The Skin Effect: Why High Frequency Current Stays on Top
How RF frequencies change the rules of electrical resistance.
In DC circuits, electricity flows through the entire cross-section of a wire. But as frequency increases, a strange phenomenon occurs: the current is pushed toward the surface of the conductor. This is known as the skin effect, and it is a fundamental constraint in the design of everything from coaxial cables to high-power transmitters.
What Causes the Skin Effect?
The skin effect is caused by eddy currents induced by the changing magnetic field of the AC signal. These eddy currents oppose the flow of the main current in the center of the wire but reinforce it near the surface. As frequency goes up, the magnetic field changes faster, pushing the current further and further toward the 'skin' of the metal.
Defining Skin Depth (δ)
Skin depth is the distance from the surface where the current density has dropped to about 37% (1/e) of its surface value. At 60 Hz (power lines), the skin depth in copper is about 8.5 mm. At 1 GHz (mobile phones), it is less than 2 micrometers (0.002 mm). This means at RF, the center of a thick copper wire is completely useless for carrying current.
AC Resistance vs. DC Resistance
Because the current is restricted to a small area, the 'effective' resistance of the wire increases dramatically with frequency. A wire that has almost zero resistance at DC may have high loss at GHz frequencies. This is why RF engineers use waveguides (hollow pipes) or silver-plated conductors to minimize these losses in high-frequency systems.
The Power of Plating
Since RF current only travels in the top few micrometers, we don't need the whole wire to be made of expensive silver. Engineers often use copper or even steel wires and plate them with a thin layer of silver or gold. This provides the low resistance of a precious metal at a fraction of the cost. Our Skin Depth calculator helps you determine the required plating thickness for your design.
FAQ
Does skin effect happen in PCB traces?
Yes, absolutely. Skin effect increases the loss in PCB traces as signal speeds increase. This is why high-speed digital and RF boards use smooth copper foils and specialized dielectrics to minimize both conductor and dielectric losses.
Is Litz wire useful for RF?
Litz wire consists of many thin, insulated strands woven together to increase surface area. It is very effective for frequencies up to about 1 MHz (like in switching power supplies), but at RF/Microwave frequencies, it actually performs worse than a solid conductor due to parasitic capacitance.
Why use gold plating if silver is more conductive?
Silver is the best conductor, but it tarnishes (oxidizes) over time, which increases surface resistance. Gold is slightly less conductive than silver but is chemically inert, meaning it won't tarnish, providing stable performance over many years.