Skin Depth at mmWave Frequencies
Conductor skin depth is a measure of the depth to which current will penetrate from the surface of a conductor, as a function of frequency. For the frequencies of interest here (up to 1THz), the skin depth, δ is well approximated by
where ρ is the conductivity of the conductor material; μ. the permeability of the conductor material; and f, the operational frequency.
See Figure 1 below for a plot of the skin depth of copper for microwave and mmWave frequencies. Note that at 100GHz, the skin depth is of the order of 200nm. At these dimensions, typical surface roughness of copper can have a significant impact on losses.
Figure 1. Skin depth of copper for microwave and mmWave frequencies