17–27 GHz Dual Horn Receiver on the NASA 70 m Canberra Antenna
A dual beam, dual polarization, low noise receiver has been installed at a Cassegrain focus of the NASA 70m antenna near Canberra, Australia. It operates in five pairs of 1GHz bands from 17 to 27GHz simultaneously. The receiver temperature measured at the feed is 21–22K at 22GHz and, during dry winter night-time conditions, zenith system temperatures as low as 35K have been observed in the 21–22GHz band. The native polarization is linear but can be converted to circular prior to down-conversion. The downconverters have complex mixers, followed by quadrature hybrids which can be bypassed or used to convert the quadrature phase channels into an upper and lower sideband, each 1000MHz wide. For spectroscopy, four ROACH1 signal processors each currently providing 32K channel spectra across four 1000MHz bands, for 0.4km/s velocity resolution at 22GHz. Using both beam- and position-switching, the receiver achieved a noise level of 5mK r.m.s. in an hour of integration and 31kHz resolution. The NASA 70m antennas have a 45 arcsec beamwidth at 22GHz and an aperture efficiency of 35.5% giving a sensitivity of 0.49K/Jy.