Performance of the VGOS Radio Telescope with As-built Feed and Geometry Using Electromagnetic Simulations

This paper reports on electromagnetic simulations performed of the as-built VGOS (VLBI Global Observing System) 13.2-metre Radio Telescope at the Hartebeesthoek Radio Astronomy Observatory (HartRAO). Specifically, the telescope performance with a measured quadruple-ridge flared horn (QRFH) and with actual surfaces and alignment from photogrammetry, are presented from 2-14 GHz. The results are compared to an ideal Gaussian feed where the effects of the struts and primary reflector panel gaps are also characterised. The manufactured QRFH meets the specification of 50% aperture efficiency over the whole observing band and produces a minimum intrinsic cross-polarisation (IXR) of better than 15 dB. Photogrammetry results at 34 degrees elevation indicate that the telescope is structurally very rigid. The maximum main beam error within the -20 dB contour ranges between only 0.47 and 2.11% across frequency.