have more than one master site, and some RTUs are very far or partially
shadowed by hills. At these, and lower frequencies, the signal will refract around
obstructions and to a limited extent, provide a viable communications link. Using the
diffraction propagation mode for telemetry system design, calls for extreme caution.
Another noticeable phenomenon, in this frequency band, occurs in parts of the world
where large flat dry regions of land meet the ocean. The signal tends to experience long-
term fading, lasting several hours or more, because there is a significant drop in received
signal strength. This is caused by long-distance ducting due to an atmospheric tem-
perature inversion over the land when the cool air off the ocean hits the hot air on the land
and rises above it. Here the direct line of sight or diffracted signal is cancelled by a

ducted signal, which will have traveled a greater distance than the direct signal and will
arrive out of phase with the direct signal, causing a degree of cancellation.
Figure 1.26
Cancellation of signal due to ducting
It is virtually impossible to solve this problem other than to move to a higher frequency
band, where ducting does not cause as significant a problem or establish a diverse system.
1.5.2.3 High band VHF (101—
225 MHz)
High band is generally the preferred VHF band for telemetry operations. It has the lowest
natural noise characteristics and is the least susceptible to externally produced man-made
noise. It is generally more reliable when carrying higher data rates because the overall
signal availability will be higher than that of the other VHF bands.
Most manufacturers of radio equipment produce equipment that operates in VHF high
band. Therefore, mobile radio system implementation, upgrade or replacement, is
straightforward and more cost effective than with low or mid VHF bands.
Generally, high band VHF is popular because it provides a reasonable blend of the
benefits of lower VHF bands and those of UHF bands.
It will have reasonably good penetration ability through solids and reasonably good
diffraction characteristics, but neither the penetration nor the diffraction parameters will
be as noticeable as with the lower VHF bands.
In addition, the lower noise communication environment will allow good data
transmission rates, but not as good as those available in the UHF band. In built-up urban
areas, or on noisy industrial sites, the signal degradation caused by man-made noise can
still be quite severe.
The VHF high band generally does not suffer from the major fading problems described
for mid band VHF. Minor short term fading, for periods of several seconds, can occur in
some locations, but are not as noticeable.
1.5.3 UHF
The UHF band covers the frequencies from approximately 335 MHz through to 960 MHz
(the lower parts of the microwave band are also referred to as UHF – see Chapter 2). The
whole band is generally broken down into two sub-bands having slightly different
behavioral characteristics.

Radio technology 23
1.5.3.1 UHF low band 335—
520 MHz
UHF low band frequencies are the ones most commonly used in line of sight telemetry
systems. This is because degradation due to noise is less severe than that in the VHF
band. The man-made noise from switching equipment and engine ignition, most common
in urban areas and on industrial sites, has little effect on UHF frequencies. Therefore,
generally higher data rates and lower receive signal levels are possible. A combination of
lower possible receiver signal levels and line of sight communications provide a basis for
improved radio link availability.
UHF frequencies have minimal penetration ability and, depending on the type of
surface the waves are hitting, tend to either be partially absorbed or reflected off the
surface. This phenomenon affects telemetry communication systems in two ways. Firstly,
in an area such as a city or industrial site where there are a lot of buildings and objects for
the signals to reflect off, multiple signals from one transmission may arrive at the
receiver. This is referred to as multipathing.
If the transmitter to receiver path is not line-of-sight, perhaps shadowed by a building,
there is still some chance that a number of signals will be reflected to the receiver and it
will be able to lock onto the strongest reflected signal. Although multipathing can cause
cancellation of a direct signal, it is sometimes found to be very short and random in
nature and can enhance the communications link. Therefore, uninterrupted com-
munications can sometimes be carried out successfully where there is a lot of random
multipathing.
In a more open environment, for example through hilly terrain, there would be fewer
but more prominent reflection paths. A reflection off a rock or a pond of water may arrive
out of phase and cause severe cancellation of the signal.
When designing a telemetry system operating in the UHF band, the effects of
multipathing must be carefully taken into consideration. It is difficult to predict the exact
effects of multipathing.
In areas where there is heavy vegetation, the UHF signal tends to be considerably
absorbed. It is noted that the wavelength of a UHF signal is close to that of a leaf or
branch and when the tree is wet, attenuation becomes even more severe.
UHF frequencies have certain diffraction characteristics but these are significantly less
than at the VHF frequencies. Therefore, attenuation of the diffracted signal is
significantly increased at the UHF frequencies.
System designers will find that most manufacturers of radio equipment produce a good
range of equipment for the UHF band. The author’s experience with working in UHF low
band is that it is generally very cost effective.
At some locations around the world, on days when the weather is warm and still,
significant temperature inversions can occur very close to the surface of the earth and
ducting can be experienced over distances of 50 to 150 km. This sometimes causes
interference from users operating on the same frequencies at distant locations. This
phenomenon is not a major consideration and can be partially overcome using coding
techniques in receivers (See section 1.21.5).
1.5.3.2 UHF mid band (800—
960 MHz)
Frequencies in this band behave in a manner very similar to those in the lower UHF band.
In summary when compared to low band UHF they have:
• Higher free space attenuation

