Leakage Rejection in Beam-Switched CW Radars

This paper is directed to the problem of transmitter-receiver leakage rejection in CW and FMCW radars which employ stepped-scanning antennas. An IF carrier-elimination filter has been devised and analyzed which provides leakage-rejection capabilities limited only by leakage variations during stationary beam positions. Leakage rejection is provided by coherently reducing the signal to zero IF, rejecting the leakage components by a combination synchronous switch and high-pass filter, and remodulation onto a second IF carrier. The filter operates in a linear manner. Positive and negative senses of Doppler signals are retained. The spectrum of the input signal is faithfully reproduced in the output with the exception of a sharp notch at the input carrier frequency. In one configuration, the minimum notch bandwidth is limited by the on-time of each beam, and is approximately equal to one-half the reciprocal of this on-time. A more complex arrangement removes this restriction, permitting a minimum notch width which is limited only by the cutoff frequency of a high-pass RC filter. The simpler configuration has been used with the AN/APN-118 Doppler Navigation Radar, a system which was developed under contract with the U. S. Signal Corps, Fort Monmouth, N. J., by the Sperry Gyroscope Company.