Isak P. Theron

Recent progress on the antenna simulation program FEKO

This paper describes recent progress with FEKO, an antenna simulation program. FEKO uses the method of moments (MoM), and also offers users a hybrid MoM/physical optics for suitable electrically large problems. Current work on UTD hybridisation is outlined, as well as work on the pre- and post-processors. The performance of FEKO in high-performance computing environments is discussed. The incorporation of a rigorous stratified media treatment is reviewed and current plans for FEM hybridisation briefly outlined.

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.

L-Band Feed Horn and Orthogonal Mode Transducer for the KAT-7 Radio Telescope

The design, simulation and measured performance of a broad bandwidth L-band wide flare angle corrugated conical horn and its orthogonal mode transducer are presented. The feed horn is used on prime focus parabolic reflector antennas of a seven-element interferometer, which is part of the Karoo Array Telescope project. The horn was designed to maximize the sensitivity of the radio telescope over a frequency band of 47.6% relative bandwidth. The orthogonal mode transducer design has low insertion and transmission losses and good isolation, and because of its compact size requires a very small cryogenic cooling system.

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.

The design of the MeerKAT L-band feed

The design methodology of the MeerKAT L-band feed which consists of a wide flare angle axially corrugated conical horn, a horn matching section and a compact orthomode transducer (OMT) is presented. The dish is a classical Gregorian offset dual-reflector antenna system. The predicted results show stable radiation patterns over the frequency band with an ideal antenna efficiency above 73 %. The feed has losses below 0.05 dB resulting in very good system sensitivity.

Extensions to the hybrid method of moments/uniform GTD formulation for sources located close to a smooth convex surface

We present an extension to the uniform geometrical theory of diffraction (GTD) for reflection from smooth curved surfaces. This approach allows the source to be much closer to the reflecting surface than the conventional uniform GTD formulation and does not require a Hertzian dipole source. In essence, the field point is mirrored in the plane tangential to the specular (reflection) point; the incident field is then calculated at the mirror point and the uniform GTD reflection coefficients are used to mirror this field to the original field point. This formulation reduces exactly to the conventional uniform GTD if the incident field is ray optical. The application to a hybrid method of moments (MoM)/GTD code is outlined and results computed using this code are presented for a dipole radiating in the vicinity of a cylinder.

Compact low loss L-band orthomode transducer

The design of a wideband orthomode transducer (OMT) with an integrated directional coupler for radio astronomy applications in the frequency range 900 MHz to 1760 MHz is presented. The OMT has very low insertion and transmission losses, good isolation, and is of small size.

Improving the bandwidth of a slotted waveguide array by using a centre-feed configuration

The use of a centre-feed position (instead of the more conventional end-feed) was investigated to improve the reflection coefficient bandwidth of a slotted waveguide array. The results demonstrated improved bandwidth performance, but at the cost of a degradation in the pattern and a more difficult design procedure.

Understanding the impact of beamshapes on radio interferometer imaging performance

Primary beam shapes are emerging as a vital consideration both in the calibration of the new generation of radio interferometers, and in the design of future instruments such as the SKA. However, their impact on interferometer performance is far from obvious, especially since calibration techniques addressing these effects are still in their relative infancy. We present the results of recent work aimed at developing a comprehensive interferometric simulations methodology which can be used to quantify the impact of beamshapes on radio interferometer sensitivity. We show the results of applying this methodology to a comparative study of several possible MeerKAT dish designs. In particular, two beamshape-related effects that have been virtually ignored in older (lower-sensitivity) telescopes are far sidelobe confusion noise (FSCN), and calibration artefacts due to pointing errors (PEs). These effects are quite subtle and often completely non-intuitive, so exploring them via simulations can lead to interesting new insights into radio telescope design.