Abstract: A radar receiver has a main channel 2,3 and 4, and an auxilliary channel 7,8 and 12. The main channel is fed from a directional antenna 3 and the auxilliary channel from a relatively non-directional antenna 7. A side lobe canceller consisting of components 5,6 and 9 receives signals from the main and auxilliary channels and produces a correction signal D which is applied to the main channel so as to cancel signals received via the side lobes of the antenna 3 and therefore attributable mainly to jamming signals.In order that the signals B' fed from the main channel to the side lobe canceller should be free from clutter, as is necessary for the side lobe canceller to operate effectively, a moving target discriminator 11 is used so as to suppress any signal components due to stationary and slow moving objects, including any clutter features. In order to compensate for any frequency filtering and phase change effects of the circuit 11 a similar circuit 12 is inserted in the auxilliary channel.