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Chapter 3: Waveguides and Transmission Lines

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1 Chapter 3: Waveguides and Transmission Lines
ELCT564 Spring 2012 Chapter 3: Waveguides and Transmission Lines 4/16/2017 ELCT564

2 Waveguides Metal Waveguides Dielectric Waveguides 4/16/2017 ELCT564

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. 4/16/2017 ELCT564

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 4/16/2017 ELCT564

6 Parallel Plate Waveguide
TEM Waves 4/16/2017 ELCT564

7 Parallel Plate Waveguide
TM Waves 4/16/2017 ELCT564

8 Parallel Plate Waveguide
TE Waves 4/16/2017 ELCT564

9 Summary of Results for Parallel Plate Waveguide
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10 Rectangular Waveguide
TE Waves 4/16/2017 ELCT564

11 Rectangular Waveguide
TM Waves 4/16/2017 ELCT564

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. 4/16/2017 ELCT564

14 Example II 4/16/2017 ELCT564

15 Circular Waveguide TE Waves 4/16/2017 ELCT564

16 Circular Waveguide TM Waves 4/16/2017 ELCT564

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. 4/16/2017 ELCT564

19 Example II 4/16/2017 ELCT564

20 Attenuation of Waveguides
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21 Coaxial Line Higher Order Modes 4/16/2017 ELCT564

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 4/16/2017 ELCT564

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 4/16/2017 ELCT564

24 Strip Line 4/16/2017 ELCT564

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. 4/16/2017 ELCT564

26 Microstrip Line 4/16/2017 ELCT564

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. 4/16/2017 ELCT564

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. 4/16/2017 ELCT564

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 4/16/2017 ELCT564

30 Other Lines and Guides 4/16/2017 ELCT564

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