Optimum feeds for all three modes of a monopulse antenna I: Theory

In a monopulse antenna for use in a tracking radar, the requirements for the sum and difference modes are not the same. For the amplitude-comparison type of monopulse having an antenna whose main aperture is illuminated by a feed, these independent requirements can be met by a feed designed to fulfill two conditions. One is that excitation of the feed aperture in the difference modes be effectively about twice as wide as in the sum mode. The other is that the shapes of all the feed excitations be free of any avoidable irregularities. The amount of improvement available when these conditions are met depends on the optimization point desired for each mode, as well as the design of the reference antenna. In a typical case, the gains and slopes in the difference modes should increase by several db. Furthermore, the near sidelobes and the spillover radiation in the difference modes should decrease by an order of magnitude. A small increase of gain in the sum mode is also available, and finally, the criticalness of positioning the feed may be appreciably reduced.