EMC Question of the Week: May 1, 2023

Interface between two 125 -ohm transmission lines with different dielectrics

A signal propagates as a plane wave in a transmission line with a 125-Ω characteristic impedance. It reaches an interface with another transmission line that has the same characteristic impedance, but a different wave impedance. Is there a reflected plane wave?

  1. No
  2. Yes
  3. Impossible because Z0 = η
  4. Impossible because η = 377 Ω


The best answer is “a.” If there is no change in the characteristic impedance, the reflection coefficient is zero and there is no reflected wave.

Electric field lines in the vicinity of a transmission line interfaceThe question is interesting though, because it describes a situation where a plane wave encounters an interface between two different dielectrics. Plane wave shielding theory suggests that a reflection must occur.  The tangential electric and magnetic fields have to be continuous at the interface. 

This apparent paradox is resolved by realizing that the fields are distorted at the interface. This means the fields are no longer consistent with those of a plane wave at the boundary. The field distortion satisfies the boundary conditions without producing a reflected wave. The distorted field can be viewed as an evanescent (non-propagating) mode that quickly diminishes with distance from the interface.

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