In order to reduce the skin effect at UHF

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Option A: Copper tubes with silver plating are used

Explanation & Concepts:
At Ultra High Frequencies (UHF), the skin effect causes current to concentrate on the extreme outer surface of a conductor, significantly increasing effective resistance. Using silver-plated copper tubes maximizes the high-conductivity surface area while minimizing material usage, as the interior of the conductor carries negligible current.

Working Principle: The skin depth $\delta$ is inversely proportional to the square root of frequency $f$. As $f$ increases, $\delta = \sqrt{\frac{1}{\pi f \mu \sigma}}$ decreases, pushing current to the surface. By plating the surface with silver (a superior conductor) and using a hollow tube, we reduce the AC resistance and material cost simultaneously.

Key Formulas: $\delta = \frac{1}{\sqrt{\pi f \mu \sigma}}$ — Skin depth equation defining penetration depth; $R_{AC} = \frac{l}{\sigma A_{eff}}$ — Effective AC resistance equation

Key Points: Silver has higher electrical conductivity than copper, reducing surface resistance.; Hollow tubes are used because the center of the conductor carries almost zero current at UHF.; Skin depth decreases rapidly as frequency increases.; UHF range is typically 300 MHz to 3 GHz.

Advantages: Reduced ohmic losses; Lower mass and material cost compared to solid conductors; Improved signal transmission efficiency

Disadvantages: Higher manufacturing complexity for silver plating; Corrosion of silver plating over time (tarnishing)

Applications: Waveguides; UHF resonant cavities; High-frequency transmission lines

Additional Info: The skin effect is defined by the depth at which the current density drops to $1/e$ (approx 37%) of the surface value.. Option B is incorrect because a solid rod does not provide the weight and cost efficiency of a tube, and the center is underutilized.

Answer: A is correct — Copper tubes with silver plating provide an optimal low-resistance surface for high-frequency current flow while reducing unnecessary conductive material.

Exam Tip: Always remember that at high frequencies, the interior of a conductor is essentially 'dead weight'; focus on surface properties and skin depth calculations.

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Option B: Copper roads with silver plating are used

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Option C: Anodized conductors are used

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Option D: Painted conductors are used

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