Miniature dielectric-loaded personal telephone antennas with low SAR

Abstract

The twisted loop or bifilar helix antenna is designed to minimise the specific absorption rate (SAR). It is basically a loop antenna which is balance fed at the top by a co-axial feed which passes axially through the centre. The lower portion of the antenna is a sleeve balun which creates a transition between single-ended and balanced drive so that the loop antenna can be fed with balanced currents. Currents at the centre conductor feed point of the antenna pass round a 360/spl deg/ path including the rim of the sleeve balun and then back to the outer conductor of the co-axial cable feed.