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Radar System Engineering

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SEC.9.3] FAN BEAMS 275<br />

The cosecant-squared fan beam is used not only in airborne navigational<br />

radar but also in surface-based radar for the detection of airplanes,<br />

In this case the fan is inverted, the intention being that a target airplane<br />

FIci. 9.6.—A fan beam which enablesan airborne radar to scan the surface of the earth.<br />

FIG. 9.7.—The energy distribution to a square.root scale as a function of the depree+sion<br />

anglein an airbornen~vigational radar,<br />

flying at any constant altitude will be indicated with a constant intensity,<br />

independent of range.<br />

Several antenna types have been developed for producing a cosecantsquarecf<br />

fan beam; those most widely used can be discussed in terms of

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