Hollow metallic waveguides — practical frequency range of use Hollow waveguides are generally used as transmission lines at which frequencies?

Introduction / Context:
Hollow metallic waveguides (rectangular or circular) guide electromagnetic waves above a cutoff frequency determined by their cross-section. Compared to cables, they exhibit very low loss at microwave frequencies and high power-handling, but they are impractical at low frequencies due to size and cutoff constraints.

Given Data / Assumptions:

• Standard rectangular and circular metallic waveguides.
• Focus on typical engineering practice, not exotic oversized guides.

Concept / Approach:
The dominant cutoff (TE10 for rectangular, TE11 for circular) scales inversely with the guide dimension. To operate, frequency must exceed the dominant cutoff; at lower frequencies the guide does not support propagation. For commonly available sizes, this places practical operation in the microwave regime, typically above ~1 GHz and often several GHz. Below this, guides would need to be physically huge to pass energy, making coaxial lines preferable.

Step-by-Step Reasoning:

Verification / Alternative check:
Standard WR-series guides (e.g., WR-90, WR-62) cover bands starting a few GHz upward, reaffirming the microwave-band use case.

Why Other Options Are Wrong:

• B: “above 5 GHz” is too restrictive; waveguides are widely used from ~1–2 GHz upward.
• C/D: 10 MHz or 1 MHz are far below practical waveguide operation unless dimensions are impractically large.

Common Pitfalls:
Assuming a single fixed threshold; actual threshold depends on size, but sub-GHz usage requires extremely large guides.

Final Answer:
above 1 GHz