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In this paper we describe a method of obtaining nulls in a phased array system using sub-array based phase-only nulling concepts. Phase-only nulling in phased-array antennas is appealing because the phase shifters can be employed for the dual purpose of beam-steering and nulling of unwanted interference such as jamming, radio frequency interference and clutter. The technique is applicable to transmit as well as receiver nulling in the situation when the amplitude control is not available. The application here is on transmit nulling for jammers decoying and surface clutter suppression in the upper beams. Instead of phase of the entire array, a small set of sub-arrays on the periphery of the array are considered. This implementation allows minimal modification of the phase control structure of the phased array system. Phase-only nulling also has an obvious advantage for transmitting nulling if we want to weight uniformly for maximum output. The problem is solved by formulating it as an optimization problem with side constraints. The method is quite general and can be applied to symmetric as well as non-symmetric arrays with real or complex beams. Based on the analysis the system is then designed and results illustrated for a case similar to the proposed advanced Aegis system. The system simply consists of sub arrays which can be part of the main array clustered together, each cluster receiving only a single phase-change instruction for nulling in presence of jamming, in real time, involving very minimal computation, once the location of jammers are determined. The null-depths achieved shows a performance of about 50 dB's or more. Presently there are many radars which can b represented as circular arrays, with minor modifications, can be retrofitted with hardware for nulling in presence of multiple jammers or to suppress clutter.
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