Compact and High-Gain Magnetic Dipole Antennas Utilizing Superdirective Array Excitation Condition

In this communication, two compact and high-gain magnetic dipole antennas (MDAs) with and without a metallic ground are developed by utilizing an endfire superdirective array excitation condition. The two MDAs, which are referred to as MDAs #1 and #2 in this work, implant a simple and effective design methodology of exciting two directly connected zero-phase-shift-line (ZPSL) loops via a single port. The directly connected topology ensures an almost equal excitation amplitude. Different loop perimeters provide an essential excitation phase difference, thereby complying with the superdirective array excitation condition and achieving gain improvement. The two MDA prototypes are fabricated and measured. The measured results for the uniplanar MDA#1 without ground demonstrate 8.2% of -10-dB impedance fractional bandwidth (FBW), 6.57-dBi realized gain (RG) at 2.45 GHz, and 7.11-dBi peak directivity, which is close to the theoretically superdirective boundary of 7.20 dBi. The corresponding overall size is 0.44 x 0.23 x 0.01 lambda(3)(0), with an interelement distance of 0.13 lambda(0) . Retaining only half size according to the symmetry, MDA#2, the metal-backed version, has an approximately 3-dB gain improvement, radiating a vertically polarized tilted beam. Its measured peak RG, FBW, and RE are 9.34 dBi, 9.8%, and 91.9%, respectively, with a radiator footprint of 0.29 x 0.25 lambda(2)(0) and an interelement distance of 0.15 lambda(0) .