4.2 - Department of Electrical and Computer Engineering...

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Department of Electrical and Computer Engineering ECSE 352 Electromagnetic Waves and Optics 2 Basic waveguide operation 4.2 Basic waveguide operation References: Section 14.2, 14.3 @AGK 4.2-1 Leaning outcomes After taking this class you should be able to: • Define what is meant by a TE or TM mode for a waveguide efine the cut ff frequency Define the cut-off frequency • Explain why waveguide modes have a cut-off frequency • Calculate phase and group velocity in conducting waveguides ECSE 352 4.2-2 Parallel plate transmission line • TEM wave on a parallel plate line: • E and H are both orthogonal to the direction of travel g •T h e y a r e b o t h transverse y d + + + + + -- - - - ++ + + + + y E H + + - - - - -- - - - - ρ s ε,μ d E y z H x ECSE 352 4.2-3 z 0 J s x w Disadvantages of transmission lines • Transmission lines work well at moderate frequencies (MHz-GHz depending on distance) Loss increases with frequency, due to surface Rs 2 π f μ Lf q y , f resistance. E.g. for a parallel plate TL: R = 2 w = w c σ c Ω /m ( ) nce wavelength onductor spacing general Once wavelength conductor spacing general waveguiding can occur his results in lower loss but introduces other ECSE 352 4.2-4 This results in lower loss, but introduces other factors
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Waveguiding • Waves are confined via: dts ( t l l i i d s ) l id t i s – Conductors (metallic waveguides): applied to microwaves – Dielectrics (glass or plastic): applied to optical waves • Waves travel in a variety of modes, not just TEM Rectangular λ /2 λ λ waveguide 1. 2. 3. Parallel plate waveguide ECSE 352 4.2-5 Modes Dielectric waveguide From transmission line to waveguide EM wave between conductors = TEM wave between conductors Transmission line k ω με = Non-TEM wave between conductors = waveguide k = u k d k ud kk ωμε == u k d k E H u k d k E H ECSE 352 4.2-6 Transverse electric (TE) Transverse magnetic (TM) TM and TE modes • In a waveguide, E and H may no longer be purely transverse They may have a field component ( E z or H z ) in the direction of propagation ( z ) y y E y E x z x z E y H y H x z H x z E x z E z H z ECSE 352 4.2-7 TEM Transverse Magnetic (TM) Transverse Electric (TE) Slab waveguide modes 1.5 2 TM 2 mode for a slab waveguide 0.5 1 g 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 1 E H TEM mode ECSE 352 4.2-8 What is different ? 0
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4.2 - Department of Electrical and Computer Engineering...

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