Frequency-independent amplitude difference patterns in broadside arrays

A technique is presented for obtaining a frequency-independent amplitude-difference pattern from two broadside arrays. Broadside arrays have a beamwidth that varies as a function of frequency. This frequency dependence causes an equivalent frequency dependence in the amplitude-difference pattern generated by the pair of antennas. To achieve the desired effect, the pointing of each array beam is made frequency dependent. This is accomplished by applying an arithmetically progressive frequency-independent phase shift to the array elements. An additional degree of freedom can be gained by also applying a frequency-dependent phase shift to each array. This permits the adjustment of the amplitude difference pattern to a particular slope. This technique was applied to an -band antenna system consisting of two arrays of three cavity-backed planar spiral antennas. The corrected and uncorrected amplitude-difference patterns obtained from a computer calculation are presented. These results are then compared with measured data obtained from the -band antenna system.