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This paper presents an investigation into the practical performance of a direction of arrival (DOA) estimation method for sparse regular arrays, called VEXPA. Its ability to estimate the number of incoming signals through cluster analysis is compared to the traditional method used by dense estimation techniques. It is seen that the usefulness of the cluster analysis method depends on the underlying estimation method used by VEXPA. The effect of quantisation errors introduced by the use of an ADC is also investigated. It is found that VEXPA performs satisfactorily with one-bit data for a maximum of one signal.

Accurate placement of elements in large antenna arrays is a difficult and costly process. We explore the use of the validated exponential analysis (VEXPA) technique that was previously formulated to solve a direction-of-arrival (DOA) estimation problem, to find the antenna element positions in an array after the installation phase, so that cost-savings can be realised during placement of the antenna elements. Measurements are taken from harmonically related input signals transmitted from an Unmanned Aerial Vehicle (UAV) for which the position in the sky is known. It is shown how the UAV's zenith angle can be manipulated to generate parameters required for VEXPA's de-aliasing step. A simple simulation illustrates the functioning of the proposed method.

Both the SKA and ngVLA radio telescopes (will) use shaped offset Gregorian reflector antennas, in the feed low configuration, with a variety of different feeds to cover their full operating bandwidth. It is foreseen that both systems will most likely employ variations of quad-ridge flared horns (QRFH) to cover receiver bands with about 3.5:1 bandwidth – possibly wider [1] , [2] . For both systems, the most important performance parameter is the receiving sensitivity, and it is critical that any feed horn employed in the telescope has a carefully controlled radiation pattern, which is specifically designed to maximize the receiving sensitivity over the entire operating bandwidth.

The National Radio Astronomy Observatory (NRAO) is planning to build a new radio astronomical instrument, called the Next Generation Very Large Array (ngVLA). The core will be built at the existing VLA site in New Mexico, USA. The reference design [1] for this instrument specifies offset Gregorian dual reflector antennas of which approximately 244 antennas will have an 18 meter aperture diameter and 19 antennas a 6 meter aperture diameter. This work will focus on the 18 meter reflector system.

The design of an all-metal quadruple-ridge flared horn (QRFH) feed antenna for the current nominal ngVLA optics is presented. The antenna is required to operate over the 3.5 GHz - 12.3 GHz band with a reflection coefficient of better than −15 dB, while maximizing the receiving sensitivity over the band. Analytical profiles for the horn and ridges are employed to reduce the design space dimensionality (over that of spline profiled antennas). Simulated results suggest sensitivity performance to within 10% of that achievable with octave band corrugated horn antennas.

Feed network loss and reactive loading are major sources of inefficiency in mm-wave patch array antennas. To address this, the application of non-resonant radial combiners to antenna feed networks is investigated for a 16-element E-band uniform circular microstrip patch antenna array. The radial feed network is compared in full-wave simulation to a corporate feed network with identical array and element structure. It is found that the feed network with a radial combiner based feed achieved 15% better radiation efficiency and 72% reduced shift in $f_{c}$ , at the expense of 89% reduction in −15 dB port impedance bandwidth.

Recently, a new algorithm for robust direction of arrival (DoA) estimation, using regular sparse array antenna systems, was described in [1]. Aliasing (grating lobes) is removed through the use of two sparse arrays - one a shifted version of the other. Minimum spacing between any two elements is not required to be smaller than a half wavelength. The method completely removes the dense Nyquist spacing requirement, allowing improved resolution for a fixed number of sensors and reduced mutual coupling between antenna elements. Because of a shared structured linear system of equations between the two arrays, as a consequence of the shift between the two, the analysis of both is automatically paired, thereby avoiding a computationally expensive matching step as is required in the use of so-called coprime arrays. Furthermore, the method provides an accurate estimate of the number of signals impinging on the system without any prior knowledge, while there is no requirement that the respective signals be uncorrelated. Distinct, fully coherent signals, such as those that occur in a multipath environment, can thus be accurately distinguished. Several examples were presented in [1], investigating both the performance in increasing noise, as well as increasing mutual coupling scenarios. In all of these, the new method performs better than standard techniques on a variety of metrics.

