Electromagnetic analysis and optimization of a cryogenic receiver for VLBI applications

Receivers for radio-astronomical applications, in particular for very long baseline interferometry techniques, where a metal feed is often used to illuminate a reflector antenna, usually operates over wide bandwidths and cooled to low temperatures. In particular, the possibility of cooling not only the low noise amplifier, but also the entire feed is attractive to lower the equivalent noise temperature. However, this is usually achieved placing the feed within a Dewar, thus imposing an impact on the radiation properties of the feed itself. In addition, the feed performance can be further affected by the presence of a noise injection probe antenna, often required for calibration purposes. This paper presents the electromagnetic study of a cryogenic receiver, composed by a Quad Ridge Feed Horn (QRFH), a wideband noise injection antenna probe, and a cryogenic Dewar. The receiver works from 2 GHz to 14 GHz, intended for VLBI applications. The simulated and measured results show that a minor impact on the QRFH radiation patterns is expected from the Dewar and the probe antenna, while achieving the required coupling factor between the QRFH ports and the probe antenna.