Curved microstrip lines as compact wideband circularly polarised antennas

A new technique for the inherent generation of circular polarisation from compact microstrip antennas is described, based on radiation from curved microstrip transmission lines supporting a single travelling wave. The radiation is analysed in terms of effective magnetic current sources to give an estimate of the decay coefficient of the wave, which is found to be in reasonable agreement with experiment. Good characteristics are obtained from both circular and spiral antennas, with a useable bandwidth of up to 40% being available at 10 GHz for an average efficiency of 50%, owing to power absorbed in the line termination. The advantages obtained over conventional microstrip circular-polarised antennas are the avoidance of band-width limitations owing to resonant matching and the elimination of feed networks to produce circular polarisation.