Maintaining a Radio Frequency Interference (RFI) free site is an important aspect in the performance of the Square Kilometre Array (SKA) radio telescope. Regular on-site RFI measurements are required to verify compliance and identify RFI culprits over the full operating band of the telescope. For these measurements, hyperband directional antennas to cover all the frequencies of the first phase of SKA-MID are required. Three antennas are thus investigated because of their bandwidth and gain: a Double Ridged Guide Horn (DRGH); a Transverse ElectroMagnetic (TEM) horn; and a Valentine antenna. Because of their symmetry, a wideband coaxial to parallel-plate transition is explored as a feeding structure for the Valentine and TEM antennas. The three antennas are modeled and simulated in CST Microwave Studio and prototyped. The simulated and measured return loss and gain results are presented. The return loss has further been optimized and bandwidth ratio of the three antennas has been improved to at least 16:1. A Surrogate Based Optimization (SBO) scheme using a Kriging model and Expected Improvement (EI) was employed to optimize the antennas.

Current research in the development of a sparse-regular aperture array concept is presented for use in the Square Kilometre Array’s (SKA) Mid Frequency Aperture Array (MFAA). Advantages offered by the sparse array regime are explored, while considering methods to overcome its inherent problems. In particular, the prevalence of grating lobes in visible space poses a significant design challenge, which is carefully considered and addressed. The work is a collaborative effort between South African and Dutch institutions affiliated with MFAA, and involves parallel investigations of both the overall system and the antenna element design. First-stage solutions are presented, providing a foundation on which a more complete system prototype may be developed.

This article presents the design of an ultra-wideband coaxial Marchand balun for the pyramidal sinuous antenna. The focus of the design is related to the fabrication process and the nuances involved. Closed-form equations are used to determine the element values of the balun, which are then tuned to provide a desired equi-ripple response. The designed balun, with a 300 Ω load termination, showcases a simulated return loss better than 10 dB from 1.1 − 4.8 GHz. The initial design produced amplitude and phase balance results within ±2.5 dB and 9.8°, respectively. Updated designs achieve an amplitude balance of ± 0.2 dB and a phase balance of 1.5°.

This article presents the design of an ultra-wideband planar Marchand balun for the pyramidal sinuous antenna. Ground plane slots, placed beneath the coupled-line sections, increase the coupling factor of the coupled-line sections and ensure tight coupling. This results in wider bandwidths compared to standard implementations and relaxes the requirements on the trace width and gap spacing. The balanced output lines are placed on the same side of the substrate to facilitate straightforward integration with the antenna. The designed balun showcases a simulated return loss better than 10 dB, and an insertion loss better than 0.84 dB across the band from 2-6 GHz. The simulated amplitude and phase balance are better than 0.8 dB and −12° respectively. Manufactured performance corresponds well with simulated predictions with minor differences being attributed to manufacturing tolerances.

Unmanned aerial vehicle based field measurements have been proposed as a possible solution to provide calibration data for large ground based radio telescope arrays, such as the Mid Frequency Aperture Array planned for the Square Kilometre Array project. This paper considers planar near-field scanning using a quad-copter equipped with a differential GPS (DGPS). The effect of random positional errors associated with DGPS on two different near- to far-field (FF) transformations, namely the planar plane wave expansion (PPWE) and the Fast Irregular Antenna Field Transformation Algorithm (FIAFTA), are investigated in the frequency range 450 MHz-1450 MHz. It is shown that FIAFTA drastically outperforms the PPWE for the given situation and produces adequate results over the band for a positional error of 1 cm. However, considering positional inaccuracies of 5 cm, both algorithms produce a predicted FF pattern which is corrupted beyond being of any practical use.

Global surrogate models are presented for antenna elements in large phased arrays, to cheaply and accurately model scan-dependent impedance and radiation responses for systems with wide-angle scanning requirements. Adaptive sampling combined with rational and Kriging interpolation is found to produce accurate, convergent models within 50 full-wave EM simulations of a patch element within a planar PBC unit cell.

