The Effect of the OMT in Electromagnetic Simulations of MeerKAT Primary Beams

Abstract

Until recently, full-wave electromagnetic (EM) primary beam simulation results for MeerKAT’s offset-Gregorian reflector antennas in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> -band (856–1711 MHz) differed systematically and inexplicably from high-precision field measurements. The measured beam patterns exhibit unexpected frequency- and polarization-dependent pointing offsets (squint) in both horizontal (H) and vertical (V) directions for the copolarization patterns, as well as distinctive asymmetries in the horizontal direction for the cross-polarization patterns in the upper half of the band. These effects were not predicted by design stage simulations that are based on the excitation of the fundamental waveguide mode only. Conventional large reflector simulation strategies used in the literature idealize the orthomode transducer (OMT) and waveguide and exclude this module from primary beam simulations entirely. This research shows that the inclusion of MeerKAT’s wide-bandwidth OMT and waveguide in the simulation is critical to the excitation of the TE21 mode, which results in the asymmetric features observed in the measurements. It is found that the characteristic frequency-dependent asymmetries of the primary beams are a consequence of the design of the OMT and cannot be attributed to manufacturing tolerances, reflector collimation errors, nor the coupling to conductive structures near the feed horn.