Enhanced dual-circular polarised four-arm Archimedean spiral antenna with low-profile cavity backing

A wideband dual-circular unidirectional four-arm Archimedean spiral antenna operating from 0.6 to 2.4 GHz is presented. Without a cavity, the antenna radiates bi-directionally and the dual-circular polarisation of the spiral antenna at boresight is limited to a frequency range from 0.6 to 1.5 GHz. After adding a low-profile, 15 mm deep metallic cavity, the dual-circular polarisation at boresight is extended to more than 2.4 GHz and uni-directional radiation is realised. The four-arm spiral antenna is excited through a feed network consisting of a wideband four-way power divider and four wideband digital 2-bit phase shifters. This feed network facilitates instantaneous switching between right hand circular polarisation and left hand circular polarisation. The measured input return loss of the system is more than 10 dB for the entire two-octave bandwidth. The measured boresight gain from 0.6 to 2.4 GHz is -2.8 to 11.2 dBi and -3.1 to 8.7 dBi for RHCP and LHCP, respectively.