Step-scanned circular-array antenna

A circular-array radar antenna designed for operation over a 20-percent bandwidth has demonstrated improved performance over that of linear arrays. A major advantage is that the beam pointing angle is independent of frequency. The array developed and tested provides 128 beam positions with a beam crossover of about -2 dB, a half-power beamwidth on the order of 4 or 5 degrees, and a sidelobe level of -25 dB. Step increments of one element per beam position allow practical implementation of step scanning, i.e., simple commutation of the current distribution. Two approaches are possible for feeding and scanning the ring array: 1) a vector-transfer system involving 3-bit phase and amplitude boards which are capable of amplitude attenuation (VT scan) and 2) a lens feed system employing an parallel-plate region, with the spacing between the plates less than 1/2 a free-space wavelength, so that only the electric-field component perpendicular to the plates is propagated. Energy is launched and extracted from the lens by means of monopoles mounted 1/4 wavelength in front of the circumferential ground plane enclosing the lens. Computations involved in determining current distribution are based on [1] and [2].