Wideband Cryogenic Receiver for Very Long Baseline Interferometry Applications

One of the critical points concerning radio-astronomy applications regards the power received by the radio telescope, most often extremely faint. Beyond approaches aimed at increasing the collecting area of the radio telescope (at microwave frequencies almost always a reflector antenna), such as for example the use of very long baseline interferometry (VLBI) techniques, a key point is the installation of cryogenic receivers to reduce the system noise temperature. To achieve the lowest noise temperature, it is desirable to cryogenically cool also the feed of the reflector antenna. This is achieved by enclosing the feed in a Dewar to provide the required thermal isolation between the feed and the external environment. However, this enclosure inevitably affects the radiation properties of the feed. This letter presents the development of a cryogenic receiver, composed of a 2-14 GHz quad ridge flared horn and a Dewar, for a notable case where the Dewar is designed to be as compact as possible to minimize not only the thermal and mechanical load, but also the illumination blockage. Indeed, the receiver is intended to be installed on a 12 m reflector antenna for VLBI applications, whose optics require an installation of the feed itself very close to the subreflector. A prototype of the cryogenic receiver has been fabricated and measured, discussing the impact of the Dewar on the feed radiation patterns, input matching, isolation, antenna efficiency, and showing the overall system noise temperature.