Figure 7.22 Power Distribution at Rectenna The most obvious candidates for power beaming antennas are large reflector antennas or array antennas. A reflector antenna uses one or several feeder elements to illuminate a reflector surface. The feeder is positioned so that the output of the reflector represents an equal-phase wave front in the direction of the receiver. Reflector antennas are efficient and widely used. However, they require mechanical steering in order to direct the beam in a given direction. For large antennas in space this represent a serious limitation in their usage. For precise beam control applications, large arrays of discrete elements are found to be more feasible. An array antenna consists of individual radiating elements equally distributed over the radiation surface. The spacing between each element is approximately half the wavelength in order to avoid grating sidelobes. By controlling the phase of each element the mainlobe direction can be steered without physically changing the orientation of the antenna. If the transmitting antenna is mounted on a spacecraft which moves with respect to the rectenna, some sort of beam pointing must be used. For low accuracy pointing (e.gxl.O") mechanical steering of the antenna can be utilized. For power transmission much higher pointing accuracy is required. In general we require pointing accuracy to be significantly lower than the beam width of the mainlobe (A/Dt rad). As an example, a 100 m antenna transmitting at 2.45 GHz will require pointing accuracy better than 0.05”. Pointing accuracy requirements increase with frequency and antenna size. It is therefore desirable to use electronic steering of the beam. Large phased array antennas already exist for C-band. The Radarsat SAR antenna operates with a pointing accuracy within 0.2”.[Raney, 1991] It is expected that the same technology will be available for phased arrays operating at higher frequencies in the near future (e.g. 35 GHz). Several different types of antenna radiators can be used in an array configuration. The elements must, however exhibit a certain omnidirectionality in order to steer the phased beam within a reasonable angle around the antenna boresight. Regular dipoles mounted on a ground plane have been used as well as slotted waveguides or waveguide horn antennas. Waveguides have high power handling capability and they can easily be connected to klystron converters with low insertion losses. Figure 7.23 shows the configuration of a slotted waveguide radiator. Most modern phased array antennas utilizes microstip patch antennas where each element is fed from solid state hybrid power amplifiers or microwave integrated circuits (MIC's). This scheme eliminates the need for distribution of RF power within the antenna structure. Solid state devices operate at a temperature lower than 130 “C thus limiting the permissible power density on the antenna surface. At present solid state amplifiers operate with an efficiency below 50% and efficiency decreases with increased operating frequency.
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