9.2 ANTENNA APPROACHES The requirements for reception and rectification of microwave power from a transmitter in synchronous orbit are listed below. a. Non-directive aperture b. High absorption efficiency c. High rectification efficiency d. Very large power handling capability e. Passive radiation of waste heat f. High reliability g. Long life h. Low radio frequency interference (RFI) i. Capable of being constructed in large aperture size j . Easy mechanical tolerance requirements k. Low cost These requirements must be matched up with the capabilities of various approaches to performing this function. Each candidate approach must consist of a means of collecting the microwave power and then converting it into dc power. While there are a number of existing technologies that can be used to collect the power, there is only one existing rectifier technology that is at all practical and that is the semiconductor Schottky barrier diode. The diode may be used singly or grouped in large numbers to provide the load for any collection approach, although it is obvious that auxiliary cooling will be necessary if large numbers are grouped together. The number of ways in which the power may be collected is limited. It may be collected by an array of contiguous horns with independent microwave load, an array of contiguous reflectors and feed horns with independent microwave load, a phased array of small-aperture elements with a common microwave load, or an array of small aperture elements with independent microwave load (rectenna). There is a basic objection to horn or reflector-horn collectors because of their inability to collect close to 100% of the power that impinges upon them. The near uniform power density of the microwave power impinging upon them will result in uniform illumination of the aperture and this will not match the natural aperture
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