Polarisation reconfigurable, centre-fed, and low-profile Archimedean spiral antennas with unidirectional broadside patterns

摘要

A low-profile, dual circularly polarised, and centre-fed four-arm Archimedean spiral antenna with broadside radiation patterns is investigated in this paper. It is shown that both right- and left-hand circular polarisations (CPs) are generated in the four-arm spiral antenna, whose circumference is less than three wavelengths, by properly exciting the spiral arms from their inner ends. Similar analogy can be used to extend the concept to any multi-arm spiral antenna. The proposed compact antenna, which is backed by a planar reflector surface, is designed and extensively investigated to study main antenna specifications including gain, cross polarisation, and pattern symmetry, for dual-polarised wireless communication systems. A 3 dB axial ratio frequency bandwidth of about 1.8-5.3 GHz is realised for both clockwise and counter-clockwise CP with axial beams. Furthermore, superior pattern symmetry with equal beamwidths is attained for both senses of polarisation. A prototype antenna is fabricated and tested. Excellent agreement between the measured and computed results is observed. An asymptotic equation is also developed to estimate the antenna peak gain over the frequency band for both senses of CPs.