Ultra-wideband phased array antennas for low frequency radio astronomy

Esta tesis se centra en el desarrollo de antenas diferenciales de banda ultra ancha para arrays de apertura, como los que han sido propuestos por el consorcio internacional para cubrir las bandas bajas de frecuencia del telescopio (SKA-AAlo (0.07-0.45 GHz) y SKA-AAhi (0.3-1 GHz)). La tesis comienza con una introducción al campo de la radioastronomía y una presentación del estado del arte de la teoría y las tecnologías de arrays de antenas de banda ultra ancha para radioastronomía. Los capítulos principales del documento se centran en explicar el proceso de diseño y medida de 3 prototipos (2 para SKA-AAhi y 1 para SKA-AAlo), que representan propuestas española, holandesa y británica respectivamente para cubrir las citadas frecuencias. Se hace especial hincapié en las di cultades en el diseño de antenas diferenciales de banda ultra ancha y que además han de ser de bajo coste, ya que el telescopio contara con miles y hasta millones de elementos. Estas di cultades incluyen, entre otras, el análisis de los materiales, la medida de los prototipos, la eliminación de anomalías de banda estrecha o el ruido del sistema. Finalmente se presenta una técnica para la simulación electromagnética de arrays de antenas irregulares de gran tamaño, como los que se podrán encontrar formando parte de las estaciones del SKA. En las conclusiones del documento original de la tesis se resaltan las aportaciones realizadas a las tecnologías para radioastronomía (incluyendo los 3 prototipos que cumplen las especi caciones del SKA) y tecnologías de antenas en general y se presentan las líneas futuras de trabajo.-------------------------------------------------------------------------------------------------------------------------In the context of the SKA and the SKADS, in this thesis I explore antenna technologies to cover the sub 1 GHz frequency band of the telescope. I study and design array antennas to cover the high frequency band of the aperture array proposal for the SKA (300 MHz to 1 GHz) using dense arrays and the sub 450 MHz band using sparse arrays. Special effort is put on analyzing possible issues, such as narrow band anomalies, related to the use of ultra-wideband differential technology. Low-cost and high sensitivity are the main objectives of the designs I present in this thesis. The chapters are arranged in chronological order. First I elaborate on dense arrays of Tapered Slot Antennas (TSAs) to cover the SKA-AAhi band. This work was done during my 4 months internship in the Netherlands Institute for Radio Astronomy in 2007, the time I worked for the Yebes Astronomical Center since January 2006 until September 2007 and the time I have been a graduate student at the Universidad Carlos III de Madrid since October 2005. After, I present the work I have done on sparse arrays of bow-tie antennas for low-frequency radio astronomy since I started my work at the Cavendish Laboratory, University of Cambridge, UK, in September 2007. Finally I conclude with a chapter on characterization of large irregular arrays of ultra-wideband antennas for low-frequency radio astronomy. This work started in 2009 when I was already in Cambridge and it was done in close collaboration with the Université Catholique de Louvain, Belgium

A beamforming approach to the self-calibration of phased arrays

In this paper, we propose a beamforming method for the calibration of the direction-independent gain of the analog chains of aperture arrays. The gain estimates are obtained by cross-correlating the output voltage of each antenna with a voltage beamformed using the other antennas of the array. When the beamforming weights are equal to the average cross-correlated power, a relation is drawn with the StEFCal algorithm. An example illustrates this approach for few point sources and a 256-element array

The effects of the antenna power pattern uncertainty within a global 21 cm experiment

Experimental 21 cm cosmology aims to detect the formation of the first stars during the cosmic dawn and the subsequent epoch of reionization by utilizing the 21 cm hydrogen line transition. While several experiments have published results that begin to constrain the shape of this signal, a definitive detection has yet to be achieved. In this paper, we investigate the influence of uncertain antenna-sky interactions on the possibility of detecting the signal. This paper aims to define the level of accuracy to which a simulated antenna beam pattern is required to agree with the actual observing beam pattern of the antenna to allow for a confident detection of the global 21 cm signal. By utilising singular value decomposition, we construct a set of antenna power patterns that incorporate minor, physically motivated variations. We take the absolute mean averaged difference between the original beam and the perturbed beam averaged over frequency ($\Delta D$) to quantifying this difference, identifying the correlation between $\Delta D$ and antenna temperature. To analyse the impact of $\Delta D$ on making a confident detection, we utilize the REACH Bayesian analysis pipeline and compare the Bayesian evidence $\log \mathcal{Z}$ and root-mean-square error for antenna beams of different $\Delta D$ values. Our calculations suggest that achieving an agreement between the original and perturbed antenna power pattern with $\Delta D$ better than -35 dB is necessary for confident detection of the global 21 cm signal. Furthermore, we discuss potential methods to achieve the required high level of accuracy within a global 21~cm experiment

