VARIABLE BEAMWIDTH REFLECTOR ANTENNA BY FEED DEFOCUSING

The beamwidth of a reflector antenna may be varied by defocusing the f eed. The radiation characteristics are studied for both axisymmetric a nd offset single reflector antennas using both geometric optics for un derstanding the operation and physical optics for a parametric study. The beamwidth may be increased by over 200% compared to the focused re flector before the main beam bifurcates, but the on-axis gain is reduc ed. Better performance is obtained with an offset reflector antenna. T he study quantifies the limits of beam broadening which can be achieve d. Measurements on a 30 GHz experimental antenna show good agreement w ith theory.