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.
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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Inverse rectennas for two-way wireless power transmission
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Overview
The document discusses advancements in wireless microwave power transmission through the development of inverse rectennas, which can function as both receivers and transmitters. Conducted by researchers Richard M. Dickinson from Caltech and James O. McSpadden from Texas A&M, this work was supported by NASA's Jet Propulsion Laboratory.
The core concept revolves around rectennas, which are devices that convert microwave energy into direct current (dc) power. Traditionally, rectennas have been used in receiving mode, achieving high energy-conversion efficiencies—up to 91 percent in some cases. However, the research introduces an innovative approach where rectennas can also operate in an inverse mode, converting dc power back into radiated radio frequency (RF) power. This dual functionality could significantly enhance the efficiency of wireless power transmission systems.
In the experiments, a rectenna element designed for 2.45 GHz was tested, achieving an approximate 10 percent efficiency in converting dc to RF power while oscillating at 3.3 GHz. The document highlights the challenges associated with this technology, particularly the low efficiency of IMPATT (Impact Avalanche Transit Time) oscillators, which typically convert only about 10 percent of the dc input power into RF output, with the remainder dissipated as heat. Effective heat sinking is crucial for practical applications of inverse rectennas.
The rectenna design includes a half-wave dipole antenna, a low-pass microwave filter, and a GaAs Schottky-barrier diode rectifier. The system is optimized for both receiving and transmitting modes, with specific configurations to enhance performance. The research emphasizes the need for further optimization to improve the dc to RF conversion efficiency without compromising the RF to dc efficiency.
Overall, this work represents a significant step toward the realization of two-way wireless power transmission systems, which could have applications in various fields, including space exploration, high-altitude platforms, and potentially even terrestrial power distribution. The findings suggest that with continued research and development, inverse rectennas could play a pivotal role in the future of energy transmission.