• Slightly less signal degradation due to noise and therefore able to carry high
data rates at a better availability
• Less penetration ability
• Slightly more reflection ability
• More absorption in vegetation areas – suffer from very high attenuation in
dense wet vegetation
• Less diffraction characteristics and higher diffraction attenuation
• More susceptible to reflection cancellation
Equipment manufactured for this band is not as common as for the lower UHF band,
but is sufficient and diverse enough to warrant using this band, if required.
1.5.4 Frequency selection
The choice of frequency band that should be used when establishing a telemetry system
will depend upon a careful evaluation of a number of criteria. These criteria include:
• Distance to remote sites
• Terrain type
• Vegetation type
• Climate and weather patterns
• Noise environment
• Availability of frequencies
• Availability of equipment
• Required data rates
• Costs
Each of these criteria will now be discussed in more detail.
1.5.4.1 Distance to remote sites
This has been discussed under the sections of propagation, diffraction, surface waves, and
the characteristics of different bands. It was seen that for longer distances, lower
frequencies should be used and for shorter distances higher frequencies be used. As a
rough guide, it could be concluded that for:
• Distances greater than 60 km Use HF
• Distances between 60 and 30 km Use VHF
• Distances up to 35 km Use UHF
Greater distances can be obtained in the VHF and UHF frequency bands depending on
the equipment configuration and required link availability (refer to section 4.3).
1.5.4.2 Terrain type
This is a relatively complex consideration. Rarely is the transmission path just a smooth
surface, except perhaps over coastal or inland water or over very flat countryside. If the
terrain is rolling hills, it is best to use a lower frequency. If the region is mountainous it is
often more appropriate to use high UHF frequencies to make use of multipathing effects.
Care must be taken to ensure that severe shadowing of an RTU does not occur in a
location where no reflected signal can reach.

