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Large ground-based radio telescopes are increasingly being equipped with multibeam feed systems to enable simultaneous observation of a larger part of the sky. The aim of the One Centimetre Receiver Array-f project, part of the European Commission funded FARADAY Programme, was to develop a high-quality science instrument to survey the sky at 1-cm wavelength, while developing technologies that would be applicable to future development of systems with 100 beams or more. A complete ten-beam Ka-band radiometer system is described, together with measured results at cryogenic temperatures. Performance compares well with that obtained with similar (non monolithic microwave integrated circuit) instruments produced for the Wilkinson Microwave Anisotropy Probe and Planck satellites. Index Terms--Cryogenic, InP, Ka-band, monolithic microwave integrated circuit (MMIC), radio astronomy, radiometry.

The PLANCK mission of the European Space Agency is devoted to produce sky maps of the cosmic microwave background radiation. The low-frequency instrument is a wide-band cryogenic microwave radiometer array operating at 30, 44, and 70 GHz. The design, test techniques, and performance of the complete differential radiometer at 30 GHz are presented. This elegant breadboard 30-GHz radiometer is composed of a front-end module (FEM) assembled at the Jodrell Bank Observatory, Cheshire, U.K., and a back-end module assembled at the Universidad de Cantabria, Cantabria, Spain, and Telecomunicacio, Universitat Politecnica de Catalunya, Barcelona, Spain. The system noise temperature was excellent, mainly due to the very low noise performance of the FEM amplifiers, which achieved an average noise temperature of 9.4 K. Index Terms--Cosmic microwave background (CMB), cryogenic, low-noise amplifier (LNA), microwave radiometer, PLANCK low-frequency instrument (LFI).