expensive and would take too much time to develop, so that this pointing technique must be discarded for this demonstration. As this pointing technique is the only one able to correct any flatness inaccuracies, the mechanical flatness requirements are tightened. An alternative would be to use a simpler design composed of four isotropic antennae placed in the center of the phased array as shown in Figure 10.2.7. An isotropic antenna placed in the center of the rectenna emits a pilot signal at a different frequency than the power transmission. The four elements of the rectenna receive signals which are carrying the direction information of the incoming signal in their relative phases. This information could be extracted by multiplying the signals two by two, in order to get the sine of their phase difference, as shown in Figure 10.2.8. The results are sent to the control unit, which computes the direction of the incoming signal. The correct phase commands can then be determined and applied to the phase shifters. Further investigations should be done to specify this system. In order to avoid any phase ambiguity, the antennae of the interferometer should be spaced less than half a wavelength apart (of the pilot signal). No distance measurement is needed as the pilot signal transmitter is in the far field of the interferometer. Figure 10.2.8 Phase Control Transmission Efficiency The transmission efficiency has been estimated as follows. It is assumed that the rectenna always stays within a cone with an opening angle which corresponds to the half gain angle 03dB as shown in Figure 10.2.9. Figure 10.2.7 Antenna System Layout
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