An antenna for limited scan in one plane: Design criteria and numerical simulation

An analytical and numerical study has been performed on a novel design scheme for an antenna system for limited one-dimensional scan. The system has a number of control elements approximately equal to the minimum theoretically compatible with the aperture size and field of view (FOV). The radiating structure consists of a "bootlace" lens with linear outer and circular inner profiles. This geometry plays a basic role in determining excellent scan performance over a moderate frequency band. A linear array whose size depends critically upon the scan requirements and the lens focal length is located on the focal plane and is focussed onto the inner lens profile. The array is fed by a hybrid network (HN) performing a spatial Fourier transformation. The input ports of the HN are fed by the output ports of a beam forming network (BFN) through a set of variable phase shifters. The BFN has separate input ports for the sum and difference patterns, controlled independently, The system works as follows. The antenna illumination is synthesized as the weighted superposition of components illuminations or "overlapping subarrays" each of which is due to the excitation of one of the ports of the HN. The amplitudes of the subarray excitations are fixed and determined by the power divisions provided by the BFN. Their phases are controlled by the set of variable phase shifters. A desirable feature of the scheme is that for a fixed phase shifter setting neither the beam scan nor its width changes for a moderate frequency variation. Through a suitable design of the BFN, ultralow sidelobes outside the FOV can be achieved at the expense of a slight reduction of the illumination efficiency, which is, however, always high, since the aperture is fully used. Extensive numerical computations for an antenna having a half-power beamwidth of
1.2\deg
show that the sum beam can be scanned in a sector greater than
\pm3
beams, on a band of 20 percent with excellent performance form the viewpoint of gain and sidelobes-the scan sector being slightly less for the difference beam.