Compact, Superdirective, Mixed-Multipole Antennas Enabled by a Custom-Designed Feed Structure

Two compact, low-profile, broadside-radiating unidirectional multipole antennas with exceptionally high gain performance are presented. Both consist of three parallel radiating elements. A custom-designed cross-connecting feed structure excites them with precisely controlled current distributions. A theoretical model of the first system, which consists of three electric dipoles, is analyzed to derive the necessary amplitude and phase of the currents required to excite each element to attain the optimal performance of the system. A single feed structure is then designed to achieve those excitations. The simulated results predicted a high realized gain (RG) of 7.86 dBi and a radiation efficiency of 99.3% even though the antenna occupies a compact size with $ka =1.67$ at 1.21 GHz. The fabricated prototype’s performance exhibited outstanding agreement with those simulated results. A RG of 7.72 dBi and an overall efficiency (OE) of 96.45% were measured. The second optimized system evolved from the first. It is electrically small, consisting of two modified bowtie elements and an Egyptian axe dipole (EAD). Its size is compact: $ka =0.99$ ( $0.31\times 0.21\times 0.09\,\,\lambda ^{3}$ at 750 MHz), and it exhibits a measured superdirectivity of 7.7 dB, RG of 6.82 dBi, front-to-back ratio (FTBR) of 7.94 dB, and OE of 82%.