Compact Dual Circularly Polarized Primary Feeds for Symmetric Parabolic Reflector Antennas

Four-arm Archimedean spiral antennas, capable of generating both senses of circular polarization with broadside radiation patterns, are investigated as a primary feed in symmetrical parabolic reflector antennas with circular rims. Unidirectional, center-fed, and compact spiral antennas are studied, backed by a planar reflector as a ground plane. Both right- and left-handed circularly polarized fields with broadside radiation patterns, according to the first and third modes of the spiral, are realized by confining the radiation zone to less than a three-wavelength circumference. The proposed feed exhibits excellent pattern symmetry and axial ratios well below 0.5 dB at the boresight direction for both senses of polarization. In this letter, the performance of the proposed spiral antenna is fully addressed as primary feeds in symmetrical parabolic reflector antennas. Different antenna parameters such as gain, cross polarization, and efficiency are studied for paraboloids, having different focal-length-to-diameter ratios, for both senses of circular polarization. Efficiencies of 70% and higher are obtained over a frequency band of 2.5-5.0 GHz.