Practical Superdirectivity With Resonant Screened Apertures Motivated by a Poynting Streamlines Analysis

Abstract

Fundamental limits on the degree of practical achievable superdirectivity for antennas include narrow bandwidth, large antenna loss, and sensitivity to excitation and fabrication errors. Superdirective antennas are commonly considered as transmitters, but thinking of the antenna as a receiver may help to understand and overcome some of these limits on practical superdirectivity. We directly model a superdirective antenna in receive mode by calculating streamlines of Poynting vector field near the receiving antennas. Superdirectivity is achieved by expanding the shape of the effective area beyond the antenna physical aperture area. The degree of superdirectivity is parameterized by an effective area expansion distance. The theory predicts that superdirectivity is practical for electrically small and middle size antennas and electrically large antennas with a large aspect ratio. With this motivation, a $1.5\lambda \times 2\lambda $ superdirective horn antenna with resonant screen structure in front of the antenna aperture is designed and fabricated. The superdirective horn achieves a measured antenna efficiency of 115% over a reasonable bandwidth and with moderate sensitivity to fabrication errors.