Abstract
This paper establishes the practicality of terrestrial microwave power beaming at distances >1 km. To beam microwave power along the surface of the earth, it is necessary to consider and, where possible, exploit the effects of microwave propagation across terrain. Key design considerations include the effect of scattering from surface topography, approaches for characterizing the beam in the presence of multipath, safety, and efficiency. A rapid demonstration at the US Army Research Field in Blossom Point, MD, delivers 1.6 kW of electrical power at a 1046 m standoff from a 5.4-m-diameter X band transmitter. The transmitter is a reflector antenna with a linearly actuated feed horn that can focus the power density at specific standoff distances. Experimental results over cluttered, irregular terrain achieve a 2.3 dB (70%) enhancement in power density at the target site by deliberate exploitation of a ground bounce. A 4 m<sup>2</sup> rectenna receiver produces the 1.6 kW of output power at a 73% RF-to-DC conversion efficiency, which exceeds the current state of the art at X band. In addition, a test to destruction of 4 large rectenna arrays demonstrates how overvoltage protection circuits can improve the RF power handling of rectenna arrays by >1 dB. A final experiment demonstrates >1 kW delivered at >1 km distance to a light display. Since these results should readily scale to higher levels of power and performance, recommendations are provided to mature the technology for operational use.
Highlights
Microwave power beaming is the efficient point-to-point transfer of electrical energy across free space by a directive microwave beam
A recent publication [1] provides an overview of the progress to-date in the field and motivating factors – including, prominently, the eventual use of microwave power beaming for space-to-earth wireless power transmission (WPT)
EXPERIMENTAL RESULTS The experimental results provided demonstrate: (i) the usefulness of terrain effects for enhancing transmit power density at the receiver, (ii) >1.6 kW of electrical power delivered wirelessly to a receiver at >1 km standoff with excellent RF-to-DC conversion efficiency, (iii) overvoltage protection circuits for rectenna arrays, and (iv) a second demonstration at higher average power driving an light emitting diode (LED) display
Summary
Microwave power beaming is the efficient point-to-point transfer of electrical energy across free space by a directive microwave beam. This paper explores the application of microwave power beaming to energy transfer along the surface of the earth. Prior terrestrial demonstrations at distances > 500 m have minimized terrain scattering effects by significantly elevating the transmit aperture and rectenna array [1], [8], [9] and by positioning the beam’s target across a chasm on a mountainside [8]. This paper describes a rapid demonstration of microwave power beaming across irregular, inhomogeneous terrain. To the authors’ knowledge, this is the first power beaming demonstration to exploit the effect of “ground bounce” to enhance power density at the target location, and the first demonstration exceeding 1 km in range and 1 kW in received power at a frequency >2.5 GHz. The paper is organized as follows: Section II describes the test site, the transmitter, and the rectenna array.
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