Hi, Resistance: I think you can look at the standard EM theory on plane wave incident into a metallic plane. You can solve the field inside the metallic plane. You will see the E and H field will be decaying exponentially when you are going into the metallic structure. Since J = sigma*E. It means that most of J is concentraing on the surface of the metallic plane. The concentration is more severe at higher frequency and it will cause more loss because the current is passing through an equivalently narrower cross-section. That is the so-called skin effect. As I mentioned, its implication is that there will be more loss at high frequency because the current is more concentrated. Regards.