Power from space, by means of microwave power transmission using the sun as the energy source, has been a vision of many for decades. The power system would employ large solar- power satellites in geostationary earth orbit, and the system would consist of a solar energy collector to convert solar energy in direct current electricity, a DC-to-microwave converter, a large antenna array to beam the microwave power to the ground, and a receiving antenna on the earth. The solar energy collector can be either photovoltaic cells or a solar thermal turbine. The DC-to-microwave converter can be either microwave tube or semiconductor based. The final part of the system is called a rectenna, because it contains a rectifying system as well as an antenna.

Satellite systems for power transmission have been reported since early research in the 1970s.1-11 Typical parameters of the transmitting antenna of the solar power satellites have been summarized in the table from the NASA (National Aeronautics and Space Administration), US Department of Energy (DOE), and the Japan Aerospace Exploration Agency (JAXA). The main points that influence microwave power transmission from space are the selection of operating frequency and choices for the four main components in the system, including the earth-based antenna.

The Industrial-Scientific-Medical (ISM) frequency bands available for microwave transmission and accepted by international regulatory authorities include the 2.45-, 5.8-, and 24.125-GHz bands. The US DOE, Raytheon Co., NASA's Jet Propulsion Laboratory (JPL), the Canadian Communications Research Center, and various European agencies have taken part in development programs for the components needed for microwave power transmission at 2.45 GHz, due to the low atmospheric attenuation at that frequency. The 5.8-GHz band is preferred by a number of research organizations, including the David Sarnoff Research Center, SRI International, NASA, and JAXA, because of the reduction in component size at that smaller wavelength compared to 2.45 GHz. In addition, the attenuation loss through the atmosphere does not increase appreciably at 5.8 GHz compared to at 2.45 GHz.

The rectenna is a key component in a power transmission system since it must handle high power levels with high efficiency in order to conserve as much as the received energy from space as possible. Early work on a thin-film rectenna at 2.45 GHz was reported by Brown.12-13 The rectenna achieved conversion efficiency of 85 percent at S-band. Bharj et al.14 reported a highly efficient antenna array at 5.87 GHz, comprised of 1000 dipole elements. These rectenna dipole elements exhibit RF-to-DC efficiency exceeding 80 percent with a uniform illuminated aperture. The rectification element consists of a silicon Schottky diode quad bridge with high reverse breakdown voltage.

McSpadden et al.15 developed a 5.8-GHz rectenna with RF-to-DC conversion efficiency of 82 percent. The dipole antenna and filtering circuitry were printed on a thin duroid substrate. In that design, a silicon Schottky-barrier mixer diode with low breakdown voltage was used as the rectifying device. This type of antenna element is very sensitive to reflected RF power. In the current report, a printed-circuit rectenna element was developed using optimized impedance matching to achieve conversion efficiency of better than 88 percent. The antenna element was first realized with coaxial cable as a "proof-ofconcept" design and then produced in printed-circuit form on 10-mil-thick RT/duroid 5880 substrate material from Rogers Corporation.

To evaluate the performance of the rectenna element, the overall efficiency (EFo) can be defined as7, 15,

EFo = (Pdc)/(Prf) (1)

where Pdc = DC output power and

Prf = incident RF power.

The conversion efficiency (EFc) can be found from

EFc = (Pdc)/((Prf) Pr) (2)

where Pref = the reflected RF power.

More DC output power can be produced from a microwave transmission system by increasing the conversion efficiency. This can be done by means of optimized impedance matching of the rectenna element. Matching can be optimized by means of tuning stubs in the dipole element structure. Figure 1 shows a basic dipole rectenna element.7, 14, 15 The dipole antenna and coplanar stripline are printed on one side of 10-mil-thick RT/duroid 5880 printed-circuit-board (PCB) substrate material. The length of the dipole is approximately 2.66 cm. This length is determined by using computeraided- engineering (CAE) simulation software and optimizing the design according to simulated results.7

The input resistance of the resonant dipole is about 92.5 Ω in the optimum condition. The gap between the dipole terminals is determined by the characteristic impedance of the coplanar stripline. The calculated characteristic impedance for a 0.27-cm gap and 0.158-cm-wide conductive strips is 255 Ω . The diode input impedance is maintained at 255 Ω to reduce losses associated with the built-in voltage of the diode. A lowpass filter is printed on the opposite side of the coplanar stripline transmission line used to form the resonant dipole. The filter appears as three printed strips in Fig. 1. The filter passes signals at 5.8 GHz and rejects higher-order harmonic signals produced by the rectifying diode. The lowpass filter is also designed to transform the 92.5- dipole impedance to the 255-Ω diode impedance.

