Dirk I. L. de Villiers

Recent experiments in cosmology, particularly those aimed at detecting the faint, redshifted, global 21[ ]cm hydrogen line (depth [ ], [ ]), have imposed stringent new requirements on radiometer calibration. In this work, we present a framework for circuit modeling and parameter inference to strengthen these calibration pipelines. This new approach enables in situ characterization of otherwise immeasurable systematics using physically motivated models. A combination of frequentist and Bayesian techniques are employed in a pipeline that supports iterative modeling, robust parameter estimation, and detailed uncertainty quantification. The framework is applied to the REACH telescope, where the precise correction of variations in the radio signal paths arising from component aging or environmental effects is critical. Circuit models of REACH’s calibration sources are developed, with the goal of predicting source temperature corrections that are conventionally obtained from laboratory measurements. By fitting the models to measured data using a convolutional cost function, a strong agreement with RMS residuals no worse than [ ] is obtained. However, Bayesian inference reveals that the resulting temperature corrections can have uncertainties on the order of 1 to [ ], caused by reflection coefficient degeneracies, measurement noise, and errors in the models. To combat this, posteriors obtained from laboratory measurements are employed as updated priors, reducing correction uncertainties down to [ ]. Ultimately, the framework provides a means of dynamically accounting for drift in system non-idealities over time, addressing the increasing precision demands of global 21 cm radio astronomy.

This paper presents an efficient beam pattern modeling method for reflector antennas where the performance of a specific feed antenna, as a function of frequency and geometric variations of the antenna, is known on a different (but similar) reflector antenna. Instead of modeling patterns for the desired reflector directly, the difference patterns between the desired and previously modeled reflectors are employed. Scaling and normalization measures are applied to the patterns to simplify their modeling as Characteristic Basis Function Patterns (CBFPs), which can be accurately interpolated over the desired design parameter space. The CBFP coefficients drive an adaptive sampling algorithm that aims to accurately obtain the antenna’s performance across the entire parameter space with as few electromagnetic simulations as possible. Two examples are presented using basic reflector optics and a simple choke horn and demonstrate the advantage of using a reference design compared to directly modeling the reflector patterns, showing that fewer simulations are required to obtain a certain level of accuracy.

Reflector antennas often exhibit frequency ripples in key performance metrics, including aperture efficiency and antenna noise temperature. These ripples stem from corresponding fluctuations in the electric field pattern, due to interference between fields radiated, reflected or diffracted by the feed and reflectors, and it is important that they are characterised. In this paper, we introduce an innovative approach for reconstructing the electric fields of reflector antennas using simulated data across a range of frequencies. By leveraging the power of sub-Nyquist sampling, the number of required samples can be reduced, leading to increased computational efficiency. This presents an advantage to traditional methods such as the Fourier transform that requires Nyquist sampling. We demonstrate how these reconstructions can be used to characterise the frequency contents of resulting reflector properties, such as aperture efficiency and antenna noise temperature.

This paper presents an efficient beam pattern modeling method for reflector antennas where the performance of a specific feed antenna, as a function of frequency and geometric variations of the antenna, is known on a different (but similar) reflector antenna. Instead of modeling patterns for the desired reflector directly, the difference patterns between the desired and previously modeled reflectors are employed. Scaling and normalization measures are applied to the patterns to simplify their modeling as Characteristic Basis Function Patterns (CBFPs), which can be accurately interpolated over the desired design parameter space. The CBFP coefficients drive an adaptive sampling algorithm that aims to accurately obtain the antenna's performance across the entire parameter space with as few electromagnetic simulations as possible. Two examples are presented using basic reflector optics and a simple corrugated horn and demonstrate the advantage of using a reference design compared to directly modeling the reflector patterns, showing that fewer simulations are required to obtain a certain level of accuracy.

Reflector antenna systems used in applications such as radio astronomy and radiometry are required to have a stable and predictable receiving sensitivity response as a function of frequency, among other characteristics. It is well known that a chromatic aberration, or ripple, occurs in the frequency response of the these systems due to the multiple reflections between the feed antenna and reflector surface. In clear aperture optics, such as an offset Gregorian system, this effect is drastically reduced by removing the feed out of the optical path. However, this paper shows how diffraction mechanisms may cause further ripples in the response, whose structure is a strong function of reflector pointing angle. For the analysis herein the reflector/feed interaction is completely removed from the simulation to highlight the diffraction interference effects, which are illuminated by a graphical illustration of the mechanisms at play.

Reflector antennas often exhibit frequency ripples in key performance metrics, including aperture efficiency and antenna noise temperature. These ripples stem from corresponding fluctuations in the electric field pattern, due to interference between fields radiated, reflected or diffracted by the feed and reflectors, and it is important that they are characterised. In this paper, we introduce an innovative approach for reconstructing the electric fields of reflector antennas using simulated data across a range of frequencies. By leveraging the power of sub-Nyquist sampling, the number of required samples can be reduced, leading to increased computational efficiency. This presents an advantage to traditional methods such as the Fourier transform that requires Nyquist sampling. We demonstrate how these reconstructions can be used to characterise the frequency contents of resulting reflector properties, such as aperture efficiency and antenna noise temperature.

Reflector antenna systems used in applications such as radio astronomy and radiometry are required to have a stable and predictable receiving sensitivity response as a function of frequency, among other characteristics. It is well known that a chromatic aberration, or ripple, occurs in the frequency response of the these systems due to the multiple reflections between the feed antenna and reflector surface. In clear aperture optics, such as an offset Gregorian system, this effect is drastically reduced by removing the feed out of the optical path. However, this paper shows how diffraction mechanisms may cause further ripples in the response, whose structure is a strong function of reflector pointing angle. For the analysis herein the reflector/feed interaction is completely removed from the simulation to highlight the diffraction interference effects, which are illuminated by a graphical illustration of the mechanisms at play.

The X-Band All-Sky Survey (X-BASS) is a planned radio astronomy experiment aimed at mapping the sky in both temperature and polarisation. The main purpose of X-BASS is to aid observations of the cosmic microwave background (CMB) by removing various contaminating foreground emissions present in the galaxy, such as synchrotron and thermal dust emissions. When designing the antenna for such an instrument, one must rely on traditional figures of merit (FoMs) used to describe the performance of antenna radiation pattern characteristics. However, these FoMs often have a conflicting nature, the optimal trade-off of which may not be immediately apparent. What is more, traditional FoMs are usually developed from a telecommunications standpoint or determined heuristically from other radio astronomy projects with differing goals. Thus, the exact effect of antenna pattern characteristics on the end science goals of the experiment may be unclear. As such, this paper seeks to determine the limitations posed by antenna noise temperature, beamwidth and cross-polarisation on the end science goal of the experiment, which is the determination of various cosmological parameters through observation of the CMB. From this, a new FoM was developed which provides a single metric to be used in the optimisation of the antenna structure. This FoM was then used in the preliminary design of a corrugated horn antenna to feed the X-BASS reflector system.

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.