What increases the resistance of wire at high frequencies?

• The resistance does depend on the frequency. The reason is the 'skin effect’.
• When an alternating current is passed through a small portion of the conductor, the skin depth carries the current.
• The value of skin depth is inversely proportional to frequency. As the frequency is increased, the skin depth decreases.
• But the value of ac resistance is directly proportional to frequency, or in other words, inversely proportional to skin depth.

Thus, at higher frequencies, ac resistance is higher.

Important Notes:

• The skin depth is a measure of the current density and is defined as the distances from the outer edges of a conductor to the point at which the current density falls to 1/e of the value at the surface.
• The non-uniform distribution of electric current over the surface or skin of the conductor carrying a.c is called the skin effect.
•  The concentration of charge is more near the surface as compared to the core of the conductor.
• The ohmic resistance of the conductor is increased due to the concentration of current on the surface of the conductor.