Radio technology 25
If the terrain is halfway between mountainous and smooth, rolling hills (referred to as
rough terrain), the final determination as to what frequency to use will depend on the
earth type and vegetation. If the land is arid, dry, and rocky, it is better to use UHF
frequencies. If the land has moderate to heavy vegetation, it is better to use VHF
frequencies.
Another factor to consider is the location of stretches of flat land, or water, between the
master station and RTUs. These can be sources of significant reflected signals that can
cause severe phase cancellations at the receiver.
For example, a radio link operating over a salt lake may operate perfectly well until it
rains, at which point the lake will turn into a perfect mirror. The reflection off the lake
may cancel the direct signal, to the point that the link drops out.
1.5.4.3 Vegetation type
As was discussed earlier in this section, the denser the vegetation, the lower the frequency
that should be used. For example, if transmission is directly through several kilometers of
thick, wet forest, it may exhibit an attenuation of 2 dB for a frequency of 30 MHz. For
the same section of forest the signal attenuation at 900 MHz may be 40 dB or more.
For thin dry vegetation, the attenuation is noticeable but significantly less than through
wet forest, e.g. 5–10 dB at 900 MHz.
1.5.4.4 Climate and weather patterns
The major weather condition that affects the propagation of radio waves in the UHF and
VHF bands is the degree of moisture on vegetation; the more rain that falls, the higher the
attenuation through vegetation. However, wet surfaces on buildings or on hard rocky
mountains, increase the reflective properties of the surfaces and increase the multipath
effects, which in most cases would improve the probability of successful communication.
In hot dry regions, significant temperature inversions can occur in the lower parts of the
atmosphere during still weather conditions, allowing significant ducting of radio signals,
which may cause interference problems at distant receivers.
The presence of fog generally infers a temperature inversion that can cause the same
problems.
1.5.4.5 Noise environment
Noise that affects the performance of telemetry systems operating in the VHF and UHF
bands is primarily man-made electrical noise. Atmospheric noise only starts to degrade
the radio receiver as frequencies go down into low band VHF and HF bands.
Man-made electrical noise can come from switching equipment, relays, rectifiers,
inverters, ignition systems, generators, high power ac lines, and numerous other sources.
Lightning and atmospheric static build up, produce the worst degree of environmental
noise. Static noise is a major problem in the tropical areas of the world.
All noise sources cause degradation of the radio signal at the receiver. They can cause
interference with the signal to the point where errors occur in the data. Noise also has the
effect of increasing the minimum useable receive signal level, which decreases the
availability of the radio path. Therefore, the overall performance of the system degrades.
The degrading effect of man-made noise is worse at lower frequencies. For example,
engine noise is quite severe in the 1–225 MHz bands, significantly less severe in the
335–520 MHz band and has virtually no effect in the 800–960 MHz band.

The level of noise is very dependant on the environment. For example, the level of
noise in an industrial environment at 450 MHz would be approximately equal to the level
of noise in a quiet rural environment at 80 MHz.
Solar noise (also referred to as galactic noise) is noise from space. This is generally
considered to be of a low level and only affects, to a small extent, frequencies below
80 MHz. This is worse during the day, when the sun is radiating direct noise, than during
the night.
Figure 1.27
Mean values of man-made noise power for different environments (Reference – CCIR Doc 6/167 E/F/s)
Figure 1.27 illustrates relative power levels of man-made noise for different areas (the
noise level being relative to thermal noise at 15°C).
Another source of noise occurs when a number of radios operate, within close
proximity, on different frequencies. A summation of these frequencies and their
associated harmonics form frequencies that interfere with existing frequencies. This is
referred to as intermodulation interference and will be discussed in detail in section
1.15.2. It is important that all potential intermodulation noise is determined during the
design stage, so that frequencies can be appropriately selected to avoid interference
problems.
1.5.4.6 Availability of frequencies
This subject is discussed in detail in section 1.19 under the heading ‘Regulatory licensing
requirements for radio frequencies’.
1.5.4.7 Availability of equipment
As was discussed in the previous section, equipment operating in lower UHF, high VHF,
and HF bands is readily available from different manufacturers, while in mid VHF and
high UHF it is sometimes a little harder to obtain and in low VHF band, there is a definite
lack of good available equipment on a competitive basis.
During the initial stages of system design, it is essential that the designer determines the
availability and range of equipment that can be easily purchased and is fully supported
and maintained in his region.

Radio technology 27
1.5.4.8 Costs
The costs of radio equipment in the 335–520 MHz, 60–100 MHz, and 101–225 MHz
bands are generally very competitive. Equipment in the 31–59 MHz and 800–960 MHz
band slightly more expensive and equipment in the 1–30 MHz band generally more
expensive again.
1.5.5 Summary
The following tables summarize the information that was discussed in this section.
Low band
VHF
Mid band
VHF
High band
VHF
Propagation mode Mostly L.O.S.
some surface wave
L.O.S. minimal
surface wave
L.O.S.
Data rates 1200 baud 2400 baud 4800 baud
Diffraction properties Excellent Very good Good
Natural noise
environment
High Medium Low
Affected by man-
made noise
Severe Bad Some
Penetration of solids Excellent Very good Good
Fading by ducting Long term Medium term Short term
Absorption by wet
vegetation
Negligible Low Some
Equipment
availability
Minimal Reasonable Excellent
Relative equipment
cost
High Medium Low
Uses – In forested areas
– Mostly mobile
– Very hilly
– Very hilly
forested areas
– Mostly mobile
– Over water
– Long distance /
L.O.S./hilly
areas
– L.O.S links
– Mobile
– Borefields
– Over water
Table 1.1