Brown13 has shown the loss in conversion efficiency due to the diode's built-in voltage (Vv) as,

Efficiency loss = (Vv)/(Vdc + Vv) (3)

where Vdc = the voltage measured across the resistive load.

As this relationship shows, in order to minimize the loss associated with the diode's built-in voltage, the diode must be selected for a larger input impedance. In the current rectenna design, a model MA40150-119 silicon Schottky diode from M/A-COM Technology Solutions was used. The measured builtin and breakdown voltages at 10 A are 0.225 V and 4.44 V, respectively. The power-handling capabilities of the diode are limited by the breakdown voltage. A 47-pF chip capacitor was used to effectively short the RF energy and pass the DC power to a resistive load. The lowpass filter also serves to transform the input impedance of the dipole antenna to the input impedance of the diode. Tuning stubs were added to the design to improve the impedance matching of the antenna and other components. The chip capacitor was also used to maximize the conversion efficiency of the diode. The distance between the diode and the chip capacitor constitutes an inductance that also tunes the capacitance of the diode.

Figure 2 shows a block diagram of the test setup used to evaluate the rectenna, using a synthesized test source and impedance tuner. The size of the matching stubs has been optimized for the lowest possible VSWR with the dipole rectenna, lowpass filter, Schottky diode, RF short-circuit capacitor, and end load. In terms of measuring the performance of the rectenna element, Hannan et al.16 developed a technique for simulating a phased array antenna in a waveguide in order to determine the performance of the radiating elements. Bhirj et al.14 also used a waveguide simulator to measure the efficiency of a 1000-element array antenna. McSpadden et al.15 also tested a rectenna element in a waveguide at a frequency range of 5.7 to 5.9 GHz. They used a taper section to expand the dimensions of the waveguide for the test system.

A similar type of evaluation system was designed for testing the current dipole rectenna, using a step transformer to expand the size of the waveguide instead of a tapered section. The main advantage in using a step rather than a tapered approach is that the length of the waveguide is smaller. Figure 2 shows a block diagram of the measurement setup. In the setup, a synthesized microwave sweep generator is used to provide input power. A coaxial isolator is used minimize the reflection of signal power back to the signal generator. Slotted WR137 waveguide is used to measure reflected power. VSWR measurements were made by means of a detector on the carriage along with a voltmeter. A step transformer transition connects the WR137 waveguide to the square output of the waveguide simulator. The square waveguide simulator houses an adjustable reflecting plane to tune the dipole rectenna element.

Figure 3 shows plots of VSWR versus frequency for the dipole rectenna element with and without matching stubs. Measurements were performed from 5.7 to 5.9 GHz with 50 mW input power. Curves A and B show the lowest and optimum values of VSWR, respectively.

Figure 4 shows conversion efficiency for the rectenna dipole element versus frequency from 5.7 to 5.9 GHz. The bottom curve (C) denotes the lower efficiency without matching stubs while the top curve (D) denotes higher conversion efficiency using matching stubs. The conversion efficiency is better than 88 percent at 5.8 GHz with matching. Without matching, the conversion efficiency is better than 81 percent at 5.8 GHz. Based on these results, the 5.7-to-5.9- GHz dipole rectenna element will be used to create a full antenna array of about 1000 elements for future evaluation at 5.8 GHz.

REFERENCES

1. W. J. Robinson and W. C. Brown, "The NASA effort in high efficiency free-space microwave power transmission," Proceedings of the Microwave Power Symposium May 1974, pp. PS1-2/1 2/2.

2. A. Kumar, technical communication with Session Chairman W. C. Brown, Microwave Power Generation, Transmission, and Rectification, Marquette University, Milwaukee, WI, May 1974.

