Suitable rectennas under reverse bias can be made to act as transmitters.

The term "inverse rectenna" may seem like an oxymoron at first glance. However, preliminary experiments have demonstrated that a rectenna of suitable design can be made to operate in an inverse mode, in which radio-frequency (RF) power is generated in the rectenna rectifier circuits and radiated by the rectenna antenna elements.

This experimental finding provides encouragement for the use of rectennas as bidirectional (both transmitting and receiving) devices in developmental microwave wireless-power-transmission systems. Heretofore, a bidirectional microwave terminal for a typical conceptual wireless-power-transmission system might have included (a) a transmitter comprising a transmitting antenna connected to a magnetron or klystron oscillator or perhaps an impact avalanche transit-time- (IMPATT)-diode oscillator, plus (b) a receiver comprising a separate rectenna. If only one device — a rectenna capable of operating in transmitting as well as receiving mode — could be used at each end of a microwave power link, then the cost of the link could be reduced. Potential applications for inverse rectennas lie in the microwave wireless transmission of power between any two of the following: ground stations, airships, aircraft, and spacecraft.

Image In the rectenna used in the experiments, the rectifier circuits included low-pass microwave filters and microwave resonators connected to GaAs Schottky-barrier diodes (see figure). Inverse operation was obtained by doing little more than treating the dc-output terminals as dc-input terminals. By simply applying reverse-polarity dc bias to these terminals, the rectifier circuits were made to function similarly to IMPATT-diode oscillators.

Approximately, 1 percent dc to RF conversion efficiency was obtained, with oscillations at 3.3 GHz. In previous research, rectenna energy-conversion efficiency as high as 91 percent had been achieved in the receiving mode. However, IMPATT oscillators are typically only about 10-percent efficient; in other words, about 90 percent of the dc input power becomes heat, which must be removed. Special provisions for heat sinking were made for the experiments. The issues of energy-conversion efficiency and heat sinking would have to be addressed in developing practical inverse rectennas.

This Rectenna Array consists of multiple identical units, each containing dipole microwave antenna elements, microwave circuitry, and a half-wave rectifier in the form of a GaAs Schottky-barrier diode. In the traditional mode of operation, the array acts as a receiver, converting incident RF power to dc power. In the recently discovered inverse mode, the array acts as a transmitter, converting dc power to radiated RF power.

This work was done by Richard M. Dickinson of Caltech and James McSpadden of Texas A&M, NASA Center for Space Power, for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Components & Circuits category. NPO-20321

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