Mode-Based Beamforming with Closely Spaced Antennas

Beamforming with closely coupled antennas are investigated based on the orthogonal radiation modes of the coupled antenna system. Through a multiport decoupling network, these modes can be separated into different independent driving ports and matched individually; hence, although the strong mutual coupling still exists between the antennas, the mutual coupling between the modes are destructively canceled. The close antenna spacing impacts on the radiation impedances and the radiation Q factors of the higher order modes rather than the existence of the modes themselves. This finding overcomes the degradation caused by strong mutual coupling and leads to practical supergain arrays for beamforming. A pair of closely coupled monopole antennas is built as an example to validate this concept. Simulation and experiment have shown, with a pair of quarter-wave monopoles at 0.1 lambda spacing, 4~5 dB gain increase of a single quarter-wave monopole can be achieved, which is very close to the ideal theoretical directivity.