UHF 1 UHF 2
Propagation mode L.O.S. L.O.S.
Data rates 9600 baud 19 200 baud
Diffraction properties Some Minimal
Natural noise environment Low Negligible
Affected by man made noise Low Very low
Penetration of solids Low Negligible
Reflection absorption by
solids
Good (enhanced
multipathing)
Excellent (excellent
multipathing)
Absorption by wet vegetation High Very high
Interference by ducting Some Some
Equipment availability Excellent Reasonable
Relative equipment costs Low Medium
Uses –Telemetry
– Mobile
– Telemetry
– Mobile
– Links
Table 1.2
1.6 Modulation and demodulation
The frequency, at which a radio system operates, is referred to as the carrier wave
frequency. If the system has been allocated a frequency of 452.725 MHz, then this is the
carrier wave frequency. All information that is to be transferred from the transmitter to
the receiver is imparted on to the carrier wave.
Modulation is the process of varying some characteristic of the carrier wave, in
accordance with the information signal to be transferred.
Demodulation is the process of deriving the information signal back from the
modulated carrier wave.
A modulator is a device that takes the information signal and modulates it on to the
carrier. A demodulator, conversely, takes the modulated carrier signal and extracts the
information signal out again.
There are four main variations of the modulation techniques used in radio. The first
three involve varying the amplitude, frequency, or phase of the carrier in accordance with
the information signal. The fourth method is to turn the carrier wave on and off in a
digital manner. In any radio system, only one modulation technique is normally used
(with a few exceptions). Each of these techniques will now be discussed in detail.
1.6.1 Carrier wave modulation
Carrier wave modulation is where the carrier wave is switched ON and OFF in a digital
format. This is used in the simple Morse code telegraphy system.

Radio technology 29
Figure 1.28
Carrier wave modulation
This type of modulation is also referred to as continuous wave (or CW), or ON–OFF
keying.
Because of the slow rise and fall times of the output stages of the transmitter, the data
speeds possible are severely limited. Generally, this technique is only used for very slow
data rates of 50 or 100 baud.
1.6.2 Sidebands and bandwidth
All methods of modulation of a carrier wave produce frequencies that are above and
below the carrier frequency. These frequencies are called the sideband frequencies.
Bandwidth is the term used to describe the maximum distance the sideband frequencies
are allowed from the carrier. For example, if the allowed bandwidth for an amplitude
modulated radio system, working at 1 MHz, is 10 kHz, then the maximum distance the
sidebands can extend either side of the carrier is 5 kHz. This is illustrated in Figure 1.29.
In this example, the maximum frequency component allowed in the information signal is
5 kHz.
Figure 1.29
Illustration of bandwidth
Normally when describing an operating bandwidth (i.e. that at which a communication
system is required to operate on), the outer frequency limits of this bandwidth are the
points, where the sideband frequencies have dropped to a maximum power level 3 dB
below the maximum central frequency power level.

Figure 1.30
3 dB bandwidth
The regulatory government bodies and private organization that allocate frequencies
and bandwidth to radio users generally define bandwidth from a different perspective. As
an example, they may allocate a new frequency to a user every 25 kHz. An example
performance specification would typically specify that for a 25 kHz bandwidth, the output
frequency spectrum sideband levels of a transmitter should:
a) For +3 kHz to –3 kHz either side of the carrier have a relatively flat amplitude
response
b) For +3 kHz to +6 kHz and –3 kHz to –6 kHz either side of the carrier, the amplitude
shall not exceed the levels between +3 kHz and –3 kHz
c) At +6 kHz and –6 kHz either side of the carrier, the amplitude shall be a minimum of
6 dB down on the amplitude of the sidebands at +1 kHz and –1 kHz
d) At frequencies beyond +6 kHz and –6 kHz, the amplitude shall fall off at a rate of
14 dB per octave
e) At ±12.5 kHz, the levels are normally 50 to 70 dB below the ±1 kHz sidebands
Figure 1.31
Frequency response for 25 kHz and (12½ kHz) bandwidth radio

