Application of frequency scan to circular arrays

The problem of obtaining focusing from a circular array together with 360-degree scanning by frequency variation is considered. It is shown that uniform variation of the line length between elements of a tapped serpentine transmission line allows good focusing from a circular array. The radiating aperture is limited to the appropriate sector of the circle by using filters in the coupling junctions between the serpentine line and the radiating elements. The relation between bandwidth and transmission-line folding factor is determined, and limitations imposed by element coupling, element spacing, and overlapping apertures at the band edges are found. Accuracy of focus is determined as a fourth-power function of aperture size relative to diameter. Aperture amplitude distribution is related to the filter characteristics and efficiency of coupling. The design of directional filters for partial coupling is described, and the effect of the transfer phase characteristic is calculated. Performance is estimated for a sample design covering the frequency range 2 to 4 kmc with a six-foot-diameter array of 100 elements.