Experimental radio cosmology

Abstract:

We describe a novel heterodyne interferometer currently under construction at Oxford. The instrument employs new techniques in heterodyne interferometry, with the aim of achieving very high brightness sensitivity in the millimetre band. It is a single-baseline tracking interferometer for operation in the frequency range 185-275 GHz with two 0.4m offset parabolic antennas separated by a 0.5 m baseline. Each antenna feeds an SIS mixer with a 2-20 GHz IF band, driven by a phase-switched LO source. The IF signals from the mixers are processed by a 2-20 GHz analogue complex correlator. The primary science goal of this instrument is to measure the spectrum of the Sunyaev-Zel'dovich effect in galaxy clusters. In particular we intend to measure the frequency of the S-Z null near 217 GHz, which allows the gas temperature of the cluster to be determined. Measuring the spectrum of the S-Z effect requires very high brightness sensitivity with moderate spatial and spectral resolution.

Abstract:

We present the design of a 230 GHz finline SIS mixer with a 2-20 GHz IF bandwidth. The mixer is intended for use in a prototype high brightness sensitivity, low spatial resolution heterodyne interferometer currently under construction at Oxford[1]. The sensitivity of the instrument will be sufficient for measuring the spectrum of the Sunyaev-Zel'dovich effect in the brightest galaxy clusters. The mixer design is based on a previously reported 230 GHz finline mixer design[2], with a number of improvements and features added to achieve the very demanding IF bandwidth requirements. An RF bandpass filter is included on the chip to isolate the IF signals from the finline transition, and the mixer tuning circuits, RF choke and IF connections have been carefully designed to exhibit very low parasitic reactances in the IF band. The first two batches of these mixers were recently fabricated at Cologne University, and are currently being tested.

Abstract:

We describe the design and testing of an easy-tomachine smooth-walled horn which exhibits excellent beam circularity and low cross polarisation over a relatively large bandwidth. The design comprises three coaxial conical sections and two flare discontinuities joining the three sections together. The discontinuities generate appropriate higher order modes which combine to give a circular field distribution at the aperture. The positions and sizes of these discontinuities were calculated using a genetic algorithm. The horn was fabricated either by using the well known electroforming method or simply by a drill tool, shaped into the horn profile, and a standard mill. The measured radiation patterns or the electroformed horns show good excellent circularity and agree well with the calculated curves. They also show that the three-section horn has a substantially wider bandwidth than the conventional Potter horn. Preliminary measurements of the drilled horns patterns are also shown and compared with theory.