LIST OF ILLUSTRATIONS 1-1 First experiment in the efficient transfer of power by means of microwaves at the Spencer Laboratory of Raytheon Company, in May 1963. In this experiment microwave power generated from a magnetron was transferred 18 feet and then converted with DC power with an overall efficiency of 16%. A conventional pyramidal horn was used to collect the energy at the reviewing end and a close-spaced thermionic diode was used to convert the microwaves into DC power. The DC power output was 100 watts. 5 1-2 Microwave powered helicopter in flight 60 ft above a transmitting antenna. The receiving array for collecting the microwave power and converting it to DC power was made up of several thousand point contact silicon diodes. DC power level was approximately 200 watt. 6 1-3 The first rectenna. Conceived at Raytheon Company, it was built and tested by R. George of Purdue University. Composed of 28 half-wave dipoles spaced one-half wavelength apart, each dipole terminated in a bridge-type rectifier made from four IN82G point-contact semiconductor diodes. A reflecting surface consisting of a sheet of aluminum was placed one-quarter wavelength behind the array. 10 1-4 The special rectenha made for the first microwavepowered helicopter. The array is 2 feet square and contains 4480 IN82G point-contact rectifier diodes. Maximum DC power output was 200 watts. 12 1-5 Greatly improved rectenna made from improved diodes (HP2900) which are commercially available. The one foot square structure weighs 20 grams and can deliver 20 watts of output power. 12 1-6 Test set-up of microwave power transmission system at Marshall Space Flight Center in 1970. The magnetron which converts de power at 2450 MHz is mounted on the waveguide input to the pyramidal horn transmitting antenna. The rectenna in the background intercepts most of the transmitted power and converts it to de power. Ratio of de power out of rectenna to the rf power into the horn was 40. 8%. Overall dc-to-dc efficiency was 26. 5% 13
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