Experimental verification of the multifocal properties of reflecting zone plate antennas

The reflecting version of the Fresnel zone plate is currently finding increasing applications, principally in the domestic satellite TV market. Products based on this technique are now commercially available at both Ku and C-band in sizes up to 3 m aperture. The immediate attraction of such antennas arises from their portability and for flush mounting on walls or roofs facilitating their concealment and consequently enhancing their environmental acceptability. This form of antenna offers a further advantage over the conventional circular parabola, or dish, in that whereas in the latter the received signal is focused by reflection to a point, in a zone plate the focusing is performed by diffraction, which results in much wider tolerances in both focal distance and off axis angle in the position of the feed horn. This offers, in principle, four distinct, novel and potentially useful antenna configurations. The results of an extensive experimental investigation of these antennas features has been carried out by Portasat Ltd at the satellite monitoring site of the UK´s Radio Communication Agency at Baldock, using quarter-wave zone plates to receive signals from the main European Ku-band satellites