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View File - University of Engineering and Technology, Taxila

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FIGURE 2.15Feed array elements ðy 1 ; ...; y n Þ for a multibeam antenna system.The feed-elements array consists <strong>of</strong> radiating elements <strong>and</strong> a beamformingnetwork, which are either air stripline or waveguide. The selection <strong>of</strong> the type<strong>of</strong> elements is determined by the achievable gain, scan loss, cross polarization,frequency b<strong>and</strong>width, size, <strong>and</strong> weight.Nearly all earth station antennas with a diameter greater than 4 m are <strong>of</strong>the dual-reflector Cassegrain type [12]. The disadvantage <strong>of</strong> the front-fed configuration,apart from the obvious energy blockage, is that for large antennas along length <strong>of</strong> waveguide will be required to carry signals from the focus to thelow-noise amplifier. The attenuation <strong>of</strong> this waveguide raises the noisetemperature <strong>of</strong> the antenna by about 7 K for every 0.1-dB loss. A typicalconnection <strong>of</strong> the antenna-feed to a high-power amplifier (HPA) <strong>and</strong> low-noiseamplifier through an orthogonal-mode transducer (simply orthomode transducer,or OMT) is shown in Fig. 2.16. An OMT is a three-port waveguidedevice.FIGURE 2.16device.A transceive paraboloidal antenna connection with a three-port OMTCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.

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