1. Chapter 3: Waveguides and Transmission Lines
ELCT564 Spring 2012
Chapter 3: Waveguides and Transmission Lines
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2. Waveguides
Metal Waveguides
Dielectric Waveguides
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3. 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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4. General Solutions for TEM, TE and TM Waves
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5. General Solutions for TEM, TE and TM Waves
TEM Waves
TE Waves
TM Waves
Attenuation due to Dielectric Loss
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6. Parallel Plate Waveguide
TEM Waves
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7. Parallel Plate Waveguide
TM Waves
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8. Parallel Plate Waveguide
TE Waves
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9. Summary of Results for Parallel Plate Waveguide
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10. Rectangular Waveguide
TE Waves
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11. Rectangular Waveguide
TM Waves
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12. Summary of Results for Rectangular Waveguide
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13. 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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14. Example II
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15. Circular Waveguide
TE Waves
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16. Circular Waveguide
TM Waves
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17. Summary of Results for Cirular Waveguide
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18. 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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19. Example II
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20. Attenuation of Waveguides
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21. Coaxial Line
Higher Order Modes
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22. Coaxial Line: Example =563.4 m-1 =18.15GHz
Consider a piece of RG-401U coaxial cable, with inner and outer conductor diameter of ’’ 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
Field lines for TE11 mode of a coaxial line
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23. 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
2.4mm
50GHz
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24. Strip Line
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25. 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 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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26. Microstrip Line
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27. 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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28. 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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29. Summary of Transmission Lines and Waveguides
Characteristic
Coax
Waveguide
Stripline
Microstrip
Modes: Preferred
Other
TEM
TE, TM
TE10
TM, TE
TM,TE
Quasi-TEM
Hybrid TM, TE
Dispersion
None
Medium
Low
Bandwidth
High
Loss
Power Capacity
Physical Size
Large
Small
Ease of Fabrication
Easy
Integration with Others
Hard
Fair
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30. Other Lines and Guides
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