Predicting the Magnetic Fields from a Twisted-Pair Cable

A theory that predicts the magnitude of low-frequency magnetic fields near a current-carrying twisted-pair cable is developed. By asymptotically expressing the theoretical results, it is shown that the magnetic fields from a twisted-pair cable of pitch distance p decrease exponentially with the radial distance from the center of the cable. The asymptotically expressed result is verified experimentally for a radial distance as large as (3/2)p. At such a distance, the maximum fields from the cable are shown to be 50 dB below that from a two-wire line (two parallel wires), even though both the cable and the wire line are carrying the same amount of current.