Superdirective Magnetic Dipole Array as a First-Order Probe for Spherical Near-Field Antenna Measurements

The theory as well as numerical and experimental results are presented for a superdirective array composed of closely spaced electrically small resonant magnetic dipole elements. The array operates on a metal ground plane and can exhibit a maximum directivity of 11.5 dBi, 15.2 dBi, and 17.8 dBi (including 3 dB due to the ground plane), for 2, 3, and 4 magnetic dipoles, respectively. The array is self-resonant and is directly excited by a 50-ohm coaxial cable through the ground plane. The array radiates essentially the vertical bar mu vertical bar = 1 spherical modes, which, despite a narrow bandwidth, makes it an excellent first-order probe for spherical near-field antenna measurements at low frequencies.