Super-Directive Antenna Arrays: Fundamentals and New Perspectives

Abstract

Wireless power transfer between an array of antennas and a single-antenna terminal is governed by matrix circuit theory. A complex-valued, non-conjugate symmetric impedance matrix constitutes a complete quantitative description of the system. The real-part of the impedance matrix is non-negative definite. The efficiency with which power can be transferred-either down-link (from the antenna array to the single-antenna terminal) or up-link (from the terminal to the array) is equal to the ratio of received power to transmitted power. The maximization of power transfer efficiency with respect to the joint transmit/receive activities yields the fact that the optimized up-link efficiency is equal to the optimized down-link efficiency. The typical operation of antenna arrays seeks to minimize mutual coupling among the constituent antennas by spacing the antennas at least half of a wave-length apart, which yields an array gain proportional to the number of antennas. By utilizing closer spacing, and deliberately creating strong mutual coupling, in principle it is possible to realize considerably higher array gains for the same number of antennas, a phenomenon called super-directivity. Practical super-directivity would benefit not only wireless communications, but also wireless power transfer.