2. 9.1 Overview
9.2 Two-wire line
9.3 Coaxial Cables, Terminations, Connectors and
Transitions
9.4 Coaxial Attenuators:
Phase Shifters
Baluns
3. 9.1 OVERVIEW
• The coaxial cable is a transmission line consisting of two coaxial
cylindrical conductors, separated by a dielectric (see Fig. 2.1).
• The two conductors, here shown as homogeneous, are often made of
braided small diameter copper wires.
• If Er denotes the relative permittivity of the insulator, the line
parameters are given by:
4. • Coaxial Cable.
• The Field Lines Of The Electric
Field Are Shown By Solid Lines,
Those Of The Magnetic Field
By Dashed Lines.
• Geometry Of The Coaxial Cable.
5. • The Maximum Freq For Which The Coaxial Cable Is Single Mode Is
Approximately:
• Where V (z) Is The Voltage.
• Hence The Maximum Electric Field,
Not To Be Exceeded In Order To Avoid
Sparks, Is On The Surface Of The Inner
Conductor And Has The Value:
6. • Worked Example
•
• Compute the parameters of a cable, with inner conductor diameter d
=1.6 mm, outer conductor diameter D = 5.8 mm, ²r = 2.3.
• Solution:
• Applying the previous formulas we get L = 0.26 µH/m, C = 99.35 pF/m,
Z∞ = 50.92 Ω, Vf /c = 1/ √ Er = 65.9%, fmax = 17.0 GHz.
• Plse kindly Very these Answers,
• The normalized maximum electric field is Emax = 485.3V/m if the voltage
V is 1V.
8. 9.2 Two-wire line
• Two-Wire Lines The two-wire transmission line consists of two parallel
cylindrical conductors of radius a separated by distance d from each other,
as shown below
• Examples of TL: (a) coaxial cable, (b) two wire line, (c) optical fiber, (d)
microstrip , (e) stripline.
9. • Shielded two-wire line
• To avoid the electromagnetic compatibility problems of the two-wire line,
the structure below can be used.
• Shielded two-wire line and field configuration of the symmetric (balanced)
TEM mode.
10. • Note That This Is A Three Conductor Line (Two Plus A Grounded One).
• In This Case There Are Two TEM Modes,
• A Symmetric (Balanced) One Where The Potentials Of The Two Inner Conductors Are
Symmetric With Respect To That Of The Outer One, Connected To Ground,
• And An Asymmetric (Unbalanced) One, With Different Parameters.
• The Parameters For The Symmetric Mode Can Be Computed From The
Following Equations:
11. 9.2 COAXIAL CABLES, TERMINATIONS, CONNECTORS &
TRANSITIONS
COAXIAL CABLES
• The Coaxial Cable Is A Transmission Line Consisting Of Two Coaxial
Cylindrical Conductors, Separated By A Dielectric.
• The Two Conductors, Here Shown As Homogeneous, Are Often Made Of
Braided Small Diameter Copper Wires.
• The Line Parameters Are Given By:
12. • Coaxial Cable
• The Field Lines Of The Electric Field Are
Solid Lines Shown By ,
• Those Of The Magnetic Field By Dashed Lines.
• Observe That:
• The Electric Field Configuration Is That Of A Cylindrical Capacitor,
• Consistently With The Fact That The TEM Mode Has Zero Cutoff Frequency.
• If The Operation Frequency Increases,
• A Point Is Reached In Which Higher Order Modes Start To Propagate.
13. • The Maximum Freq For Which The Coaxial Cable Is Single Mode Is Approx:
• Where V (z) Is The Voltage.
14. • The Maximum Electric Field, Not To Be Exceeded In Order To
Avoid Sparks,
• Is On The Surface Of The Inner Conductor @ Has The Value:
• Note That, Coaxial Cable Is An Unbalanced Line,
• Meaning That,
• The Return Conductor Is Connected To Ground.
• Hence, The Voltage Of The Inner Conductor Is Referred To Ground.
• Please kindly See Worked Example in the Handout.
15. Coaxial Cables
• These Are Used To:
• Transmit Electrical Energy, Or Signals, From One Location To Another, i.e.,
• To Connect A Source To A Load, Such As A Transmitter To An Antenna.
• This Is Accomplished With A Transverse Electrometic (TEM) Wave Field
Distribution Propagation Within The TL, Or Coaxial Cable,
• Governed By TL Theory.
• Coaxial Cable (Coax) Is Typically Identified Or Classified According To Its
Impedance Or Rg-type.
• E.g., A 50-ohm Coax Or An RG-8 Type Coax.
