Experimental radio cosmology

Abstract:

We describe the design and performance of an easy to machine horn which exhibits excellent beam circularity and low cross polarisation over a relatively large bandwidth. No grooves are machined into the horn walls but, alternatively, flare angle discontinuities are generated along the horn profile. In other words, the horns will have several flare angles or sections instead of one. For example, if the horn consists of two flare angles, it could then be considered as a conventional Potter horn. As can be seen below, even with this simple design, excellent radiation patterns can be obtained over 15% bandwidth. The bandwidth could be further increased by adding more subsections with 30% bandwidth obtained when the profile is based on 4 sections. The operation of the horn is based on generating higher order modes at the correct amplitude ratio and phase with respect to the incident TE11 mode in the circular waveguide, which is achieved by accurate determination of the magnitude and location of the flare steps. This in turn yields a field distribution at the horn aperture that has low sidelobes and cross polarization in the radiation pattern. A key component in the design package is the optimization software that searches for the correct magnitude and location of the flare discontinuities. We have generated a software package based on the combination of modal matching, genetic algorithm (GA) and simplex optimization. The genetic code is first used to locate the proximity of the global minimum. The set of parameters obtained are then used as a starting point for the simplex method, which refines the parameters to the required accuracy. We shall illustrate our method by showing radiation patterns using two and three step discontinuities and also patterns for a spline profiled horn based on work by other investigators who used different optimization techniques.

Abstract:

We describe the objectives, design and predicted performance of Clover, a fully-funded, UK-led experiment to measure the B-mode polarisation of the Cosmic Microwave Background (CMB). Three individual telescopes will operate at 97, 150 and 225 GHz, each populated by up to 256 horns. The detectors, TES bolometers, are limited by unavoidable photon noise, and coupled to an optical design which gives very low systematic errors, particularly in cross-polarisation. The telescopes will sit on three-axis mounts on a site in the Atacama Desert. The angular resolution of around 8 ́ and sky coverage of around 1000 deg2 provide multipole coverage of 20<ℓ<1000. Combined with the high sensitivity, this should allow the B-mode signal to be measured (or constrained) down to a level corresponding to a tensor-to-scalar ratio of r = 0.01, providing the emission from polarised foregrounds can be subtracted. This in turn will allow constraints to be placed on the energy scale of inflation, providing an unprecedented insight into the early history of the Universe.

Abstract:

We describe the design and performance of Potter horns at millimetre and submillimetre wavelength employing a novel software package that we have developed, using Genetic Algorithm. The horn is easy to fabricate and exhibits excellent beam circularity and low cross polarization over a 15% bandwidth which is sufficient for many applications. Excitation of the required higher order modes is done by either a step or a flare discontinuity at the horn throat. In each case we provide design curves that give the optimum parameters of the horn geometry as a function of frequency and beamwidth. The range of values provided covers the parameters required for the design of horns for telescope feeds and various other instruments. The design curves show clearly that the flare-step performance is superior to the traditional groove-step Potter horn. The simulations for designing these horns were carried out at millimetre and submillimetre wavelengths but the results can be scaled to lower or higher frequencies. A key component in the design method is the optimization software that searches for the correct magnitude and location of the flare discontinuities. We have developed a software package based on the combination of modal matching, a genetic algorithm (GA) and downhill simplex optimization. The genetic code is first used to locate the proximity of the global minimum. The set of parameters obtained are then used as a starting point for the simplex method, which refines the parameters to the required accuracy. © 2007 Springer Science+Business Media, LLC.

Abstract:

We present the design and test results of a compact C-band orthomode transducer which comprises four rectangular probes orthogonally arranged in a circular waveguide, designed to work in the WG13 band. Measurements of the system in the frequency range 4.64 GHz to 7.05 GHz agree very well with simulation results and show a cross-polarisation level below -58 dB, a return loss of about -20 dB, and an insertion loss difference of less than 0.18 dB between the orthogonal polarisation modes across the full waveguide band.