Effect of gain and phase errors on SKA1-low imaging quality from 50-600 MHz

Simulations of SKA1-low were performed to estimate the noise level in images produced by the telescope over a frequency range 50-600 MHz, which extends the 50-350 MHz range of the current baseline design. The root-mean-square (RMS) deviation between images produced by an ideal, error-free SKA1-low and those produced by SKA1-low with varying levels of uncorrelated gain and phase errors was simulated. The residual in-field and sidelobe noise levels were assessed. It was found that the RMS deviations decreased as the frequency increased. The residual sidelobe noise decreased by a factor of ~5 from 50 to 100 MHz, and continued to decrease at higher frequencies, attributable to wider strong sidelobes and brighter sources at lower frequencies. The thermal noise limit is found to range between ~10 - 0.3 $\mu$Jy and is reached after ~100-100 000 hrs integration, depending on observation frequency, with the shortest integration time required at ~100 MHz.Comment: 23 pages, 11 figures Typo correcte

Study and Design of a Diferentially Fed Tapered Slot Antenna Array

The results of a parametric study and design of an ultrawideband dual-polarized array of differentially-fed tapered slot antenna elements are presented. We examine arrays of bunny-ear antennas and discuss the capabilities and limitations of differential antenna technology. As we focus on radio astronomical applications, the absence of a balancing-feed circuit not only reduces the first-stage noise contribution associated to losses in the feed, but also leads to a cost reduction. Common-modes are supported by the antenna structure when a third conductor is present, such as a ground plane. We demonstrate that anomalies may occur in the differential-mode scan impedance. Knowledge of both types of scan impedances, differential and common mode, is required to properly design differential LNAs and to achieve optimal receiver sensitivity. A compromise solution is proposed based on the partial suppression of the undesired common-mode currents through a (low loss) balancing-dissipation technique. A fully steerable design up to 45?? in both principal planes is achieved

A General Bayesian Framework to Account for Foreground Map Errors in Global 21-cm Experiments

Measurement of the global 21-cm signal during Cosmic Dawn (CD) and the Epoch of Reionization (EoR) is made difficult by bright foreground emission which is 2-5 orders of magnitude larger than the expected signal. Fitting for a physics-motivated parametric forward model of the data within a Bayesian framework provides a robust means to separate the signal from the foregrounds, given sufficient information about the instrument and sky. It has previously been demonstrated that, within such a modelling framework, a foreground model of sufficient fidelity can be generated by dividing the sky into $N$ regions and scaling a base map assuming a distinct uniform spectral index in each region. Using the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) as our fiducial instrument, we show that, if unaccounted-for, amplitude errors in low-frequency radio maps used for our base map model will prevent recovery of the 21-cm signal within this framework, and that the level of bias in the recovered 21-cm signal is proportional to the amplitude and the correlation length of the base-map errors in the region. We introduce an updated foreground model that is capable of accounting for these measurement errors by fitting for a monopole offset and a set of spatially-dependent scale factors describing the ratio of the true and model sky temperatures, with the size of the set determined by Bayesian evidence-based model comparison. We show that our model is flexible enough to account for multiple foreground error scenarios allowing the 21-cm sky-averaged signal to be detected without bias from simulated observations with a smooth conical log spiral antenna.Comment: 18 pages, 13 figure

Sky-averaged 21-cm signal extraction using multiple antennas with an SVD framework: the REACH case

In a sky-averaged 21-cm signal experiment, the uncertainty on the extracted signal depends mainly on the covariance between the foreground and 21-cm signal models. In this paper, we construct these models using the modes of variation obtained from the Singular Value Decomposition of a set of simulated foreground and 21-cm signals. We present a strategy to reduce this overlap between the 21-cm and foreground modes by simultaneously fitting the spectra from multiple different antennas, which can be used in combination with the method of utilizing the time dependence of foregrounds while fitting multiple drift scan spectra. To demonstrate this idea, we consider two different foreground models (i) a simple foreground model, where we assume a constant spectral index over the sky, and (ii) a more realistic foreground model, with a spatial variation of the spectral index. For the simple foreground model, with just a single antenna design, we are able to extract the signal with good accuracy if we simultaneously fit the data from multiple time slices. The 21-cm signal extraction is further improved when we simultaneously fit the data from different antennas as well. This improvement becomes more pronounced while using the more realistic mock observations generated from the detailed foreground model. We find that even if we fit multiple time slices, the recovered signal is biased and inaccurate for a single antenna. However, simultaneously fitting the data from different antennas reduces the bias and the uncertainty by a factor of 2-3 on the extracted 21-cm signal.Comment: 12 pages, 13 figures. Accepted for publication in MNRAS. Accompanying code is available https://github.com/anchal-009/SAVED21c