16. • Construction:
• A Typical Coaxial
Cable is as shown
• The Center Conductor May Be Made Of Various Materials & Constructions.
• Most Common Constructions Are Solid Or Seven-strand Conductors.
• Solid Conductors Are Used In:
• Permanent, Infrequently Handled Or Low Flex Applications &
• Stranded Conductors Are Used In Flexible Cable Appns.
• Common Materials Include:
• Copper, Tinned Or Silver Plated Copper, Copper Clad Steel & Copper Clad Aluminum, Details are in
the Handout.
17. •Electrical Properties:
• The Most Common Electrical Property Referred To In Coax Cable Is The
Characteristic Impedance, Or Simply Impedance.
• Coax Cable Is Typically Designed As 50 Ohm, 75 Ohm, And 93 Ohm
Depending Upon The Application.
• A Simple Formula To Determine The Impedance Of A Coaxial Is:
18. • The Propagation Delay in Coaxial Cable is given By:
Physical Properties:
• The Mechanical & Physical Properties May Be Critical In Selecting The Appropriate
Cable For The Appn.
• Physical Dimensions Are Critical To Assure That An Industry Standard Connector Is
Available.
• Other Odd-size Dimensions Will Require The Use Of A Custom Termination.
• Diameter Of The Center Conductor, Core, & Jacket Are Critical Values
19. • Applications:
• One Of The Broadest Uses Of Coaxial Cable Is For Video Distribution.
• From Catv Signals Around The Neighborhood To Precision Digital Signals In A
Post-production Studio,
• These Signals Are Routed On 75 Ohm Coaxial Cable.
• Widely Used In Wireless & Antenna Applications,
20. COAXIAL TERMINATIONS
• To Avoid Confusing Reflections From The Ends Of Cables, It Is Essential
That:
• All Cables Carrying Fast Pulses Be Terminated At Their Outputs By Their
Characteristic Impedance Of 50 Ohms,
oEither With A Terminating Plug On A T-connector, Or By An Internal
Termination At The Input Of A Circuit.
21. • When using coaxial cables it is usually important to avoid reflections at the
end of the cable.
• If a cable is terminated with an impedance Z ,
• The ratio of the incoming voltage Vi to the reflected voltage Vr is given by:
Vr/Vi = [Z - Zo]/[Z + Zo].
• The total voltage across the terminating resistance is V = Vi + Vr.
• Three cases are of interest:
1. If Z = 0: Then Vi = - Vr and V = 0, as you would expect for a short
circuit.
2. If Z = 8: Then Vi = + Vr and V = 2 Vi.
3. If Z = Ro: Then Vi = 0 and V = Vi.
22. •When Z = Zo = 50 Ohms,
• The Cable Is Properly Terminated And There Is No Reflected Signal.
•Proper Termination Is Also Important For Maintaining The
Line Shape For Sharply Rising Pulses;
•If The Termination Is Not Proper,
• The Rise & Fall Times For A Square Pulse Will Be Slowed &
• The Pulse Will Be Broadened.
23. 9.3 COAXIAL CONNECTORS
• For HF Operations The Average Circumference Of A Coaxial Cable Must Be
Limited To About One Wavelength,
• In Order To Reduce Multimodal Propagation & Eliminate Erratic Reflection
Coefficients, Power Losses, & Signal Distortion.
Seven types of microwave coaxial connectors are described below.
• APC-3.5 The APC-3.5 (Amphenol Precision Connector-3.5 mm.
• It provides repeatable connections and has a very low voltage standing-wave
ratio (VSWR).
24. 2. APC-7: The APC-7 (Amphenol Precision Connector-7 mm)
• Provides A Coupling Mechanism Without Male Or Female Distinction
• The Most Repeatable Connecting Device Used For Very Accurate 5o-Ohm Measurement
Applications. Its VSWR Is Extremely Low, In The Range Of 1.02 to 18 GHz.
3. BNC: The BNC (Bayonet Navy Connector) Operates Very Well At Freq Up To About 4 GHz;
• Beyond That It Tends To Radiate Electromagnetic Energy.
• The BNC can accept flexible cables with diameters of up to 6.35 mm (0.25
in.) Zo of 50 to 75 ohms.
• It is now the most commonly used connector for frequencies under 1 GHz.
Please Refer to Handout for the Remaining 4 Types
27. •COAXIAL TRANSITIONS
•
• Coaxial Transitions Are Components Used To Move From One Tx
Medium Type To A Different Tx Medium.
• This Could Be From One Waveguide Size To Another Or
• From Full Height To Reduced Height Waveguide Or From Waveguide
Tube To Coaxial Line Or From Coaxial Line To Waveguide Tube.
