In the late 1980's, a program to develop a long endurance high altitude platform called SHARP (Stationary High Altitude Relay Platform) was proposed in Canada. The idea is to float an unmanned light-weight airplane for a long period, circling at an altitude of about 21 km for the purpose of relaying radio communications signals over a wide area. To maintain the platform floating for weeks or months, a fuel-less airplane powered by microwave energy transmitted from the ground was proposed and experimented. On September 17, 1987, a 1/8-scale prototype SHARP flew on beamed microwave power for 20 minutes at an altitude of about 150 m. Figure 20 shows a photo of the prototype SHARP with a 4.5 m wingspan. The microwave beam was transmitted by a 4.5 m diameter parabolic antenna transmitting 10 kW microwave with a frequency of 2.45 GHz. Two water-cooled magnetrons each with 5 kW output power were used. The parabolic antenna mechanically tracked the airplane which flew inside a 50 degree cone. The power density at the airplane altitude was 400 W/m2. A dual polarization rectenna with two orthogonal linearly-polarized dipole arrays was developed. The rectenna diodes used in the first flight were Silicon Schottky diodes (HP2835). Its power handling capability was 1 W/element, and its microwave-to-DC conversion efficiency was about 70 %. The rectenna received sufficient power to feed 150 W to the electric motor of the 4.1 kg weight SHARP airplane. A similar project was carried out in Japan in the early 1990's. The project was called Stratospheric Radio Relay Systems (SRRS), and was studied by a working group under the Ministry of Posts and Telecommunications of Japanese government. The objectives of the SRRS are similar to those of Canadian SHARP. In the SRRS, it is planned to launch five such unmanned airplanes over Japan. In parallel with the SRRS working group, a microwave-driven airplane experiment was planned and conducted successfully on August 29, 1992 by a joint team organized by the Prof. Matsumoto. The team members were from Kyoto University, Kobe University, Communications Research Laboratory, Nissan Motor Co. Ltd., Fuji Heavy Industries Ltd. and Toshiba Co. The experimental project was called MILAX meaning MIcrowave Lifted Airplane eXperiment, and was partly sponsored by ISAS of Japan. The MILAX airplane is a balsa-based light-weight (under 4 kg) airplane with a 2.5 m wingspan and has a shape. The MILAX flew successfully for 40 seconds (or 400 m distance over a straight course for car driving test) at an altitude of about 15 m. Because of the limits of the maximum microwave power (under 1 kW) and of the aperture of the transmitting antenna (under 1.2 m) , the flight altitude had to be as low as 15 m in order to guarantee the power density of 200 W/m2 at that altitude. The microwave power beam was radiated toward the fuel-free MILAX airplane by an active phased array antenna. The MILAX active phase array transmitter was composed of five-stage Gallium-Arsenic (GaAs) semi-conductor amplifiers, 4-bit digital phase shifters and circular microstrip antennas. The transmitter is divided into 96 sub-arrays, each consisting of 3 antennas, one phase-shifter and one GaAs amplifier. Each sub-array can supply 13 W microwave output resulting in the total radiation capability of 1.25 kW. The frequency used in the MILAX was 2.411 GHz in the ISM frequency band. The transmitter system was installed on the roof of a transmitter car.
Six rectenna subarrays, each consisting of 20 rectennas are installed on the flat-bottom of the MILAX airplane. Prior to the development of the MILAX rectenna, several rectenna researches had been done in Japan. Based on these studies, the receiving antennas used for the MILAX rectenna were not of the dipole-type, like these used in the JPL/Goldstone Ground-to-Ground Power Transmission Experiment and in the MINIX and SHARP, but were of a new type of microstrip circular patch antennas. The circular patch antennas have the advantage of a non-resonant nature at integer multiple harmonic frequencies, thereby having the capability of suppressing spurious radiation from the rectennas. The disadvantage of heavier weight as compared to dipole antennas was overcome by introducing a paper honeycomb structure. The diodes used for the MILAX rectenna are eight HP5082-2350 Schottky diodes in 2-series / 4-parallel combination. The power handling capability was 1 W per element, and the microwave-to-DC conversion efficiency was about 52%.
The main reason of the adoption of the active phased array in place of a conventional parabolic antenna is its higher steerability of the microwave power beam. The power beam can be controlled and steered electronically in contrast to the mechanical control of a parabolic antenna. In the MILAX, we monitored the location of the MILAX airplane by two CCD cameras which were installed on the edge of the roof-transmitter antenna looking upward. A micro-computer, after recognizing the pattern of the airplane image and calculating the x-y coordinates and the altitude of the airplane, sends the control signals to the phase shifters of the microwave amplifiers so that the microwave beam is accurately directed toward the airplane. This system worked perfectly in the MILAX.
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