Radio technology 31
Although there is still a significant amount of 25 kHz bandwidth allocations around the
world, most countries are moving over to 12.5 kHz bandplans, due to a severe shortage of
radio spectrum that exists worldwide. In this case, the allocated bandwidth would be the
same envelope requirements as for the 25 kHz bandwidth, except that the frequencies
either side of the carrier, are halved.
Most manufacturers of radio equipment construct radios so that the transmitter spurious
signals are 50–70 dB down outside the allocated bandwidth.
1.6.3 Amplitude modulation (AM)
Amplitude modulation (AM) is the process of varying the amplitude of the carrier wave
(which is in sinusoidal form) in sympathy with the information signal. The rate of change
(speed) at which the carrier moves up and down in amplitude is directly proportional to
the frequency of the information signal. The level of magnitude to which the carrier
moves up and down is directly proportional to the amplitude of the information signal.
Figure 1.32 illustrates how a carrier signal is amplitude modulated with an information
signal and what the resultant waveform would look like.
Figure 1.32
The process of amplitude modulation
For amplitude modulation, there will be a sideband frequency, each side of the carrier
frequency, for each frequency component in the information signal. For example, if the
information signal consists of the frequencies 1 kHz and 2 kHz and it is modulating a
carrier wave of 1 MHz, then it will produce sidebands of 0.999 MHz and 1.001 MHz for
the 1 kHz signal and 0.998 MHz and 1.002 MHz for the 2 kHz signal. Figure 1.33
illustrates this effect with frequency versus amplitude graphs.

Figure 1.33
AM sidebands produced with a modulating signal that has 1 2 kHz frequency components
Note that the amplitude of the carrier frequency component does not change, just the
amplitude of the sideband frequency components.
With AM, the difference between the carrier frequency and the farthest sideband
frequency component, is determined by the highest frequency component in the
information signal.
A common term used in AM is modulation factor, which is a figure used to express the
degree of modulation. It is expressed as a percentage of modulation. Figure 1.34
illustrates a number of different modulation factors.
(a) Normal modulation (~70% modulation)
(b) Maximum modulation (100% modulation)
(c) Over modulation
Figure 1.34
Different modulation levels

Radio technology 33
Figure (a) illustrates what would be an average level input signal modulating a carrier.
The percentage modulation is measured as:
100
×
=
c
a
M
%
OR 100
×
c
b
Where a and b are normally equal.
C is normally referred to as the depth of modulation and is half the maximum modulation.
Figure (b) shows the maximum input signal allowed, which is where a = b = c or where
there is 100% modulation. Beyond this modulation-level, the RF wave becomes distorted
and will produce spurious sidebands at frequencies beyond the allocated bandwidth.
Figure (c) illustrates how over modulation appears at the RF signal.
Spurious sideband frequencies can cause severe interference to nearby receivers, and
strict government regulations restrict the user from emitting this interference. A
significant proportion of the cost of implementing a radio system can be devoted to
filtering any possible sideband interference.
With straight AM modulation a single frequency input signal produces one sideband
either side of the carrier. For this reason it is referred to as double sideband amplitude
modulation (DSB-AM).
As the carrier and the two sidebands are used to transmit a single piece of information,
this is very wasteful of bandwidth resources, considering that all the information is
actually contained in a single sideband. Two methods are used to improve the efficiency
of the AM system.
With the first method, the carrier is suppressed and only the two sidebands transmitted.
This is referred to as double sideband suppressed carrier amplitude modulation (DSBSC-
AM). The major advantage of this system is the reduced power requirement at the
transmitter output to amplify and transmit the RF signal. In general, this represents a 66%
reduction in power requirements. (In a normal DSB-AM circuit, each sideband is a
maximum of 25% of the power of the carrier.)
The second method used to increase the efficiency of the radio is to remove the carrier
and one of the sidebands. Since both sidebands are carrying the same information, the
removal of one sideband does not affect the integrity of the information. This method of
modulation is referred to as single sideband suppressed carrier amplitude modulation
(SSBSD-AM).
Figure 1.35 – (a) Double sideband amplitude modulation (DSB-AM)

