S/X-band beam waveguide for the Onsala radio telescope

The antenna of the Onsala radio telescope is a Cassegrain antenna with a parabolic main reflector of 19.810 m diameter and a hyperbolic subreflector of 1.83 m diameter. The focal length is 8.990 m. The secondary focus is located at the apex of the paraboloid. This antenna was built primarily for radio astronomical research, but is temporarily reconfigured for geodetic VLBI by placing a S/X-band feed system in the secondary focal zone of the Cassegrain antenna. However, the old feed system can no longer be used because the secondary focus of the Cassegrain will be permanently occupied by a millimeter wave imaging array. In the present report we study a new beam waveguide feed system which will not interfere mechanically with the imaging array and which can be removed when the latter is to be operated. The new feed system consists of a S/X-band corrugated conical horn illuminating a dual-offset-reflector beam waveguide structure. One reflector deflects the received beam away from the secondary focus and the rest of the feed system is located beside this region. There will be access to it through a hole in the main reflector surface. The present paper describes the Gaussian beam synthesis and the results of PO analysis of the complete four reflector system. The present study included also an investigation of single reflector feed solutions.