Coaxial microwave line – Practical single-mode operating range For a coaxial transmission line (supporting TEM as the fundamental mode), at what frequency range should normal operation be restricted to ensure single-mode propagation and avoid higher-order mode excitation (e.g., TE11)?

Introduction / Context:
Coaxial lines support a TEM fundamental mode that has no cutoff, but they also support higher-order modes (like TE11) above specific cutoff frequencies. For low-distortion, predictable measurements and links, designers must keep operation in the single-mode region.

Given Data / Assumptions:

• Standard coaxial geometry with inner and outer conductors.
• Desire is to avoid multimode propagation.
• TEM mode is desired; TE11 is the first higher-order mode with a finite cutoff.

Concept / Approach:

Although the TEM mode has no cutoff, once the frequency approaches the TE11 cutoff, fields can couple to TE11. Above that cutoff, the line can carry multiple modes, causing dispersion, mismatch errors, and unpredictable coupling. Therefore, the usable range is constrained to frequencies below the TE11 cutoff (often with margin), ensuring only the TEM mode propagates.

Step-by-Step Solution:

Verification / Alternative check:

Vendor datasheets specify a maximum single-mode frequency for each connector/line size, always below the TE11 cutoff, validating the rule of thumb.

Why Other Options Are Wrong:

Operating above or at cutoff risks TE11 excitation; “much higher than cutoff” ensures multimode propagation; “much lower” (option C) is overly restrictive wording not required by the question.

Common Pitfalls:

Confusing “no cutoff” of TEM with “unlimited bandwidth.” In practice, higher modes and connectors limit usable bandwidth.

Final Answer:

below the cutoff frequency of the TE11 mode.