Circular waveguide at millimetre wavelengths — favoured low-loss propagation mode At millimetre wavelengths, which mode in a circular waveguide is typically preferred for long, low-loss transmission?

Introduction / Context:
In circular waveguides, different modes exhibit different attenuation, power handling, and field symmetry. At millimetre wavelengths, long low-loss links and high-power transport often prioritize a mode with particularly low wall-current losses and minimal mode conversion. This question asks which mode is favoured for such purposes.

Given Data / Assumptions:

• Metal circular waveguide, smooth inner wall.
• Millimetre-wave operation (approximately 30–300 GHz).
• Interest in low attenuation over distance, not just lowest cutoff.

Concept / Approach:
While TE11 is the dominant (lowest-cutoff) mode in circular waveguide, the TE01 mode is often preferred for long-haul millimetre-wave power transmission because it has very low conductor loss due to circumferential surface current distribution and no longitudinal electric field at the wall, reducing ohmic loss and breakdown risk. TE01 also offers improved power-handling and reduced susceptibility to bends, provided suitable mode converters are used at interfaces.

Step-by-Step Solution:

Verification / Alternative check:
Millimetre-wave literature and handbooks document TE01 lines for low-loss transmission and high-power gyro-device outputs, confirming its favored status for these specific applications.

Why Other Options Are Wrong:

• TE11: dominant, but not lowest loss for long transport.
• TE20, TE21: higher-order modes with higher cutoffs and typically higher attenuation.

Common Pitfalls:
Equating “dominant” with “preferred for long-haul.” The engineering preference may differ when loss and power handling dominate the design criteria.

Final Answer:
TE01