This paper presents a discussion and examples of using the Characteristic Basis Function Pattern method for the modeling of radiation patterns of large, wideband reflector antenna systems. It is shown that beam normalization should be applied to reduce the number of required modes to achieve a specified accuracy in the reconstructed interpolated field, and that care should be taken to sample finely enough in frequency to capture any chromatic aberrations if high levels of accuracy are required.

This work investigates the effects of the number of array feed elements on the overall system $G/T_{sys}$ for phased array feed systems, in the context of scanning along a single angular plane. It is shown that the negative impact of receiver noise caused by larger feeds on overall system sensitivity can outweigh the contribution to system gain in noisy systems. A numerical study is conducted which shows how a linear array can achieve superior performance to planar arrays with realistic receiver noise parameters.

Gap waveguide filters, designed using coupled resonator theory, typically require a significant amount of post-design tuning in simulation to obtain the desired response before manufacturing. This process can be time consuming due to the large number of computational electromagnetic simulations required. This paper presents a simple space mapping approach to automate this final tuning process, which requires a significantly reduced number of full wave simulations. A third order groove gap waveguide filter at 14 GHz is presented as an example to illustrate the method and its utility.

A surrogate-based modelling study of an infinite, regular aperture array of bowtie antenna elements is presented. The sensitivity per element is predicted using global data-driven surrogate models, in order to efficiently determine a design that maximises receiving sensitivity over multiple scan angles and a 4.5:1 bandwidth. The model is obtained across a broad parameter space, and its accuracy is verified with the use of k-fold cross validation. Sensitivity performance of the design obtained using this technique exceeds previously published results.

This paper compares the interpolation based modeling of antenna radiation pattern response features when the models are built directly on the response feature of interest, or on an expansion coefficient describing the full radiation pattern. It is shown that by interpolating the expansion coefficients of an orthogonal set of basis functions for the full radiation pattern, the expected rational response in frequency (and some geometric parameters) can be exploited to construct accurate interpolation models using very few training data. In many cases it is not possible to predict the functional form of the response features, and therefore interpolation directly on these data requires more training samples than for the expansion coefficient case.

This paper presents a wideband pyramidal sinuous antenna, over a conducting ground plane, for use as a reflector antenna feed. Specific attention is given to comparisons with a similar structure where the sinuous shape is instead projected onto a cone. The pyramidal structure is significantly simpler to manufacture, and measured results confirms this where close agreement with simulations is observed.

We present a fully automated method to predict the full radiation patterns and S-parameters of antennas over a large frequency bandwidth using the knowledge of the simulated results at a few frequency points. The frequency points are adaptively selected in regions of fast variations of radiation pattern and S-parameters, as well as in large unsampled regions. The method has a built-in absolute error control measure and is guaranteed to converge to an accurate model in a modest number of iterations. The proposed method is validated through pertinent examples.

A method is presented for the fast space mapping based optimization of aperture-coupled rectangular patch antennas using transmission line models. The procedure is used to optimize the antenna for a significantly wider bandwidth than that for which the transmission line model provides an accurate approximation. A case study is provided, whereby the suggested method produces an antenna with bandwidth in excess of 6 % for a −10 dB reflection coefficient. The simulation cost is equivalent to only a few high fidelity full wave electromagnetic simulations of the structure, and the method can be easily automated to require minimal designer input.

A method for the design of a conical to coaxial transmission line transition with an arbitrary smooth impedance profile is presented. This type of transition is often used in conical line combiners, and has previously either had a constant impedance or an impedance profile that could not be explicitly designed. The method presented herein provides the designer with more design freedom by allowing an arbitrary impedance profile to be chosen or optimized, with the geometry following from the designed impedance profile. A conical combiner with significantly improved bandwidth performance is designed using the presented method. The performance predicted by the circuit model is in good agreement with full-wave simulations and measurements.

This paper presents a space mapping approach to parametric modeling of the gain and sidelobe level in blocked aperture reflector antenna systems. The well-known aperture field integration method is used to provide a fast coarse model of the main beam responses, while full wave or multi-reflection physical optics simulations provide the accurate fine model responses. A formal implicit space mapping formulation is used to align the coarse and fine models over some low dimensional parameter space describing variations in the reflector geometry. The method is validated through two illustrative examples, showing significantly improved accuracy over the classical aperture integration method while requiring only a few full simulations of the structure.