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A HALF CENTURY AMONG THE
SIAMESE AND THE LĀO

A HALF CENTURY AMONG
THE SIAMESE AND THE LĀO
AN AUTOBIOGRAPHY
By
DANIEL McGILVARY, D.D.
WITH AN APPRECIATION BY
ARTHUR J. BROWN, D.D.
ILLUSTRATED

New York Chicago Toronto
Fleming H. Revell Company
London and Edinburgh

Copyright, 1912, by
FLEMING H. REVELL COMPANY
New York: 158 Fifth Avenue
Chicago: 125 N. Wabash Ave.
Toronto: 25 Richmond St., W.
London: 21 Paternoster Square
Edinburgh: 100 Princes Street

AN APPRECIATION
Missionary biography is one of the most interesting and instructive
of studies. It is, however, a department of missionary literature to
which Americans have not made proportionate contribution. The
foreign missionary Societies of the United States now represent
more missionaries and a larger expenditure than the European
Societies, but most of the great missionary biographies are of British
and Continental missionaries, so that many Americans do not realize
that there are men connected with their own Societies whose lives
have been characterized by eminent devotion and large
achievement.
Because I regarded Dr. McGilvary as one of the great missionaries
of the Church Universal, I urged him several years ago to write his
autobiography. He was then over seventy-five years of age, and I
told him that he could not spend his remaining strength to any
better advantage to the cause he loved than in preparing such a
volume. His life was not only one of unusual length (he lived to the
ripe age of eighty-three), but his missionary service of fifty-three
years covered an interesting part of the history of missionary work in
Siam, and the entire history, thus far, of the mission to the Lāo
people of northern Siam. There is no more fascinating story in fiction
or in that truth which is stranger than fiction, than the story of his
discovery of a village of strange speech near his station at
Pechaburī, Siam, his learning the language of the villagers, his long
journey with his friend, Dr. Jonathan Wilson, into what was then the
unknown region of northern Siam, pushing his little boat up the
great river and pausing not until he had gone six hundred miles
northward and arrived at the city of Chiengmai. The years that

followed were years of toil and privation, of loneliness and
sometimes of danger; but the missionaries persevered with splendid
faith and courage until the foundations of a prosperous Mission were
laid.
In all the marked development of the Lāo Mission, Dr. McGilvary
was a leader—the leader. He laid the foundations of medical work,
introducing quinine and vaccination among a people scourged by
malaria and smallpox, a work which has now developed into five
hospitals and a leper asylum. He began educational work, which is
now represented by eight boarding schools and twenty-two
elementary schools, and is fast expanding into a college, a medical
college, and a theological seminary. He was the evangelist who won
the first converts, founded the first church, and had a prominent
part in founding twenty other churches, and in developing a Lāo
Christian Church of four thousand two hundred and five adult
communicants. His colleague, the Rev. Dr. W. C. Dodd, says that Dr.
McGilvary selected the sites for all the present stations of the Mission
long before committees formally sanctioned the wisdom of his
choice. He led the way into regions beyond and was the pioneer
explorer into the French Lāo States, eastern Burma, and even up to
the borders of China. Go where you will in northern Siam, or in many
sections of the extra-Siamese Lāo States, you will find men and
women to whom Dr. McGilvary first brought the Good News. He well
deserves the name so frequently given him even in his lifetime
—“The Apostle to the Lāo.”
It was my privilege to conduct our Board’s correspondence with
Dr. McGilvary for more than a decade, and, in 1902, to visit him in
his home and to journey with him through an extensive region. I
have abiding and tender memories of those memorable days. He
was a Christian gentleman of the highest type, a man of cultivation
and refinement, of ability and scholarship, of broad vision and
constructive leadership. His evangelistic zeal knew no bounds. A
toilsome journey on elephants through the jungles brought me to a
Saturday night with the weary ejaculation: “Now we can have a day
of rest!” The next morning I slept late; but Dr. McGilvary did not; he

