In microwave networks, a uniform waveguide section behaves like which type of filter around its cutoff?

Introduction / Context:
Metallic waveguides support propagation only above their cutoff frequency. Below cutoff, fields are evanescent and power does not propagate. This frequency-selective behavior mirrors a classic filter response in RF design.

Given Data / Assumptions:

• Uniform rectangular or circular waveguide section.
• Single-mode region around the dominant mode cutoff.
• Losses assumed small compared with cutoff effects.

Concept / Approach:

For a given mode, the propagation constant β is real for f > fc and imaginary for f < fc. Thus, the section 'passes' energy above fc and 'blocks' it below fc, which is the hallmark of a high-pass filter behavior.

Step-by-Step Solution:

Verification / Alternative check:

Plotting S21 versus frequency for an ideal waveguide shows near-zero transmission below fc and increasing transmission above fc, matching a high-pass characteristic.

Why Other Options Are Wrong:

• Low-pass: Opposite of waveguide behavior.
• Band-pass / Band-stop: Would require two edges; a simple uniform guide exhibits a single cutoff edge.
• All-pass: Not true; magnitude and phase vary strongly around cutoff.

Common Pitfalls:

Confusing cavity resonators (band-pass) with uniform guides; overlooking that mode conversion or discontinuities can create additional filtering not present in a straight section.

Final Answer:

High-pass filter