The effects of mounting plates and horn chokes in feeds for reflectors in space applications

In many space reflector applications the desired radiation characteristics are stringently specified. A common feature of a reflector feed assembly is the use of a mounting plate to which the horn is attached. As is demonstrated in this paper, the mounting plate can have a significant, and possibly detrimental, impact on the primary pattern, and thus, the secondary pattern. In the traditional feed models, structures to the rear of the feed, such as the mounting plate, are ignored because the analysis techniques do not allow for the inclusion of their effects. For this work, a pyramidal feed horn is modeled using a two dimensional method of moments program. Patterns in the elliptical plane cuts are generated and these patterns are combined to form a three dimensional pattern. This approach, while having the limitation of being based on a two dimensional analysis, has the advantage of rigorously including the effects of structures external to the horn. In this paper, the effects of a mounting plate on the primary pattern of a pyramidal horn, and on the associated secondary pattern, are presented. It is demonstrated that a mounting plate can have significant impact. Also, a quarter wavelength choke in the E plane is analyzed which acts to mitigate the effects of the mounting plate.