The skin depth (penetration depth) of electromagnetic wave in a good conductor is inversely proportional to:

Explanation

Skin depth δ is the distance into a conductor at which electromagnetic wave current density drops to 1/e (approx 37%) of its surface value. It is calculated as δ = √(2 / (ω * μ * σ)) = 1 / √(π * f * μ * σ), showing inverse proportionality to √f.