Relationships between squinted, sum and difference radiation patterns of amplitude monopulse antennas with mutual coupling between feeds

The relationships between the squinted beams and sum and difference beams of amplitude monopulse antennas are usually given on the basis that mutual coupling between feeds is negligible. These relationships can lead to gross conceptual errors regarding the basic capabilities of amplitude monopulse antennas. This paper develops similar relationships which include the effects of mutual coupling. It is found that the sum and difference beams are still proportional to the sum and to the difference of the squinted beams but the constants of proportionality, which are related to the VSWR's at the sum and difference ports before matching, are generally different. These constants of proportionality, or VSWR's, can be evaluated in terms of the squinted beams in the case of lossless antennas by applying the principle of conservation of energy. The matched sum pattern gain on boresight and the matched difference pattern slope on boresight are computed for a lossless antenna with squinted beams of the form
\sin k\theta/k\theta
versus squint angle. The results obtained are drastically different from previously published results. Both of these antenna parameters are maximum in the limit of zero squint angle. However, for these squinted beam shapes, a small squint angle implies that a very high VSWR is to be matched at the difference port. It is noted that a given pair of matched sum and difference patterns can be produced by different pairs of squinted beams. These pairs of squinted beams can differ in their amplitudes and shapes. However, a single pair of squinted beams corresponds to inherently matched sum and difference ports.