Endfire Circularly Polarized Planar Antennas: A review of their development

This article reviews and summarizes the developments of endfire circularly polarized (CP) planar antennas. The difficulty in designing such antennas is that there is not a large radiating aperture on a substrate to produce strong enough vertically polarized (VP) radiation to match the horizontally polarized (HP) radiation. Mainly based on the structures of radiation and feeding, endfire CP planar antennas are classified into several groups. The mechanisms of these antennas are identified, their performance is compared with tables, and the relation between performance and structures is explored. It is indicated that the distance between the phase centers of VP and HP radiators is a crucial parameter to affect the axial ratio (AR) bandwidth and beamwidth of an endfire CP antenna. It is addressed that it is essential to have strong VP radiation for a high-gain and wideband endfire CP antenna when substrates are thin.