Directive properties of parasitic helix and its application to circularly polarised antenna

By using an integral-equation method, a 6-turn parasitic helix with a tapered section is analysed to reveal its operation as a director for a circularly polarised wave. It is demonstrated that a parasitic helix, illuminated by a circularly polarised plane wave, maintains directive properties over a wide range of frequencies from 1.0–1.7 λ. On the basis of this result, an antenna system composed of a rectangular spiral antenna and a parasitic helix is discussed. It is found, both numerically and experimentally, that the gain of the antenna system is increased above that of the isolated spiral, while the inherent input impedance of the spiral is maintained. The axial ratio of the antenna system is better than that of the spiral.