Design and Experiments of a High-Conversion-Efficiency 5.8
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 46, NO. 12, DECEMBER 1998 2053 Design and Experiments of a High-Conversion-Efficiency 5.8-GHz Rectenna James O. McSpadden, Student Member, IEEE, Lu Fan, Member, IEEE, and Kai Chang, Fellow, IEEE Abstract—A high-efficiency rectenna element has been designed the transmitting frequency of choice due to its advanced and and tested at 5.8 GHz for applications involving microwave- efficient technology base, location at the center of an industrial, power transmission. The dipole antenna and filtering circuitry scientific, and medical (ISM) band, and its minimal attenuation are printed on a thin duroid substrate. A silicon Schottky- barrier mixer diode with a low breakdown voltage is used through the atmosphere even in heavy rainstorms. as the rectifying device. The rectenna element is tested inside The conversion efficiency of the rectenna continued to a waveguide simulator and achieves an RF-to-dc conversion increase from the 1960’s through the 1970’s at 2.45 GHz. Con- efficiency of 82% at an input power level of 50-mW and 327- version efficiency is determined by the amount of microwave load. Closed-form equations are given for the diode efficiency and power that is converted into dc power by a rectenna element. input impedance as a function of input RF power. Measured and calculated efficiency results are in good agreement. The antenna The greatest conversion efficiency ever recorded by a rectenna and circuit design are based on a full-wave electromagnetic element occurred in 1977 by Brown, Raytheon Company [7]. simulator. Second harmonic power levels are 21 dB down from Using a GaAs–Pt Schottky barrier diode, a 90.6% conversion the fundamental input power. efficiency was recorded with an input microwave-power level Index Terms—Detectors, nonlinear circuits, power conversion, of 8 W. This rectenna element used aluminum bars to construct rectennas, rectifying antennas. the dipole and transmission line. Later, a printed thin-film class of rectenna design was developed at 2.45 GHz where I. INTRODUCTION conversion efficiencies of 85% were achieved [8]. VER 100 years ago, the concept of wireless power Components for microwave-power transmission have tradi- Otransmission began with the ideas and demonstrations by tionally been focused at 2.45 GHz. To reduce the transmitting Tesla [1]. Although Tesla was unsuccessful at implementing and rectenna aperture areas and increase the transmission his wireless power transmission systems for commercial use, range, researchers at ARCO Power Technologies, Inc., Wash- he did transmit power from his oscillators that operated up ington, DC, developed a 72% efficient rectenna element at 35 to 100 MV at 150 kHz. For demonstration purposes, Tesla GHz by 1991 [9]. 35 GHz was targeted due to a decrease in the would hold lighted light bulbs, which derived power from atmospheric absorption around this frequency. However, com- the uncollimated radiated energy. After Tesla’s experiments, ponents for generating high power at 35 GHz are expensive researchers in Japan [2] and the U.S. [3] continued efforts to and inefficient. promote wireless power transmission in the 1920’s and 1930’s. To decrease the aperture sizes without sacrificing compo- The modern era of wireless power transmission began in the nent efficiency, technology development at the next higher 1950’s with the advancement of high-power microwave tubes ISM band centered at 5.8 GHz has been investigated. This by Raytheon Company, Waltham, MA [4]. By 1958, a 15-kW frequency is appealing for beamed power transmission due average power -band cross-field amplifying tube was devel- to smaller component sizes and a greater transmission range oped that had a measured overall dc-to-RF efficiency of 81% over 2.45 GHz. [5], [6]. The first receiving device for efficient reception and In 1992, the first -band rectenna achieved a 70% overall rectification of microwave power emerged in the early 1960’s. efficiency and a 80% conversion efficiency at 5.87 GHz [10]. Conceived at Raytheon, a rectifying antenna, or rectenna, was These efficiencies were measured in a waveguide simulator developed, consisting of a half-wave dipole antenna with a with an input power level of approximately 700 mW per ele- balanced bridge or single semiconductor diode placed above ment. This -band rectenna used a printed dipole, which fed a reflecting plane. The output of the rectenna element is a Si Schottky diode quad bridge. However, little information then connected to a resistive load. 2.45 GHz emerged as is provided on the design and testing of this rectenna. In testing rectennas in a waveguide simulator, an overall Manuscript received November 23, 1997; revised March 11, 1998. This efficiency and conversion efficiency are defined as work was supported in part by the NASA Center for Space Power, by the Army Research Office, and by the Texas Higher Education Coordinating dc output power Board’s Advanced Technology Program. incident RF power J. O. McSpadden and K. Chang are with the Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128 USA dc output power (e-mail: [email protected]). incident RF power-reflected RF power L. Fan is with Texas Instruments Incorporated, Dallas, TX 75243-4136 USA. Measurements performed in a waveguide simulator can accu- Publisher Item Identifier S 0018-9480(98)09037-1. rately monitor reflected power. 