Beam broadening of a monopulse tracking antenna by feed defocusing

An investigation of defocused beam broadening by feed displacement in a parabolic reflector and its potential use as an acquisition means in a monopulse tracking antenna system is presented. The emphasis of the investigation is in the amplitude and phase characteristics of the sum and difference patterns as a function of both feed displacement and sum pattern taper. The investigation, which was performed both analytically and experimentally, shows that when feed defocusing is employed to achieve a beamwidth increase in excess of five times the defocused beamwidth, multiple nulls form in the difference pattern. Since these nulls are positioned relatively close to the beam center, acquisition is still possible under these conditions and the resulting tracking error should be tolerable for most telemetry and communication applications.