Response of a Twisted Pair Excited by the Nonuniform Electromagnetic Fields of Nearby Loops

This paper derives equations for the noise induced in a twisted pair when the pair is embedded in a nonuniform electromagnetic field of a small current loop. The derived equations are applied to calculate the noise induced in a typical 135-Ω twisted pair. In the frequency range of 10 kHz-100 MHz, the noise induced in this twisted pair is shown to be less by about 70-135 dB when compared to a similar parallel-conductor line without twist that is embedded in the same field. Included is a simplified method of applying the derived equations to the problem of finding the response of the twisted pair to distinct uncorrelated noise sources, assuming that the sources can be approximated by small current loops.