Dirk I. L. de Villiers

Design of a Ten-Way Conical Transmission Line Power Combiner

Axially symmetric power combiners, such as radial line and conical line combiners, are very effective in combining the output signals from a large number of power amplifiers over a wide band with low losses. The main problem with radial lines is the behavior of the characteristic impedance against radial distance, which makes design of radial combiners difficult and normally optimization based. In this paper, a step-by-step design procedure is presented for the design of a conical line combiner. The design strategy relies on the transverse electromagnetic properties of the conical line to eliminate the need for complex full-wave optimization in the design process. Circuit models are instead employed and optimized to achieve a wide matched bandwidth. A ten-way prototype was developed at X-band, which displayed more than an octave matched bandwidth with low insertion loss

Accurate Beam Prediction Through Characteristic Basis Function Patterns for the MeerKAT/SKA Radio Telescope Antenna

A novel beam expansion method is presented that requires employing only a few Characteristic Basis Function Patterns (CBFPs) for the accurate prediction of antenna beam patterns. The method is applied to a proposed design of the MeerKAT/SKA radio telescope, whose antenna geometry is subject to small deformations caused by mechanical or gravitational forces. The resulting deformed pattern, which is affected in a nonlinear fashion by these deformations is then sampled in a few directions only after which the interpolatory CBFPs accurately predict the entire beam shape (beam calibration). The procedure for generating a set of CBFPs—and determining their expansion coefficients using a few reference point sources in the sky—is explained, and the error of the final predicted pattern relative to the actual pattern is examined. The proposed method shows excellent beam prediction capabilities, which is an important step forward towards the development of efficient beam calibration methods for future imaging antenna systems.

The design of the MeerKAT dish optics

This paper highlights some of the trade-offs as well as the implications of the selected options during the design of the MeerKAT radio telescope antenna optics. The purpose is to achieve the best possible performance for the physical system at given cost.

Towards an optics design for the SKA

This paper describes the process to derive the SKA optics. The outcome of this process will be both the dish optics and optimised feeds (only the illuminating part, for example feed horns up to the waveguide port) for each of the respective frequency bands.

Assessment of the sensitivity of the South African KAT-7 and MeerKAT/SKA radio telescope reflector antennas

This paper assesses and compares the sensitivities of the antenna gain, cross-polarization and sidelobe levels with respect to feed and/or subreflector displacements due to estimated operational conditions as well as manufacturing tolerances. The probabilistic assessment is evaluated with the Unscented Transform, deemed computationally more efficient than other similar techniques, and employs physical optics computer simulations performed with the WebPRAC and GRASP computer codes. The procedure is applied to the KAT-7 prime focus parabolic reflector antenna, and to a proposed design for the MeerKAT offset Gregorian reflector antenna.

Deriving an Optimum Mapping Function for the SKA-Shaped Offset Gregorian Reflectors

Of primary importance for a radio telescope is its receiving sensitivity. The offset Gregorian reflector antennas, as used for the Square Kilometer Array (SKA) radio telescope, can be shaped to make efficient use of the reflectors aperture and thus to maximize sensitivity for a given sidelobe level. The shape of the reflector is controlled by a mapping function. This paper presents a method to derive the optimum mapping function given a selected set of single pixel feeds. In this case, the feeds were optimized for a geometrically similar classical conic section (unshaped) reflector system. Realistic performance results are given for reflector systems with various subreflector sizes, extensions, and subtended angles.

Offset Dual-Reflector Antenna System Efficiency Predictions Including Subreflector Diffraction

A simple analytical formula for the prediction of diffraction efficiency in offset dual-reflector antenna systems is presented, and its performance evaluated by comparison to commercial software physical optics and method-of-moments simulations. The formula only requires knowledge of the symmetry plane physical configuration of the reflectors and an analytical approximation of the feed radiation pattern, and is therefore very straightforward to implement and use as a design aid. Some example applications of the formula are presented and discussed.

Rapid Calculation of Antenna Noise Temperature in Offset Gregorian Reflector Systems

Antenna noise temperature calculations of reflector systems is often a slow process and makes direct optimization of the sensitivity of these systems a difficult and time-consuming task. This paper presents an improvement to a recently proposed method to speed up these calculations by several orders of magnitude for large dish systems. The accuracy of the improved method is tested for several types of offset Gregorian systems, and errors are shown to be in the order of a few percent. Comparisons of several layers of simplification to a standard brightness temperature model are also presented to aid the designer in the choice of model complexity to use.

Sensitivity performance of several mappings in shaped offset Gregorian reflectors

The sensitivity performance of several primary pattern to aperture field mappings in shaped offset Gregorian reflector systems is investigated. Specific attention is given to high efficiency mappings with low second side-lobe levels, as these are of interest in the design of sensitive radio telescopes where the first side-lobe is imaged in the point spread function. Comparisons are made between the predicted analytical illumination efficiencies and the side-lobe levels, as well as the physical optics simulated sensitivity of the systems. The results show that the analytical trends are closely followed in actual shaped systems of moderate electrical size. Some graphs are given showing areas of maximum sensitivity for a specified side-lobe level in the shaping parameter space.

Sensitivity performance of the SKA offset Gregorian reflector candidates with ideal feeds

A limited set of possible optics candidates for the SKA was recently proposed. This paper investigates the sensitivity performance of these offset Gregorian reflectors over tipping angle when illuminated by an ideal feed with varying edge tapers. The results show that feed down tipping together with a sub-reflector extension, that shields the feed from the ground radiation, achieves the best sensitivity performance. This is true for both unshaped and shaped reflector systems with the later performing slightly better due to higher aperture efficiencies.