Design an Omnidirectional Circularly-Polarized Antenna for Azimuth Wide-Angle Scanning Array

To achieve an omnidirectional circularly-polarized antenna mountable on metal platforms such as vehicle and aircraft is challenging work. In this paper, an omnidirectional circularly-polarized antenna is proposed. Specifically, a thin and compact open cavity loaded with a capacitive impedance surface is used to provide the horizontally-polarized electrical-field component. While the verticallypolarized electrical-field component is achieved by a thin open cavity placed above the metal ground with a small gap. Connecting the two radiating cavities with a metal plate, the omnidirectional circularlypolarized antenna can be constructed using one simple feed. Outstanding azimuth gain variation of less than 1 dB and axial ratio less than 3 dB are obtained for the proposed antenna. Owing to the proposed antenna has omnidirectional characteristics, it can be applied to a wide-angle scanning array. The proposed array consists of 1 x 16 omnidirectional circularly-polarized antennas mounted on a metal planform with half-wavelength spacing. The antenna array beam scanning is divided into broadside and endfire modes, achieving wide-angle scanning in the azimuth plane. For validation, a 1 x 16 array prototype is fabricated and experimentally tested. The measured results are consistent with the simulated results. The measured results show that the antenna can achieve good circularly-polarized radiation, with most of the axial ratio less than 3 dB at the main beam direction of the array. With the advantages of omnidirectional circularlypolarized and wide-angle scanning, the proposed antenna can be used in communications between high altitude and ground.