Thursday, July 1, 2010

What is the “skin effect?” ...

Q What is the “skin effect?” Does it take place only at RF frequencies?

A The skin effect is a fascinating phenomenon, and it takes place at all signal frequencies. Basically, a conductor’s effective resistance is not the same for alternating current as it is for direct current. With alternating current, other effects tend to force the current to flow mostly in the outer parts of the conductor. This decreases the effective cross-sectional area of the conductor, with the result that resistance increases.

At low audio frequencies, the increase in resistance is insignificant, but at radio frequencies above about 1 MHz, the skin effect is so pronounced that practically all of the current flows within a few thousandths of an inch of copper conductor’s surface. Above 10 MHz, only about the outer 0.0008 inch of the surface is used, and above 100 MHz the depth is less than 0.0003 inch. The RF resistance is consequently many times the dc resistance, and increases with frequency. In the RF range a conductor of thin tubing will have just as low resistance as a solid conductor of the same diameter, because material not close to the surface carries practically no current.

From QST July 1999