The present invention involves the use of an adaptive power injection apparatus with an existing antenna having a radiation pattern characterized by a relatively narrow main beam and a plurality of low level sidelobes for the purpose of enhancing the performance of the main antenna in a jamming evironment. The said apparatus consists of an adaptive subarray constrained to maintain a null in the direction of the main beam of the existing antenna. More specifically, the said adaptive array is composed of several individual antennae having relatively wide-beam patterns and combined with the existing antenna through individual IF stages all identical to the IF stage of the existing antenna, individual phase shifters which provide phases necessary for maximum reception from the direction of the main beam of the existing antenna and individual adjustable weights. The weights, which may consist of voltage controlled amplifiers or equivalent devices are adjusted by means of individual signals derived from correlating each corresponding auxilliary antenna signal with the combined signal at the output of the summing network and subsequently subtracting each such signal from the overall average. Setting the weights originally to have a zero average value creates a null in the direction of the main beam. Consequently, subsequent weight adjustments by means of individual adjusting signals whose combined average is zero will maintain the null in the said direction. In an MTI application, the apparatus described would leave the main beam practically intact while serving, at the same time, to mimimize interferring sidelobe power. Thus avoiding undesirable amplitude modulation of the main beam signals that would otherwise arise without the presence of the constrained null.