On the benefits (if any) of pair twisting in reducing radiated emissions from two-wire cables

An empirical investigation of the effect of cable twisting on the radiated emissions from cables is performed and presented. It is well known that cable twisting is beneficial for reducing the low frequency emissions from, and coupling to cables. However, many misconceptions have evolved with respect to the actual contribution of the cable twisting at high frequencies, in particular with respect to the electric-field and electromagnetic field emission from and coupling to cables. In fact, in certain text books, the “pair-balance” is attributed to twisted pairs, contributing a rejection of 20 dB and more. In the present paper, the actual effect of cable twisting on the reduction of radiated emissions due to common-mode and differential-mode fed signals is presented and discussed. Tests were conducted by the authors, in order to investigate the effect of pair twisting on the interference coupling to cables. Due to the duality between emission from- and coupling to cables, the results obtained by means of measurement of the field emission from the cables for common-mode and differential mode feed to the culprit cable can be applied for the estimation of the effect of twisting on the coupling to the cables of the system. It is clearly shown that for high frequency common mode emissions, absolutely no reduction in radiated emissions may be expected due to the pair-twisting. Results are also compared to the those obtained by the shielding of cables, with the shield grounded at one and both sides