• Typical Examples are the transition of RF signals from:
• Coaxial Cables to Waveguides &
• From Waveguides to A Coaxial Cables
29. Coaxial Attenuators
• A coaxial attenuator is a linear passive bidirectional TL component designed
to be inserted b/n two coaxial lines,
• In order to reduce the Pin in a matched system by a predetermined ratio without
otherwise disturbing the behaviour of the equipment.
• The ratio of input to output power is expressed in logarithmic terms where 3
dB as a power ratio is 2, 6 dB is 10, 20 dB is 100 and 30 dB is 1000 etc...
• The power absorbed by an attenuator is returned to the environment
through heat transfer by convection or conduction cooling.
31. • Attenuator Related Parameters
• "NOMINAL ATTENUATION OR ATTENUATION VALUE:
• The nominal value of attenuation is the standard value determined by
the manufacturer.
• This parameter is a representation of the power ratio b/n the input &
the ouput of the attenuator.
• It is expressed in dB :
• P out (W) Attenuation in dB = 10 log P in (W)
32. • "ATTENUATION DEVIATION
• The attenuation over full specified bandwidth does not exceed
the nominal value ¦ attenuation deviation.
• This parameter is expressed in dB.
• For example, an attenuator with 6 dB nominal value and ¦ 0.3 dB
deviation
• Do not exceed 5.7dB minimum value and 6.3 dB maximum value.
33. • " TEMPERATURE SENSITIVITY
• The maximum change of attenuation in dB per dB per °C over the
operating temperature range.
• Example:
• If temperature sensitivity is 5.10--4 dB/dB/°C,
• For a 20 dB attenuator over a 20* 20* 5.10--4 = 0.2 dB Attenuation
Variation.
34. • " FREQUENCY RANGE
• The Freq range indicated for each device is the range which RADIALL
specifies the device performance.
• " AVERAGE POWER HANDLING
• It is the maximum Continues Wave input power applied for a long time at
room temperature, or at the maximum temperature of 75°C.
• That the attenuator can handle whithout permanently changing the specifications
of the component.
• Any overpowering beyond this limit can significantly alter the input power
handling of the attenuator.
35. • " PEAK POWER HANDLING
• It Is The Maximum Peak Power Which, When Applied At Maximum
Room Temperature Under A Pulse Of One Microsecond Every
Millisecond,
• Will Not Permanently Change The Specifications Of The Attenuator.
• Any Overpowering Beyond This Limit Will Alter The Input Power
Handling Of The Attenuator.
36. •Coaxial Phase Shifters
• Phase Shifters Are Used To Change The Transmission Phase Angle (Phase Of S21) Of A Network.
• Ideal Phase Shifters Provide Low Insertion Loss, & Equal Amplitude (Or Loss) In All Phase States.
• While The Loss Of A Phase Shifter Is Often Overcome Using An Amplifier Stage, The Less Loss,
The Less Power That Is Needed To Overcome It.
• Most Phase Shifters Are Reciprocal Networks,
• Meaning That They Work Effectively On Signals Passing In Either Direction.
• Phase Shifters Can Be Controlled Electrically, Magnetically Or Mechanically.
• Can Be Analogue / Digital
38. • Coaxial Baluns
• A Balun Is An Electrical Device That Converts Between A Balanced Signal (Two
Signals Working Against Each Other Where Ground Is Irrelevant) & An
Unbalanced Signal (A Single Signal Working Against Ground Or Pseudo-ground).
• A Balun Can Take Many Forms & May Include Devices That Also Transform
Impedances But Need Not Do So.
• Transformer Baluns Can Also Be Used To Connect Lines Of Differing Impedance.
• The Origin Of The Word Balun Is “Balanced To Unbalanced”.
![• When using coaxial cables it is usually important to avoid reflections at the
end of the cable.
• If a cable is terminated with an impedance Z ,
• The ratio of the incoming voltage Vi to the reflected voltage Vr is given by:
Vr/Vi = [Z - Zo]/[Z + Zo].
• The total voltage across the terminating resistance is V = Vi + Vr.
• Three cases are of interest:
1. If Z = 0: Then Vi = - Vr and V = 0, as you would expect for a short
circuit.
2. If Z = 8: Then Vi = + Vr and V = 2 Vi.
3. If Z = Ro: Then Vi = 0 and V = Vi.](https://image.slidesharecdn.com/lectureno-250417153103-bab875f4/85/LECTURE-No-9-pptEngineeringEngineeringEngineeringEngineering-21-320.jpg)