spent an hour before breakfast in a neighbouring village, distributing
tracts and inviting the people to come to a service at our camp at
ten o’clock. It was an impressive service,—under a spreading bo
tree, with the mighty forest about us, monkeys curiously peering
through the tangled vines, the huge elephants browsing the bamboo
tips behind us, and the wondering people sitting on the ground,
while one of the missionaries told the deathless story of redeeming
love. But Dr. McGilvary was not present. Seventy-four years old
though he was, he had walked three miles under a scorching sun to
another village and was preaching there, while Dr. Dodd conducted
the service at our camp. And I said: “If that is the way Dr. McGilvary
rests, what does he do when he works?” Dr. McKean, his associate
of many years, writes:
“No one who has done country evangelistic work with Dr.
McGilvary can ever forget the oft-seen picture of the gray-haired
patriarch seated on the bamboo floor of a thatch-covered Lāo house,
teaching some one to read. Of course, the book faced the pupil, and
it was often said that he had taught so many people in this way that
he could read the Lāo character very readily with the book upside
down. Little children instinctively loved him, and it is therefore
needless to say that he loved them. In spite of his long snow-white
beard, never seen in men of this land and a strange sight to any Lāo
child, the children readily came to him. Parents have been led to God
because Dr. McGilvary loved their children and laid his hands upon
them. In no other capacity was the spirit of the man more manifest
than in that of a shepherd. Always on the alert for every opportunity,
counting neither time nor distance nor the hardship of inclement
weather, swollen streams, pathless jungle, or impassable road, he
followed the example of his Master in seeking to save the lost. His
very last journey, which probably was the immediate cause of his
last illness, was a long, wearisome ride on horseback, through
muddy fields and deep irrigating ditches, to visit a man whom he
had befriended many years ago and who seemed to be an inquirer.”
Dr. McGilvary was pre-eminently a man who walked with God. His
piety was not a mere profession, but a pervasive and abiding force.

He knew no greater joy than to declare the Gospel of his blessed
Lord to the people to whose up-lifting he had devoted his life. “If to
be great is ‘to take the common things of life and walk truly among
them,’ he was a great man—great in soul, great in simplicity, great in
faith and great in love. Siam is the richer because Daniel McGilvary
gave her fifty-three years of unselfish service.” Mrs. Curtis, the gifted
author of The Laos of North Siam, says of Dr. McGilvary: “Neither
Carey nor Judson surpassed him in strength of faith and zeal of
purpose; neither Paton nor Chalmers has outranked him in the
wonders of their achievements, and not one of the other hundreds
of missionaries ever has had more evidence of God’s blessing upon
their work.”
Not only the missionaries but the Lāo people loved him as a friend
and venerated him as a father. Some of his intimate friends were the
abbots and monks of the Buddhist monasteries and the high officials
of the country. No one could know him without recognizing the
nobility of soul of this saintly patriarch, in whom was no guile.
December 6th, 1910, many Americans and Europeans celebrated the
fiftieth anniversary of his marriage. The King of Siam through Prince
Damrong, Minister of the Interior, sent a congratulatory message.
Letters, telegrams, and gifts poured in from many different places.
The Christian people of the city presented a large silver tray, on
which was engraved: “The Christian people of Chiengmai to Dr. and
Mrs. McGilvary, in memory of your having brought the Gospel of
Jesus Christ to us forty-three years ago.” The tray showed in relief
the old rest-house where Dr. and Mrs. McGilvary spent their first two
years in Chiengmai, the residence which was later their home of
many years, the old dilapidated bridge, and the handsome new
bridge which spans the river opposite the Christian Girls’ School—
thus symbolizing the old and the new eras.
The recent tours of exploration by the Rev. W. Clifton Dodd, D.D.,
and the Rev. John H. Freeman have disclosed the fact that the Lāo
peoples are far more numerous and more widely distributed than we
had formerly supposed. Their numbers are now estimated at from
twelve to sixteen millions, and their habitat includes not only the Lāo