We present a robust method to predict full radiation patterns over a large frequency bandwidth using the knowledge of the pattern at a few frequency points. The frequency points are selected adaptively such that a set of linearly independent basis functions, called characteristic basis function patterns (CBFPs), is obtained. The model consists of a linear combination of CBFPs where expansion coefficients are parameterized with respect to frequency and interpolated to accurately yield the radiation pattern at a desired frequency within the bandwidth of interest. The method has a built-in absolute error control, given by the ratio of the smallest and largest singular values of the column-stacked matrix of CBPFs, providing a trade-off between accuracy and complexity of the model. The proposed method is validated through illustrative examples.

Large reflector antennas spread over the African continent used for satellite communications are becoming redundant with the advent of undersea fiber optic cable links. The plan is to refit these antennas for radio astronomy. An antenna in Zambia has a 100 m-high telecommunication mast erected in front of the reflector antenna. This paper investigates the effect that this mast has on the efficiency of the antenna and the size of the avoidance zone with the existence of this mast. Due to the electrical size and detail of the mast this is a very complex electromagnetic analyses problem. The results show that the size of the mast's blockage window can be up to 65° in elevation and 36° in azimuth for an allowance of 5% drop in efficiency. To minimize the effect of the mast, it should be outside the near field cylinder in front of the reflector.

This paper presents a low cost Gaussian process (GP) modeling technique for antenna responses that exhibit highly oscillatory behavior as a function of frequency. Using a judiciously constructed composite covariance function that is able to capture the quasi-periodic behavior of the responses, an accurate interpolant could be established with far fewer training points than would normally be considered necessary for representing the oscillatory behaviour. The specific example investigated here is the aperture efficiency response of an offset Gregorian reflector antenna system, which has previously been shown to possess an oscillatory response which is computationally expensive to model accurately. Initial results using the suggested method are promising, with accurate interpolants of the expected response generated using only a sparse set of randomly selected training data. Notably, predictions were accurate even in sections of the response that contained multiple contiguous cycles and no training points.

Calculation of the antenna noise temperature for an offset Gregorian reflector system is computationally intensive. The main reflector masking technique, where the main reflector is removed from the calculation domain, considerably reduces the computation cost. This technique, however, is less accurate for electrically smaller systems. Recently an improvement to the technique was proposed by introducing a diffraction compensated correction factor. In this case the error on receiving sensitivity can be in the order of a few percent. This paper further improves on the technique by introducing a power dependence to the inverse frequency diffraction compensation. The technique reduces the error of the receiving sensitivity to well below a percent.

A conical transmission line power combiner is designed using a surrogate optimization method which achieves a significant speed-up factor over previous design methods as well as improved design flexibility.

The sensitivity of dual reflector antennas can be improved by shaping the reflector system to use the aperture more efficiently. This paper explains the process to derive an optimum shaped offset Gregorian system for a given suite of single pixel feeds. The feeds are those that were optimized for a geometrically similar unshaped reflector system.

The offset Gregorian reflector system of the Square Kilometer Array radio telescope is required to operate with wideband feed (WBF) antennas down to very low frequencies, where reflectors become (electrically) relatively small. The considered WBF technologies are the log-periodic (Eleven) antenna and the quadruple-ridged flared horn. This paper investigates some of the performance degradation effects that are specific for the low frequencies and typical for these WBFs, including the feed pattern perturbations and diffraction. Several performance metrics, such as the antenna receiving sensitivity and side lobe levels are considered.

In this paper, novel frequency selective surfaces (FSS) with ultra-wideband (UWB) absorption are proposed, as well as a simple circuit model which can be used for their design. Multi-layered unit cell is realized using square ring resonators and various parameters and their effect on the FSS transmission performances have been analysed. In order to demonstrate the applicability of the proposed unit cell in the design of UWB FSS, two examples are proposed, one FSS that operates at 13.9 GHz and consists of 3 layers, and another one: a 4-layered FSS that operates at 13.3 GHz. Simulation results show that the proposed surfaces exhibit highly selective stop bands with fractional bandwidths of more than 52%, in the case of normal incidence.

