Understanding the Skin Effect in Transmission Lines

The skin effect is a phenomenon in which the AC current flowing through a conductor is concentrated near the surface of the conductor, and the deeper the current flows, the smaller the current density becomes. This is due to the magnetic field that is generated around the conductor as a result of the current flow. The magnetic field induces an opposing current in the conductor, which produces an opposing magnetic field that opposes the original magnetic field. This results in a greater resistance to the flow of current at the center of the conductor than at the surface.

The skin depth of a transmission line is defined as the the measurement of it's depth(surface area to center of the line) at which the amplitude of the signal has decayed/reduced to or 33% of the original signal amplitude at the surface. 

The skin effect has significant implications in the design of transmission lines and high-frequency circuits. At high frequencies, the skin effect can cause a significant increase in the effective resistance of a transmission line, which in turn leads to losses in the line. The skin depth, as defined above, is a measure of the extent to which the skin effect affects the transmission line. The skin depth is inversely proportional to the square root of the frequency, which means that as the frequency increases, the skin depth decreases, and the skin effect becomes more pronounced.

To mitigate the effects of the skin effect, transmission lines are often designed with a hollow core, which reduces the amount of material in the center of the line and increases the amount of material at the surface. This reduces the resistance to current flow at the center of the line and improves the overall performance of the line. Additionally, materials with high conductivity are used to reduce the overall resistance of the line and minimize the effects of the skin effect.