NONCONTACTING MICROWAVE COUPLING TO A CRYOGENIC GRAVITATIONAL-RADIATION ANTENNA

Gravitational radiation detectors must detect extremely small displacements, and thus require elaborate vibration isolation systems to attenuate the surrounding environmental noise. Contacting electrical leads can provide a direct path that will short circuit the vibration isolation, and degrade the antenna Q factor. Noncontacting microstrip transceivers provide a practical means of electromagnetically coupling to a resonant bar gravitational antenna when a microwave transducer is used as a signal readout system. Two microwave patch antennas now operate as transceivers on the University of Western Australia's 1.5 tonne niobium bar gravitational radiation antenna and are described in detail in this paper. This system allows improved vibration isolation without degradation of the acoustic Q factor of the niobium bar. Combined with a cryogenic amplifier, this system should achieve a reduction of 57 dB in the series noise power. This corresponds to a factor of 700 reduction in the effective displacement noise when referred to the transducer, from 2 X 10(-16) to 3 X 10(-19) m/square-root Hz.