Which of the following statements is true regarding the fundamental mode of the metallic waveguides shown? P: Coaxial, Q: Cylindrical, R: Rectangular

Only P has no cutoff frequency
Only Q has no cutoff frequency
Only R has no cutoff frequency
All three have cutoff frequencies

Correct answer: Only P has no cutoff frequency

Solution

A coaxial line supports a TEM fundamental mode, and TEM waves do not have a cutoff frequency. Cylindrical and rectangular hollow metallic waveguides support TE/TM modes only, which always have cutoff frequencies. Hence only the coaxial waveguide has no cutoff frequency.

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