A sidelobe blanking system is provided which uses as much of the existing radar tracking system as possible while requiring a minimal number of additional components. The sidelobe blanking system includes a guard antenna and two phase shifters the first of which is operably coupled to the main radar antenna's sum pattern. The phase shifters create signals of opposite polarities which represent the complex conjugate of the sum pattern from the main radar antenna. The system includes two summing circuits which add the signal received by the guard antenna to the complex conjugate signals of opposite polarity. The system also includes a phase detector which compares the outputs of the first and second summing circuits and generates a signal representative of the cosine of the relative phase angle. The polarity of the output of the phase detector is determinative of whether or not the target being detected is in the sidelobe region or the mainlobe region and is independent of any phase error between the main and guard antennas.