Abstract
A novel type of interferometer has been developed for precision measurements in microwave radiometry. The system employs two low-noise heterodyne receivers based on SIS (superconductor–insulator–superconductor) tunnel junction mixers in the band from 40 to 50 GHz. The rectangular horn antennas have adjacent apertures and are coaligned; the IF outputs of the receivers are multiplied together. The resulting antenna interference pattern has lobes which lead to positive output signals and lobes which give negative output. The radiometer measures the difference in the power from these lobes without beam switching or instrument motion. Other major features are: the cancellation of instrumental signals (horn emission, sidelobe pickup, etc.); very low 1/f noise in the output; and zero output signal when viewing a uniform source. The system achieves a sensitivity of ΔTrms =14.4±1.4 mK/(Hz)1/2 and is used to search for anisotropy in the cosmic background radiation on angular scales between 1° and 5°.
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