At microwave frequencies, how does the skin depth of good conductors compare with the typical physical dimensions of most feed structures and conductors?

Introduction / Context:
Skin depth δ determines how deeply RF currents penetrate into conductors. At microwave frequencies, δ becomes very small, concentrating current near the surface and impacting conductor loss, plating choices, and fabrication.

Given Data / Assumptions:

• Good conductors (e.g., copper, silver) at GHz frequencies.
• Feed structures include coaxial inner/outer conductors, microstrip, waveguide flanges.
• δ ∝ 1/√f for a given material.

Concept / Approach:

Because δ decreases with √f, at microwave frequencies δ is typically a few micrometers or less—far smaller than mechanical dimensions (mil-scale or mm-scale). Hence current flows in a thin surface layer; thick plating beyond several skin depths yields diminishing returns for RF resistance.

Step-by-Step Solution:

Verification / Alternative check:

Measured insertion loss of lines strongly correlates with surface finish at GHz even when metal is thick, confirming δ ≪ dimension scale.

Why Other Options Are Wrong:

• Same order / much larger: contradicts δ ∝ 1/√f at microwave.
• Polarization-dependent only: δ is material/frequency dependent.
• Independent of frequency: incorrect.

Common Pitfalls:

Over-plating well beyond several δ; ignoring roughness (Huray/hammer effect) which raises effective resistance.

Final Answer:

Much smaller than feed dimensions