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HIGH-EFFICIENCY RF-RECTENNA DESIGN

A “rectenna” is a rectifying antenna. At its simplest, it comprises a Schottky diode and an antenna and converts the RF power it receives at the antenna into dc voltage. The challenge is how to maximize the power- conversion effi ciency of rectennas for input-power levels of, for example, 0 dBm or less. We employed a method of directly conjugate- matching a rectifying circuit to a microstrip-patch antenna, so that the need for a matching network between the two no longer exists; as a result, the rectenna’s effi ciency improved. This matching technique automatically suppresses the reradiation of harmonics by the microstrip-patch antenna, because the harmonicswill be mismatched.

A PCB (printed-circuitboard) layout of a traditional planar-microstrip rectenna with a probefed antenna shares a common ground plane with a microstrip network (Figure A). With the aid of analytical models for the antenna and the rectifi er, we designed single-layer, internally matched and fi ltered PCB rectennae withlow input-power levels.

We analyzed the rectangular microstrip-patch antenna with a cavity model. A newly developed effective length and width take care of the fringe fi elds of the microstrip-patch antenna. A fourth-order Runge-Kutta routine solved for the packaged diode voltage and the generator current. An FFT then transformed these timedomain parameters to the frequency domain, in which, for each harmonic, a packaged diode impedance was determined for a fi xed incident-power level. After fi nding the complex diode impedance, we determined the microstrippatch edge’s feed point to obtain the conjugate impedance of the diode and thus a conjugate match between the antenna and thediode (Figure B).

With the developed analytical model, you can accurately determine the rectenna’s output voltage as a function of input power at the antenna (Figure C). This design realized an effi ciency of 52% for 0-dBm input power at 2.45 GHz, showing an improvement of more than 10% over a traditional rectenna design (Reference A). A series connection of these rectennae powers a standard household wall clock at a distance as great as 6m (Figure D). The eventual applications, however, will be in chargingbatteries at a distance.

AUTHORS’ BIOGRAPHIES

Hubregt J Visser is a scientist associated with the Holst Centre (www. holstcentre.com), TNO Science and Industry (www.tno.nl), and the Eindhoven University of Technology (www.tue.nl). J Theeuwes is a scientist associated with TNO Science and Industry and the Eindhoven University of Technology. M van Beurden is a scientist associated with the Eindhoven University of Technology. G Doodeman is a scientist associated withTNO Science and Industry.

REFERENCES

Akkermans, JAG, MC van Beurden, GJN Doodeman, and HJ Visser, “Analytical Models for Low-Power Rectenna Design,” Antennas and Wireless Propagation Letters, Volume 4, 2005, pg 187.