Time-delay feed architectures for active scanned arrays

Phased arrays are inherently narrow band antennas where the scan-angle changes as a function of frequency. This characteristic is exploited in frequency scanned arrays, but with phased arrays it restricts the use of bandwidth which is needed to enhance radar capabilities and performance. The narrow bandwidth of phased arrays is due to phase being controlled for the antenna aperture excitation rather than time-delay, and steered beams will scan towards broadside as the frequency is increased, and thus move off the target. For frequency independent beam-steering, the electrical path-lengths (and hence time-delays) from the feed to each radiating element and from there to the desired equiphase front, has to be the same. With time-delay steering, the only bandwidth limitation is that the spacing between elements should not exceed about /spl lambda//2 at the highest frequency to avoid grating lobes. Other bandwidth limitations are due to components and matching. The architectures described make multiple use of switched digital time-delay circuits and thereby significantly reduce the complexity of a system. Various possible methods are discussed and a figure of merit is derived which is the factor by which the total switchable time-delay is reduced from that of a conventional time-delay system. The methods of making multiple use of switched time-delays introduce losses and are therefore mainly applicable to active arrays, where the losses can be compensated by amplification of both transmitted and received signals.