Dual-spiral slot antennas

Two types of dual-spiral slot antennas characterised by their simple feed are studied; one is a bidirectional radiation type (Bi-DSS) and the other is a unidirectional type (Uni-DSS). The radiation characteristics of the Bi-DSS are evaluated using the numerically determined magnetic current distribution. It is found that a travelling wave magnetic current distribution becomes dominant for a particular range of slot-arm lengths, contributing to the radiation of a circularly polarised wave. On the basis of the investigation of the Bi-DSS radiation mechanism, the Uni-DSS antenna with a triplate configuration is designed. Formation of a circularly polarised, unidirectional beam is experimentally realised. A 4*8 Uni-DSS array antenna with a simple feeding system is constructed, achieving an aperture efficiency of 60% over a frequency range of 11.6 GHz to 12.0 GHz.