Robert Lehmensiek

This paper highlights some of the lessons learned during the design, production, and operation of the MeerKAT and SKA feeds and the design of the reflector surfaces.

A new radio astronomy interferometer called the Next Generation Very Large Array (ngVLA), a project of the National Radio Astronomy Observatory (NRAO), is envisaged to be built on the Plains of San Agustin in New Mexico, USA [1]. The instrument consists of offset Gregorian dual reflector antennas with approximately 244 antennas with an 18 meter aperture diameter and 19 antennas with 6 meter aperture diameter. At the reflector system’s focus is a suite of cryogenic feeds mounted on an indexer mechanism which cover the frequency range from 1.2 GHz to 116 GHz with a gap at the atmospheric absorption band between 50.5 GHz and 70 GHz.

Antenna noise temperature calculation for a reflector antenna system can be a computationally expensive operation if the structure is electrically large. Several approximate methods have been suggested to speed up the calculation. These rely on removing one or more of the reflecting surfaces from the calculation domain and compensating for the effect through geometric optics arguments. This article presents a unified multilevel approximation formalism, which allows the user the freedom to tradeoff the simulation time, implementation complexity, and result accuracy as required for a specific application. As part of the formalism, a strategy to reliably remove both reflectors in a dual-reflector system from the calculation domain and rely on ray tracing to correct the background temperature is developed. It is shown that this new method reduces calculation time by several orders of magnitude while maintaining accuracy in the final antenna temperature result of the order of 1%. It is fast and accurate enough to be used in numerical optimization loops. The method requires the implementation of a ray tracing algorithm, and thus, may present some unnecessary complexity if only rough temperature estimates are required. To this end, a simplified method is also presented requiring no ray tracing and only simple postprocessing of readily available feed pattern data. Though not as accurate as of the ray tracing-based method, temperatures may be estimated with around 10% accuracy over bandwidths well in excess of a decade.

This paper describes the design of the 18-meter shaped offset Gregorian reflector system for the Next Generation Very Large Array (ngVLA) radio interferometer and the associated 1.66:1 axially corrugated conical horn antennas. A parametric sweep determined feed horns with the goal of pattern symmetry and low spillover efficiency. Given these feeds, a mapping function for the shaped offset Gregorian reflector system was then determined, by a full factorial design, to achieve maximum receiving sensitivity with secondary parameters such as plate scale, scan loss, sidelobe, and cross-polarization at acceptable levels. A sub-reflector extension is included in a feed-down tipping configuration with improved performance over the feed-up case. The resultant optics and feeds achieve very symmetrical patterns with an aperture efficiency of around 95 %, a plate scale of 15 arcsec/mm, and first and second sidelobe levels of -18 dB and -24 dB respectively. The receiving sensitivity at 30 GHz is 8.7 m2/K and increases to above 12 m2/K at 12.3 GHz, and reduces to 1.5 m2/K at 116 GHz (ignoring efficiency loss due to non-ideal reflector surfaces). These sensitivities are close to the theoretical optimum.

This paper describes the design of the feed horns for the Karoo array telescope (KAT) experimental prototype dish. The feed horn is a dual polarized circular horn with a linearly polarized circular pattern operating from 1414 to 1670 MHz. The horn is excited with a quad-ridged ortho-mode transducer (OMT).