In a microwave waveguide system, what is the primary function of a mode filter? (Assume operation intended in the dominant mode only.)

Introduction:
Waveguides can support multiple modes if the operating frequency exceeds their cutoff frequencies. A mode filter is used to ensure only the desired (usually dominant) mode propagates. This question checks understanding of cutoff and mode control.

Given Data / Assumptions:

• Rectangular/circular waveguide intended for dominant mode (e.g., TE10 in rectangular).
• Operating frequency may be near or above some higher-order mode cutoffs.
• Goal: suppress unwanted higher-order modes.

Concept / Approach:

Each higher-order mode has a higher cutoff frequency than the dominant mode. If the operating band overlaps these cutoffs, unintended modes can appear, distorting fields, patterns, and impedance. Mode filters use geometry or loading to strongly attenuate these modes while minimally affecting the dominant mode.

Step-by-Step Solution:

Verification / Alternative check:

Examine measured S21 across modes using a mode launcher and VNA; dominant mode shows minimal attenuation while spurious modes show strong suppression.

Why Other Options Are Wrong:

• Lower cutoff modes (dominant) should not be suppressed.
• Changing mode intentionally is a mode converter’s job, not a filter’s.
• Nothing increases wave velocity beyond c; phase velocity > c is not superluminal energy transport.

Common Pitfalls:

Confusing mode filters with mode converters, or assuming absorptive loads will not affect the dominant mode—improper design can degrade desired performance.

Final Answer:

to suppress higher-order modes with higher cutoff frequencies