VSWR reduction for large millimeter-wave cassegrain radiotelescopes

Multiple reflections in large radiotelescopes used for astronomical spectroscopy cause characteristic modulations of the observed spectrum (“baseline ripple”). For a given mechanism, the magnitude of the effect depends primarily on the reflection coefficient, which for the most important paths is proportional to λ. Although ripple is thus generally negligible at mm-wavelengths, there are some instances where it may still be significant, and a recent experiment at the15 m James Clerk Maxwell Telescope was severely affected. We describe a technique which has proved effective in reducing the ripple to an acceptable level. The purpose of this research note is to draw this technique to the attention of the astronomical community. We calculate the reflection coefficient for a typical instrument and show that it can be greatly reduced over a very broad bandwidth by means of a tapered absorber in the centre of the secondary mirror. The graphical analysis of the tapered absorber also demonstrates that the scattering cones and circular absorbers widely used in radioastronomy donot in fact reduce the reflection coefficient significantly.     

Reflection and cross-polarization properties of grooved dielectric panels

The cross-polarization and reflection properties of grooved dielectric windows are calculated. Measurements on two windows in the range 8-12 GHz are found to be in good agreement with the theory. It is shown that cross-polarization is substantially reduced if the grooves on the front and back faces of the window are orthogonal to each other.     

An experimental submillimetre heterodyne array receiver

We describe an experimental array receiver intended for radioastronomical observations of the J=3→2 line of carbon monoxide. It consists of six indium antimonide mixers cooled to ～2K in a pumped helium cryostat, with rectangular feed horns arranged on a 3×2 grid at the f/9 focus of the UK Infrared Telescope (UKIRT). Each detector is mounted in a length of fundamental mode waveguide and has an independently tunable backshort. Local oscillator power is provided by a frequency tripler pumped by a klystron, and is coupled to the array via a “lossless” quasi-optical coupler which is described in detail. The beam patterns and performance of the coupler have been measured and are in good agreement with theory.     

A dual-polarization InSb receiver for 461/492 GHz

A dual-polarization InSb hot-electron bolometer-mixer receiver has been built for the James Clerk Maxwell Telescope, for operation at 461 and 492 GHz (the frequencies of theJ=4→3 rotational transition of CO and of the³ P ₁→³ P ₀ transition of neutral carbon). Receiver noise temperatures of 500K have been obtained at 461 GHz, in observing bandwidths of 3 MHz. The receiver was designed as a “common-user” or facility instrument. Here we describe those aspects of the design and construction which enabled this goal to be realized.     

Knowledge integration using problem spaces: A study in resource-constrained project scheduling

The need to develop schedules for projects with resource constraints and cash flows arises in organizational settings ranging from construction planning to research and development. Given the intractable nature of the problem, a variety of knowledge sources relevant to the project scheduling task have been identified in the Operations Management literature. These include a large number of heuristic procedures that can be used to generate feasible project schedules as well as recent neural network-based approaches that can select appropriate heuristic procedures to apply to a specific instance of the project scheduling problem. While integrated application of these knowledge sources is required to effectively support scheduling, previous work has focussed on developing and implementing them in isolation. The problem space computational model presented in this paper addresses this shortcoming by integrating these various knowledge sources, thus enabling the development of decision support systems for resource constrained project scheduling. More generally, the modeling approach used in this paper can be applied to create systems to assist knowledge intensive tasks that arise in many organizational settings.     

Phase centers of horn antennas using Gaussian beam mode analysis

The power of Gaussian beam mode analysis to accurately describe the propagation of electromagnetic beams and the location of a horn antenna phase-center is illustrated. By way of example, the case of a compensated pyramidal horn fitted with fins to produce a less abruptly tapered E-plane field distribution is discussed, and the results obtained are compared with those from an alternative method published in the literature. Excellent agreement is found     

Lenses for wideband corrugated conical horns

A dielectric lens, designed to be fed by a wide-angle corrugated conical horn, is described. The radiation pattern of the horn-lens combination is similar to that of a much longer conical horn while retaining wideband characteristics.     

Focal-plane and aperture-plane heterodyne array receivers for millimeter-wave radioastronomy—A comparison

In this paper we consider the maximization of the throughput of a single large antenna, for two possible array configurations: focal plane imaging arrays and aperture plane phased arrays. We discuss trade-offs between the two types of array in terms of field of view, sampling efficiency and time to map, a source. We also discuss limits on the number of feed elements in an imaging array imposed by the deterioration in aperture efficiency off-axis.     

Optical design of a dual-polarization receiver for 220–280 GHz

A 220–280 GHz dual polarization receiver has been built for the James Clerk Maxwell Telescope. Schottky diode mixers cooled to ～15K by a closed-cycle refrigerator are used to give DSB noise temperatures of 300K and 420K in the two channels. The optical design is based on gaussian-beam optics, and is frequency independent; it allows the significant higher order gaussian modes to propagate unhindered, thus offering the prospect of very high aperture efficiency. The receiver includes a number of novel optical components, including a completely symmetric dual polarization Martin-Puplett interferometer, used as the L.O. injection diplexer; a dielectric waveplate used as an in-line variable polarization splitter; and a dual-polarization in-line tunable Fabry-Perot SSB filter. Measurements of the performance of the optical system are presented.     

Gaussian mode analysis of Cassegrain antenna efficiency

Gaussian mode analysis is a convenient way to characterize long focal length systems, such as the Cassegrain antenna at the secondary focus. We use multimode Gaussian optics to derive several interesting results concerning the aperture efficiencies of Cassegrain antennas fed by corrugated conical horns. The highest efficiency is obtained when the antenna is illuminated by a wide-band, wide-angle horn which has its phase center at the secondary focus, or equivalently by a narrow-band diffraction limited horn with its phase-center located at a confocal tertiary focus. In both cases this corresponds to placing the horn aperture at a position where the incoming fields are frequency-independent and have limited spatial extent. We generalize these results to show that a true image of the aperture distribution can always be formed on the spherical cap in the aperture of any arbitrary scalar horn, by using a single refocusing tertiary lens or mirror. Since both the incoming fields and the horn aperture fields on this surface are frequency independent, it follows that the aperture efficiency is also independent of frequency with either of these arrangements. We suggest that devices for shaping the aperture illumination, such as shaped lenses or dual shaped subreflector systems, should, in general, be installed near an image of the telescope aperture distribution.