A technique is proposed to synthesize the ridge taper of a quadruple-ridged flared horn (QRFH) antenna. By considering the modal propagation in the horn it is possible to improve the antenna performance. A QRFH is designed for an offset Gregorian reflector system and produces an aperture efficiency above 50% across the entire operational bandwidth (6:1).

This paper investigates the performance of shaped systems with extended sub-reflectors with the goal of maximizing the sensitivity for a given feed pattern. It is shown that an extended sub-reflector may indeed provide improved sensitivity performance, especially if the system is tipped in the feed down configuration. The performance of systems illuminated with feeds with varying tapers is also investigated, with the results showing that sharper patterns provide higher sensitivity - especially when an extension is present. This result is consistent with that observed for unshaped systems.

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.

A simple equivalent circuit model for the peripheral ports of conical line power combiners is presented. Standard feeding pins present a series inductance in the feeding lines which limits the operating bandwidth of the structure. The presented circuit model is used to illustrate how stepping to a thickened feeding post in the conical line may reduce the feed pin inductance by introducing a shunt step capacitance in the feed line. The validity of the circuit model is confirmed by favorable comparisons with full wave simulations. Since the inductance of the feeding pins increase with length, the stepped feeding posts are crucial for good performance - especially for electrically large conical line combiners with higher impedances.

An ultra-wideband, dual polarized printed cavity backed sinuous antenna with a 3:1 matched bandwidth ratio is presented. The antenna is characterised as a feed for a prime focus paraboloid reflector and as a feed in an offset Gregorian configuration. A parameter study is performed using full wave simulations, and the effects of, and bounds on the main design parameters are described. A prototype antenna is designed, with a simulated aperture efficiency of about 58% for an offset Gregorian reflector antenna, whilst having a measured return loss of 10 dB or better over a 3:1 bandwidth.

A study of the directivity sensitivity to mechanical displacements in shaped offset Gregorian type reflector systems is presented. It is shown that, for small mechanical mounting errors, aggressive shaping for high directivity does not result in a more sensitive system.

TEM ahorn antennas are well known for their ultra-wideband capabilities and are widely used. However, axial symmetry of the radiation pattern over such a wide frequency band is difficult to achieve. In this paper, a modification is proposed to the exponential TEM horn in the form of a convex triangular arc at the mouth of the antenna. The modification improves the axial symmetry over a wide bandwidth.

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.

A number of new radio telescope designs, operating in the low-GHz regime, are currently under construction or in the pre-construction phase, in particular SKA precursor instruments and now SKA1-mid. This paper demonstrates the application of full wave computational electromagnetic analysis techniques - in particular the MLFMM - to the modelling of radio telescope dishes in this frequency band. Results are compared to asymptotic analyses. It has been shown that full-wave analysis can now be routinely applied to such designs.

The performance and radiation mechanism of a two element elliptical dipole array feed is investigated in detail. The performance metric considers both the maximum cross-polarization level, which is a function of the axial symmetry and cross-polarization of the feed, and the efficiency performance when evaluated on a general reflector system. It is shown that for a two element dipole array to achieve optimum performance the dipoles need to be perfectly circular and the dipole lengths need to be longer than a half wavelength to allow for a second order current mode to coexist with the fundamental mode.

This paper presents a simplified peripheral port feeding network for use in the design of N-way conical line power combiners. The traditional shorted probe type feed is replaced by an open circuit in the coaxial feed which is optimized externally in a circuit optimizer to achieve a wide operating bandwidth without the need for any full wave parameter sweeps or optimization. Furthermore, an increase in combining structure impedance can allow for more accurate assembly and thus more predictable performance. Full wave simulated results agree well with the circuit models, where a −15 dB return loss bandwidth of 70 % is achieved at X-band.

This paper presents a study of sub-reflector extension shapes to reduce the far-out side-lobes in offset Gregorian reflector configurations. It is shown that reflecting some of the energy from the extension back onto the main reflector instead of past it reduces the side-lobe past the main reflector as well as increase the aperture efficiency.