22 GNz VLBI receiver at the Metsÿhovi Radio Research Station

A dual channel, cooled low noise receiver at 22 GHz signal frequency has been designed and constructed. The noise temperature of the whole receiver measured in front of the feed horn is almost equal for both channels i.e. 60 K. The local oscillator system based originally on a 10 GHz MESFET DRO-stabilized oscil lator is phase locked using a highly stable 5 MHz maser frequency standard. The receiver has been already used successfully for several VLBI-observations at the Metsahovi Radio Research Station. The receiver consists of two separate parts: a cryogenically cooled front end containing a polarizer and two low noise HEMT preamplifiers and a dual channel, SSB-receiver operating at room temperature. The block diagram of the front end in a dewar (constructed of 15 mm thick aluminium sheets with glue joints) is shown in Figure 1. For cooling a helium refrigerator CTI Model M22 cold head and an air cooled compressor Model 8200 with cooling capacity of 1 W at 20 K is used. The signal is fed to a corrugated feed horn with a teflon lens. The horn mounting consists of a O-ring and it is rigidly screwed to the front wall of the dewar, the lens is attached to the surface of the horn with a vacuum tight connection. The polarizer input is a standard rectangular waveguide WR-42, thus a transition from a circular to rect angular waveguide is also needed.