A prototype dual-polarization microstrip patch rectenna dimensioned for an operating frequency of 8.51 GHz has been built to demonstrate a design concept for obtaining maximum output voltage from a rectenna of a given size. This rectenna contains nine square microstrip patch antenna elements in a square array. The antenna can utilize incident radiation with polarization components parallel to either or both sides of the square because the circuitry under each patch includes two independent rectifiers — one for each polarization component. The dc output terminals of the two rectifiers under each patch are connected in series, and the series rectifier pairs of all patches are connected in series. Consequently, the maximum output voltage (obtainable in the special case of circularly polarized incident radiation) can be 18 times that generated by a single-rectifier, single-polarization patch element.
Each microstrip patch antenna element is supported over a ground plane by a layer of lightweight foam with a low (1.07) permittivity. Microstrip feed circuitry, rectifier diodes, and dc conductors that collect the output power are located below the ground plane. Microwave energy is coupled from the antenna elements to the feed circuitry through apertures or coupling slots. Except for this aperture coupling, the antenna and the microstrip feed circuitry are divorced from each other.
The placement of the feed and rectifier circuitry below the ground plane makes it possible to isolate the rectifier circuitry from direct exposure to the incident microwave energy and thus prevents spurious coupling of microwave energy onto the dc conductors. The ground plane also prevents the direct reradiation of the harmonics that are generated by the diodes as unavoidable byproducts of rectification. A microstrip line filter is used to prevent radiation back through the aperture feed. Yet another advantage of mounting the rectifier circuitry below the ground plane is that the ground plane serves as an effective thermal sink for the diodes. The ground plane is not a single conductive plane but, rather, an array of dc-isolated, capacitively-coupled islands registered with the overlying antenna patches. The capacitive coupling is effected by a thin layer of copper-coated polyimide. The dc isolation of the ground-plane islands in conjunction with the orthogonality of the layout makes it possible to use these islands as parts of the series-connected dc output circuitry.
In tests at an optimized load resistance of 5.4 k Ω , the rectenna was found to function with an overall energy-conversion efficiency exceeding 52 percent over a large range of incident power densities, with a peak of 53 percent at a power density of 38.8 mW/cm2. The results of the tests also showed that a target output potential of 50 Vdc can be achieved at a power density of 25.2 mW/cm2.
This work was done by Larry Epp and Abdur Khan of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronic Components and Systems category.
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