ELCT564
Spring 2012
Chapter 3: Waveguides and
Transmission Lines
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Waveguides
Metal Waveguides
Dielectric Waveguides
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Comparison of Waveguides and Tlines
Transmission Line
Waveguide
Two or more conductors separated by some
insulating medium (two-wire, coaxial, microstrip,
etc.
Metal waveguides are typically one enclosed
conductor filled with an insulating medium while a
dielectric waveguide consists of multiple dielectrics
Normal operating mode is the TEM or quasi-TEM
mode (can support TE and TM modes but these
modes are typically undesirable.
Operating modes are TE or TM modes (can not
support a TEM mode)
No cutoff frequency for the TEM mode. Tline can
transmit signals from DC up to high frequency
Must operate the waveguide at a frequency above
the respective TE or TM mode cutoff frequency
for that mode to propogate
Significant signal attenuation at high frequencies
Lower signal attenuation at high frequencies
Small cross section line can transmit only low
power levels
Can transmit high power levels
Large cross section tlines can transmit high
power leves.
Large cross section waveguides are
impractical due to large size and high cost.
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General Solutions for TEM, TE and TM Waves
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General Solutions for TEM, TE and TM Waves
TEM Waves
TE Waves
TM Waves
Attenuation due to Dielectric Loss
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Parallel Plate Waveguide
TEM Waves
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Parallel Plate Waveguide
TM Waves
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Parallel Plate Waveguide
TE Waves
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Summary of Results for Parallel Plate Waveguide
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Rectangular Waveguide
TE Waves
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Rectangular Waveguide
TM Waves
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Summary of Results for Rectangular Waveguide
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Example I
Consider a length of Teflon-filled (εr=2.08, tanδ=0.0004) copper K-band rectangular waveguide, having
dimensions a=1.07 cm and b=0.43 cm. Find the cutoff frequencies of the first five propagating modes. If the
operating frequency is 15 GHz, find the attenuation due to dielectric and conductor losses.
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Example II
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Circular Waveguide
TE Waves
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Circular Waveguide
TM Waves
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Summary of Results for Cirular Waveguide
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Example I
Find the cutoff frequencies of the first two propagating modes of a Teflon-filled (εr=2.08, tanδ=0.0004) circular
waveguide with a=0.5cm. If the interior of the guide is gold plated, calculated the overall loss in dB for a 30cm
length operating at 14GHz.
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Example II
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Attenuation of Waveguides
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Coaxial Line
Higher Order Modes
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Coaxial Line: Example
Consider a piece of RG-401U coaxial cable, with inner and outer conductor diameter of 0.0645’’ and 0.215’’,
and a Teflon dielectric(εr=2.2). What is the highest usable frequency before the TE11 waveguide mode starts
to porpagate?
=563.4 m-1
=18.15GHz
Field lines for TEM mode of a
coaxial line
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Field lines for TE11 mode of a
coaxial line
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Coaxial Connectors
Connector Type
Other
names
Female
Male
Maximum Frequency
Phone plugs and
jacks
TS, TRS
100 kHz
RCA
Phono plugs
and jacks
10MHz
UHF
PL-259
300MHz
F
Video
250MHz to 1 GHz
BNC
2GHz
C
12 GHz
Type N
12GHz or more
SMA
12 GHz or more
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50GHz
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Strip Line
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Strip Line: Example
Find the width for a 50Ω copper stripline conductor, with b=0.32 cm and εr=2.2. If the dielectric loss tangent is
0.001 and the operating frequency is 10 GHz, calculate the attenuation in dB/λ. Assume a conductor
thickness of t=0.1mm.
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Microstrip Line
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MicroStrip Line: Example
Calculate the width and length of a 50Ω copper microstrip line, with a 90o phase shift at 2.5GHz. The
substrate thickness is d=0.127 cm, with εr=2.2.
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Wave Velocities and Dispersion
Dispersion: If the phase velocity is different for different frequencies, then the individual frequency
components will not maintain their original phase relationships as they propagate down the transmission line
or waveguide, and signal distortion will occur.
Group Velocity
Calculate the group velocity for a waveguide mode propagating in an air-filled guide. Compare this velocity to
the phase velocity and speed of light.
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Summary of Transmission Lines and Waveguides
Characteristic
Coax
Waveguide
Stripline
Microstrip
TEM
TE, TM
TE10
TM, TE
TEM
TM,TE
Quasi-TEM
Hybrid TM, TE
Dispersion
None
Medium
None
Low
Bandwidth
High
Low
High
High
Loss
Medium
Low
High
High
Power Capacity
Medium
High
Low
Low
Large
Large
Medium
Small
Medium
Medium
Easy
Easy
Hard
Hard
Fair
Easy
Modes: Preferred
Other
Physical Size
Ease of Fabrication
Integration with Others
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Other Lines and Guides
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