3. W. C. Brown, "The history of power transmission by radio waves," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 9, September 1984, pp. 1230-1242.

4. J. J. Schleesak, A, Alden, and T. Ohno, "A microwave powered high altitude platform," Proceedings of the IEEE MTT-S International Microwave Symposium Digest, 1988, pp. 283-286.

5. W. C. Brown, "Beamed microwave power transmission and its application to space," IEEE Transactions on Microwave Theory and Techniques, Vol. 40, No. 6, 1992, pp.1239-1250.

6. W. C. Brown, "Electronic and mechanical improvement of the receiving terminal of a free-space microwave power transmission system," Raytheon Co., Wayland, MA, Technical Report PT-4964, NASA Report No. CR-135194, August 1977.

7. A. Kumar, "Dipole rectenna antenna, array at 2.45 GHz," Research Report, EETC Coventry, UK, 1981.

8. A. Kumar and H. D. Hristov, Microwave Cavity Antennas, Artech House, Norwood, MA, 1989. 9. N. Shinohara, H. Matsumoto, and K. Hashimoto, "Microwave power transmission experiment on international space station," Proceedings of the ISAP Japan, 2000, pp. 5P4-5.

10. J. O. McSpadden and J. C. Mankins, "Space solar power programs and microwave wireless power transmission technology," IEEE Microwave Magazine, Vol. 3, No. 4, Dec. 2002, pp. 46- 57.

11. H. Matsumoto, "Research on solar power satellites and microwave power transmission in Japan," IEEE Microwave Magazine, Vol. 3, No. 4, 2002, pp. 36-45.

12. W. C. Brown and J. F.Triner, "Experimental thin-film, etched-circuit rectenna," Proceedings of the IEEE MTT-S, International Microwave Symposium Digest, 1982, pp. 185-187.

13. W. C. Brown, "Rectenna technology program: ultra light 2.45 GHz rectenna and 20 GHz rectenna," Raytheon Co., Wayland, MA, Technical Report PT-6902, NASA Report No. CR-179558, March 1987.

14. S. S. Bharj, R. Camisa, S. Grober, F. Wonzniak, and E. Pendleton, "High efficiency C-band 1000-element rectenna array for microwave powered applications," Proceedings of the IEEE MTT-S International Microwave Symposium Digest, 1992, pp. 301-303.

15. J. O. McSpadden, Lu Fan, and Kai Chang, "A high conversion efficiency 5.8 GHz rectenna," Proceedings of the IEEE MTT-S International Microwave Symposium Digest, 1997, pp. 547-550.

16. P. W. Hannan and M. A. Balfour, "Simulation of a phased-array antenna in waveguide," IEEE Transactions on Antennas and Propagation, Vol. 13, May 1965, pp. 342-353.

Enough Energy For The Moon?
Microwave power transmission (MPT) offers a means of extracting electricity from the sun that may be beneficial not only on Earth but perhaps on the Moon as well. Recent research on behalf of the United States Department of Energy (DoE) and NASA's Jet Propulsion Laboratory (JPL) have explored the feasibility of using rectenna arrays as part of a remote lunar power system. A study spearheaded by Professor Zoya Popvic of The University of Colorado at Boulder, for example, invesitgated the feasibility of creating a multikillowayy wireless power system to transfer power between lunar base facilities. In the conceptual system, four transmission towers power a total of five load stations, at power levels of 10 kW and distances ranging from 0.5 to 2.0 km. The feasibility study shows a better than 30-percent cost savings by using MPT technolhy rather than conventional transmission cables.

A key to the success of any MPT platform is the rectenna since it must rectify power as efficiently as possible. Work performed by James McSpadden at Teas A & M University in conjuncion with JPL has led to a 2.45-GHz rectenna element designed for more than 85-percent RF-to-DC power conversion efficiency but that could, in theory, provide considerably higher efficiency if optimized to operate while oscillating at a higher frequency, such as the 3.3 GHz suggested by the researchers. The rectenna consists of a half-wave dipole antenna, two-section input lowpass filter, GaAs IMPATT diode, and output capacitor for shorting the RF power and tuning the diode.