Effects of Shape, Proximity and Ferrite Materials On Resistive Losses in Multi-Conductor Transmission Lines and Antennas

Abstract

The sensitivity of certain near field antennas and the power handling capability of high frequency transmission lines are limited by similar geometric considerations. In particular, unwanted spatial concentrations of the electromagnetic fields lead to increased ohmic losses, to dielectric losses and to dielectric breakdown. Three dominant mechanisms by which such current concentrations occur may be enumerated: (1) The shape effect, which is exemplified by the divergence of the electromagnetic field at sharp corners of conducting parts. (2) The proximity effect which relates to concentration and eventual divergence of the electromagnetic field between two metal parts at different potential. (3) The ferrite effect, which refers to the field concentrations that result from the presence of highly permeable materials. In the past, proximity effects in parallel cylindrical conductors and shape effects in isolated conductors have been studied. This paper shows that a different picture emerges when the effects of the proximity, shape and ferrite are combined, as can easily be the case in a practical antenna. In particular, these effects may be played off against each other in order to reduce undesirable field concentrations. The argument is illustrated using exact solutions available from conformal mapping and numerical solutions calculated by a boundary element technique.