Multipole-Based Electrically Small Unidirectional Antenna With Exceptionally High Realized Gain

Electrically small, high-directivity antennas are in demand for a variety of current and future wireless applications. An electrically small directive antenna (ESDA) that requires only one specially engineered port to excite a set of multipoles is demonstrated in this article. Four 90° copper sectors are combined with additional structures and fed with a coaxial cable. Two resonant quadrupoles (equivalent to two pairs of resonant electric dipoles) and one magnetic dipole are excited. Both high radiation efficiency and good impedance matching are achieved. Theoretical calculations, numerical simulations, and experimental measurements are shown to be in good agreement. An optimized prototype is designed, fabricated, and tested. The measured results confirm that it is a supergain system. The unidirectional ESDA has a peak directivity of 6.71 dBi, a peak realized gain of 6.31 dBi, radiation efficiency of 94.5%, and a front-to-back ratio of 14.89 dB at its resonance frequency, 814 MHz. Its height is
$0.06 \lambda _{\mathrm{ res}}$
, and
$ka = 0.98$
. These measured realized gain and directivity values exceed both the Harrington and Kildal–Best
$ka$
-based upper limits.