States of northern Siam but extensive regions north and
northeastward in the Shan States, Southern China, and French Indo-
China. The evangelization of these peoples is, therefore, an even
larger and more important undertaking than it was understood to be
only a few years ago. All the more honour, therefore, must be
assigned to Dr. McGilvary, who laid foundations upon which a great
superstructure must now be built.
Dr. McGilvary died as he would have wished to die and as any
Christian worker might wish to die. There was no long illness. He
continued his great evangelistic and literary labours almost to the
end. Only a short time before his death, he made another of his
famous itinerating journeys, preaching the Gospel to the outlying
villages, guiding perplexed people and comforting the sick and
dying. He recked as little of personal hardship as he had all his life,
thinking nothing of hard travelling, simple fare, and exposure to sun,
mud, and rain. Not long after his return and after a few brief days of
illness, he quietly “fell on sleep,” his death the simple but majestic
and dignified ending of a great earthly career.
The Lāo country had never seen such a funeral as that which
marked the close of this memorable life. Princes, Governors, and
High Commissioners of State sorrowed with multitudes of common
people. The business of Chiengmai was suspended, offices were
closed, and flags hung at half-mast as the silent form of the great
missionary was borne to its last resting-place in the land to which he
was the first bringer of enlightenment, and whose history can never
be truly written without large recognition of his achievements.
Fortunately, Dr. McGilvary had completed this autobiography
before his natural powers had abated, and had sent the manuscript
to his brother-in-law, Professor Cornelius B. Bradley of the University
of California. Dr. Bradley, himself a son of a great missionary to
Siam, has done his editorial work with sympathetic insight. It has
been a labour of love to him to put these pages through the press,
and every friend of the Lāo people and of Dr. McGilvary is his debtor.
The book itself is characterized by breadth of sympathy, richness of

experience, clearness of statement, and high literary charm. No one
can read these pages without realizing anew that Dr. McGilvary was
a man of fine mind, close observation, and descriptive gifts. The
book is full of human interest. It is the story of a man who tells
about the things that he heard and saw and who tells his story well.
I count it a privilege to have this opportunity of commending this
volume as one of the books which no student of southern Asia and
of the missionary enterprise can afford to overlook.
Arthur J. Brown.
156 Fifth Avenue, New York.

PREFACE
Years ago, in the absence of any adequate work upon the subject,
the officers of our Missionary Board and other friends urged me to
write a book on the Lāo Mission. Then there appeared Mrs. L. W.
Curtis’ interesting volume, The Laos of North Siam, much to be
commended for its accuracy and its valuable information, especially
in view of the author’s short stay in the field. But no such work
exhausts its subject.
I have always loved to trace the providential circumstances which
led to the founding of the Lāo Mission and directed its early history.
And it seems important that before it be too late, that early history
should be put into permanent form. I have, therefore, endeavoured
to give, with some fulness of detail, the story of the origin and
inception of the Mission, and of its early struggles which culminated
in the Edict of Religious Toleration. And in the later portions of the
narrative I have naturally given prominence to those things which
seemed to continue the characteristic features and the personal
interest of that earlier period of outreach and adventure, and
especially my long tours into the “regions beyond.”
The appearance during the past year of Rev. J. H. Freeman’s An
Oriental Land of the Free, giving very full and accurate information
regarding the present status of the Mission, has relieved me of the
necessity of going over the same ground again. I have, therefore,
been content to draw my narrative to a close with the account of my
last long tour in 1898.
The work was undertaken with many misgivings, since my early
training and the nature of my life-work have not been the best

preparation for authorship. I cherished the secret hope that one of
my own children would give the book its final revision for the press.
But at last an appeal was made to my brother-in-law, Professor
Cornelius B. Bradley of the University of California, whose birth and
years of service in Siam, whose broad scholarship, fine literary taste,
and hearty sympathy with our missionary efforts indicated him as
the man above all others best qualified for this task. His generous
acceptance of this work, and the infinite pains he has taken in the
revision and editing of this book, place me under lasting obligations
to him.
I wish to acknowledge my indebtedness to Dr. W. A. Briggs and to
Rev. J. H. Freeman for the use of maps prepared by them, and to Dr.
Briggs and others for the use of photographs.
Daniel McGilvary.
April 6, 1911,
Chiengmai.

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