A linearly polarized rectangular dual-mode horn with a corrugated excitation region

Rectangular horns are commonly used to feed elliptical reflectors. However, it is a problem that conventional rectangular horns have very different aperture distributions and, hence, radiation pattern shapes in the E- and H-planes. The E-plane pattern has high sidelobes or shoulders within the main beam, which causes high spill-over lobes when feeding the reflector, whereas the side lobes in the H-plane are low. The purpose of the present paper is to present a linearly polarized rectangular horn that also has low side lobes in the E-plane. The side lobes in the E-plane will be reduced if the aperture distribution is changed from a uniform distribution to a tapered one like in the H-plane. The present paper is the result of a study of different approaches to obtain a tapered aperture distribution in the E-plane for a horn that must have a physical width of about 2.7 /spl lambda/ or smaller and a beam width between the /spl sim/9 dB points of 40/spl deg/ in the E-plane. The required bandwidth is 8.8%. The study considers a horn with corrugated E-plane walls, a horn with partially corrugated E-plane walls, a dual-mode horn excited by a step in the E-plane and a dual-mode horn excited by an E-plane corrugated section. The last design gave the most satisfactory performance and is documented in the present paper. The study was made by using approximate waveguide mode theories for the coarse design and a commercial mode matching program, CORREC, for the optimization.<>