A sidelobe canceller (SLC) can be efficiently employed to reduce the effects of jammers on a radar system. The paper deals with the evaluation of SLC performance, taking into account the estimation errors of the weighting coefficients of the auxiliary antennas. An explicit formula for the cancellation is given as a function of the ratio of the closed-loop bandwidth of the adaptive circuits and the radar receiver bandwidth, for one (single-sidelobe canceller, SSLC) auxiliary antenna and two (double-sidelobe canceller, DSLC) auxiliary antennas of the adaptive array and for one jammer. Therefore a relationship between the system adaption speed and the accuracy in the estimation of the jamming signal is obtained. A comparison is made between the performance of the two systems. From this analysis, a suitable selection of circuit parameters is suggested. Extensions are also made for any number of auxiliary antennas and for more than one jammer.