Active Antennas

Abstract

An active antenna is an antenna system in which the antenna is an integral part of the circuit supplying the signal to (driving) the antenna, in transmit mode, and/or the circuit receiving the signal from the antenna is an integral part of the antenna, in the receiving mode. Such an antenna system can exhibit characteristics which are quite distinct from those exhibited by an ideal linear antenna which is electrically isolated from its feed, or read‐out, network. The first section of this article gives an historical perspective on developments in radio technology before giving a qualitative discussion of transmission line modes and antenna modes and how these modes need to relate to one and another in an active configuration. The second section of this article is quantitative and first introduces the Poynting vector in order to analyze impedance matching from an electromagnetic viewpoint before presenting a circuit theoretical discussion of oscillators and their impedance matching requirements. The third section of the article discusses some applications such as active impedance matching, parametric down conversion, power boosting, proximity detection, and active retroflection as well as quasi‐optical power combining (borrowing from array concepts introduced in the second section)