This invention relates, in general, to radar antennas and, more specifically, to systems for shaping the beam of array antennas.
Array antenna systems are used in a wide variety of radar systems. Search radar systems, in particular, use array antenna systems which have a narrow beamwidth in the azimuth direction and a broad beamwidth in the elevation plane. Normally, the antenna is rotated mechanically in azimuth while the broad elevation beam sweeps the area for incoming signals. Beamwidths in the elevation plane are frequently between 20 and 60 degrees with such radar systems.
In order to improve the operation of the search radar system, it is desirable to change the frequency of operation of the radar system while the antenna is scanning. A bandwidth of 10% of the operating frequency is a typical requirement with such systems. However, changing the frequency presents problems in maintaining the desired beamwidth of the antenna. It is well known that a fixed set of signals having a particular phase and amplitude must be applied to the antenna elements in order to produce a desired pattern. Traditionally, it has been conventional practice to construct the microwave power dividers which feed the individual antenna elements in such a manner that they perform their function independently of the frequency of operation of the antenna. Unfortunately, even with maintaining the same phase and amplitude at the antenna elements, the beamwidth or beam pattern of the antenna changes with frequency simply because the physical size of the antenna and the distance between the antenna elements is a different number of wave lengths at each of the operating frequency.
It is also standard practice to construct such an antenna with components which provide the proper beamwidth at the highest frequency of operation to ensure full coverage. However, such design techniques cause the beamwidth to be larger and the overall gain to be lower when the antenna is operated at lower frequencies than the design frequency. Ideally, it is desirable, and it is an object of this invention, to provide an antenna system having a substantially constant beamwidth over a wide frequency range of operation, and to provide this capability with the least amount of apparatus.