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High and Low Beam

high beam
low beam
high beam
receiver
low beam
receiver
power
divider
transmitter

Figure 1: Radar antenna with high- and low- beam pattern

high beam
low beam
high beam
receiver
low beam
receiver
power
divider
transmitter

Figure 1: Radar antenna with
high- and low- beam pattern

High and Low Beam

Feeding a parabolic antenna with two superimposed horns results in two antenna directivity patterns, designated High Beam and Low Beam. This type of feed is mainly used in air surveillance radars of air traffic control and is intended to reduce the level of fixed target interference in the close range in the high beam channel.

Figure 1 shows the feed through the two horn radiators. This feed should be in phase. In practice, the horn radiator for the lower diagram (geometrically the upper horn) can be fed with more power than the one for the upper pattern. The transmit power is split by a power divider. During receiving time, both horn radiators feed separate receive channels.

Figure 2: ASR-NG with triple feeding of the antenna

Figure 2: ASR-NG with triple feeding of the antenna

It is also possible to feed only the radiator of the low beam with the transmitter’s power. This variant achieves a better maximum range at lower altitudes. Receiving is still possible with both channels. This is possible because the diagram limits in Figure 1 only show the boundaries with a level of −3 dB. With lower power components, however, both diagrams overlap almost completely. The power divider and the duplexer of the high beam horn are then omitted in the block diagram. Because the high beam radiator also receives some transmit energy directly from the low beam radiator, the input of the receiver channel, which does not have a duplexer, must be protected with a limiter.

In newer radars, a third horn radiator is even used to subdivide the high beam once again. However, this third horn radiator has the function to perform an elevation angle measurement with the primary radar using the monopulse method in comparison with the other two antenna directional patterns.