EMC Question of the Week: May 11, 2020
A coaxial cable has a 2-cm inner conductor diameter and a 7-cm outer conductor diameter. If the dielectric between the conductors is air, what is the ratio of the E-field magnitude to H-field magnitude for a TEM wave propagating in the cable?
- 50 Ω/m
- 50 Ω
- 75 Ω
- 377 Ω
Answer
The best answer is "d". Plane waves propagating in air or free space have a wave impedance approximately equal to 377 Ω. The propagation velocity and wave impedance of TEM waves in transmission lines is determined by the dielectric, and is independent of the conductor configuration.
The characteristic impedance of the coaxial cable described in this problem is 75 Ω. The characteristic impedance describes the ratio of the voltage to the current in a two-conductor transmission line. Characteristic impedance and wave impedance are very different concepts.
Note: Antennas don't match transmission line impedances to wave impedances. If they did, a 377 Ω transmission line would radiate all of its power without requiring an antenna. Describing an antenna as an impedance transformer is similar to telling a child that babies are delivered by storks. Both are explanations that may satisfy a certain audience, but neither is consistent with the actual physics.
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