Excessive Aperture Efficiency of Compact Antennas Demonstrated by Waveguides and Small Horns

In modeling antennas using the aperture theory the peak gain occurs when the aperture amplitude and phase distributions are uniform, where the aperture efficiency becomes 100%. This is true when one uses the effective aperture of the antenna. In compact antennas the edge diffractions and current on outside walls of the antenna extend the aperture field well beyond the physical aperture of the antenna, and the effective aperture becomes much larger than the physical one. Thus, in using the physical aperture to calculate the aperture efficiency of the antenna, it exceeds 100%. This is demonstrated here by studying the aperture efficiencies of open-ended waveguides and small horns. Their field penetrations beyond their physical aperture are also investigated.