0018–9480/98$10.00 1998 IEEE 2054 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 46, NO. 12, DECEMBER 1998 Fig. 1. The five main rectenna element components: antenna, input low-pass filter, rectifying diode, output filter, and resistive load. The rectenna element in this paper operates efficiently ( 80%) at much lower incident power levels of 20–65 mW with little reflected RF power (i.e., the overall efficiency is 1% lower than the conversion efficiency) [11]. This characteristic has two important applications in microwave-power beaming systems: 1) power can be converted efficiently at the edge of the rectenna where power densities are lower than the center elements and 2) power can be converted efficiently when the transmission distance is large and power density is low. This rectenna is the first to be designed using a full- wave electromagnetic simulator. Previous rectennas have been designed using transmission-line models, Touchstone, and Libra [10], [12], [13]. A full-wave simulator allows the rectenna’s antenna and passive circuit to be analyzed together. Also, closed-form equations for diode efficiency and input impedance agree well with the measured results. Fig. 2. Rectification cycle represented by the input fundamental and diode junction voltage waveforms impressed on the diode s curve. This model assumes there are no losses in the harmonics and a 3t 0. II. RECTENNA-ELEMENT THEORY OF OPERATION Fig. 1 shows the main components of the rectenna ele- The diode junction waveform can be expressed as ment. A half-wave dipole attaches to a low-pass filter, which transforms the dipole impedance to the diode impedance and (1) rejects higher order diode harmonics from radiating through if diode is off (2) the dipole. A diode placed in shunt across the transmission if diode is on line is the rectifier. The output filter consisting of a large capacitor effectively shorts the RF energy and passes the dc where is the output self-bias dc voltage across the resistive power. The distance between the diode and output capacitor load and is the peak voltage amplitude of the incident is used to resonate the capacitive reactance of the diode. Both microwave signal. Similar to mixers, the rectenna is a self- input and output filters are used to store RF energy during the biasing circuit. As the incident power increases, the output dc off period of the diode. A resistor is then placed across the voltage becomes more reversed biased. and are the dc output terminals to act as the load for measuring the output dc and fundamental frequency components of the diode junction power. Finally, a reflecting metal plane is then placed behind voltage, and is the diode’s built-in voltage in the forward the rectenna, typically 0.2–0.25 . bias region. Also shown in Fig. 2 are the forward-bias turn-on The typical operation of a rectenna element can be better angles of and the phase difference between and understood by analyzing the diode’s dc characteristics with . is a dynamic variable dependent on the diode’s input an impressed RF signal. Fig. 2 shows an idealized RF volt- power and is determined by age waveform operating across the diode and the diode junction voltage [13], [14]. This simple model assumes that (3) the harmonic impedances seen by the diode are either infinite or zero to avoid power loss by the harmonics. Thus, the fundamental voltage wave is not corrupted by higher order where is the diode’s series resistance and is the dc harmonic components. The rectenna conversion efficiency then load resistance. depends only on the diode electrical parameters and the circuit Fig. 3 shows the equivalent circuit of the diode used for losses at the fundamental frequency and dc. A mathematical the derivation of the mathematical model. The diode parasitic model of the diode efficiency has been derived under this reactive elements are not included in the equivalent circuit. condition [13] and expanded to account for varying input Instead, it is assumed they belong to the rectenna’s environ- power levels. ment circuit. The environment circuit is defined as the circuit McSPADDEN et al.: DESIGN AND EXPERIMENTS OF HIGH-CONVERSION-EFFICIENCY 5.8-GHz RECTENNA 2055 Fig. 3. Equivalent circuit model of the rectifying circuit consisting of the rectifying diode and dc load resistor.