Skin Depth & AC Resistance
Skin depth vs frequency, and how much it raises a conductor's resistance.
Where the current actually flows
At DC, current fills a conductor evenly. As frequency rises, the conductor's own magnetic field pushes current toward the surface — the skin effect — until it flows in a thin layer one skin depth thick:
For copper that's about 66 µm at 1 MHz and shrinks as 1/√f —
~6.6 µm at 100 MHz, ~2 µm at 1 GHz. Once the skin depth is thinner than the conductor, the
effective cross-section stops growing with size and the AC resistance climbs as
√f, even though the DC resistance hasn't changed.
This is why thick power traces don't help at RF, why hollow tubes carry RF as well as solid rods, and why losses in a controlled-impedance line rise with frequency. The onset frequency — where skin depth equals the conductor thickness — tells you when to start caring. The AC-resistance figures here are first-order estimates (current in a one-δ surface layer); real geometry and proximity effects add more.