Two double-ring cavity antennas in 19--22 dBi directivity range

Two new directional cavity antennas are proposed. Both antennas use two concentric rings in place of a single disc reflector of a short-backfire antenna (SBFA) and achieve a directivity in the 19 to 22 dBi range. These figures fill the gap between the SBFA and small parabolic-reflector antennas. The small antenna reaches a peak directivity of 19.3 dBi at an aperture efficiency of 77.5 , while the large antenna reaches a peak directivity of 21.1 dBi at an aperture efficiency of 53 . Measured 3 dB beam widths of the small antenna are 12.8 (E-plane) and 11.6 (H-plane) and 11.7 (E-plane) and 9.3 (H-plane) for the large antenna. The mechanical design of both antennas is simple and polarisation-independent.