Figure IV-3.- Output power limits. A microwave frequency of 2.45 GHz was selected for study purposes. This frequency is at the center of a 100-MHz band reserved for industrial, scientific, and medical use, so that interference with communications will be minimized. Atmospheric attenuation is also low at this frequency. A higher frequency, such as 3 GHz, offers higher gain for the same antenna diameter and should be considered, but would cause substantial interference with present users of this band. The mainbeam pattern and sidelobe characteristics of the antenna will vary with the power density taper over the antenna. Increasing the amount of taper produces a lower boresight density, a wider main- lobe, and lower sidelobes. For a given rectenna radius, the collection efficiency increases with the amount of taper. A 10-dB taper has been adopted for this study. For a no-error/no-failure condition, this gives a 90-percent collection efficiency at a rectenna radius of 4300 m. An ideal continuous taper would be too complex to be practical and was replaced in this study by a 10-step approximation that gives virtually the same performance.
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