Circular waveguide attenuation versus frequency: in which mode does attenuation monotonically decrease as frequency increases over the practical band?

Introduction:
Different circular waveguide modes exhibit different wall-loss behaviors with frequency. Some modes, notably TE01, are prized for extremely low attenuation that continues to drop as frequency rises, which is why they are used for very low-loss long-distance links.

Given Data / Assumptions:

• Air-filled circular waveguide operating above cutoff.
• Conductor losses dominate; dielectric loss in air is negligible.
• Comparison among common TE and TM modes.

Concept / Approach:

The surface current distribution determines wall loss. The TE01 mode features circumferential currents with minimal current crowding at joints, yielding exceptionally low conductor loss that improves (attenuation decreases) with increasing frequency over a wide region above cutoff.

Step-by-Step Solution:

Verification / Alternative check:

Classical charts of circular waveguide attenuation show TE01 with the lowest αc (conductor attenuation), decreasing with frequency, which motivated TE01 long-haul waveguide systems.

Why Other Options Are Wrong:

• TE11, TE21, TE05, TM01: do not offer the same favorable monotonic decrease; they have higher losses or different dependence.

Common Pitfalls:

Confusing dominant mode (TE11) with best loss; dominance concerns cutoff, not attenuation.

Final Answer:

TE01