Most anechoic chambers have background levels higher than -50 dB. For a 1% accuracy measurement, the background must be much lower, as the low level sidelobes can have a big effect on the antenna gain. One alternate approach would be to measure antenna patterns and waveguide losses separately. The antenna pattern would require a high quality anechoic chamber and many pattern cuts. Waveguide attenuation losses could be made on long sections of guide. These measurements, coupled with carefully measured impedance characteristics of the entire subarray would be sufficient to characterize the entire spacetenna performance. Harmonics Finally, the problem of harmonic interaction with the radiating structure needs to be addressed. It will not be feasible to place filters, circulators or much else between the power amplfier and the radiating element without introducing unacceptable losses. Thus, harmonic suppression on the SPS itself must be achieved with the design of the radiating elements. 7. THE RECTENNA This is an area which saw a lot of early work and development, but which has been lying rather fallow in more recent SPS history. This does not seem to be a good idea, as the present MPTS rectenna concept needs considerably more work. The major problems which the panel sees are those of weather protection, parts count, and harmonic reradiation. The demonstrated efficiencies at Goldstone have shown that the basic concept is reasonable, but have not answered the question of scaling this approach to SPS power levels and larger mass produced arrays. Harmonics The main concern of the panel was that of harmonic generation and reradiation from the individual rectenna elements. The amount of harmonic suppression possible with any economically reasonable filter placed on 10"’ individual elements does not seem to be sufficient to limit the harmonic signals to an acceptable level. The only logical approach is to look for ways to lower the number of individual receiving elements, so that more care can be exercised in their design and construction. The Rensselaer Polytechnic Institute work was pointing in that direction, and the panel feels that a lot more effort should be expended towards further developing microwave concentrating schemes such as the Boeing hogline. Weather Protection It is a certainty that some form of weather protection or radome will be needed over all of the active elements in the rectenna. This feature, which has not really been considered in present rectenna designs, must be considered from the outset as it will have significant effects on the physical construction of this costly component of the MPTS. Element Lattice Design With regard to the rectenna element lattice design, the narrow beamwidth, high
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