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A new method is proposed to analyze antenna arrays including mutual coupling, which is based on the concept of multiple scattering and relates to the iterative Jacobi and Gauss-Seidel methods. The method employs sampled far-field data of an isolated element, which can be obtained by any full-wave simulator and consists of far fields for excitation at the antenna feed and for plane waves with different angles of incidence. Mutual interactions between the array elements are modeled by approximating the incident field as a single dual-polarized plane wave taken from the spherical wave expansion of the scattered field from any other element in the array. The accuracy and run-time performances of the method are evaluated mainly by comparing simulations for several array geometries to MoM-based full-wave solutions. The method is primarily intended as a tool for the fast sequential analysis of arrays while varying the array lattice, particularly in case of irregular or sparse lattices and complex elements requiring dense meshes in full-wave simulators. The applications of the method may thus range from the systematic analysis of mutual coupling to optimization and synthesis. The article is currently available in the early-access version on IEEE Xplore. ispartof: IEEE Transactions on Antennas and Propagation vol:69 issue:1 pages:206-218 status: accepted

The new generation radio telescopes, such as the Square Kilometre Array (SKA) currently under construction, will use aperture array, technology for the low frequency regime. For SKA2, the second phase scheduled after the realization of SKA1, aperture array technology is proposed up to 1.4 GHz. The antenna element count, as well as the signal processing cost, of such a system will be high. In this paper we analyze an option to reduce the number of antenna elements by making the array sparse. To reduce the signal processing cost Fast Fourier Transform Beamforming is proposed and it performance is compared to traditional beamforming. To guide the system design a Figure of Merit for the performance cost ratio is proposed and evaluated for various levels of sparsity of the antenna array. It is concluded that, for equal front-end and back-end costs, a sparse system is only marginally better than a dense system. Only when signal processing cost is significantly lower than the front-end hardware, a sparse system can be competitive.

A new passively isolated $N$ -way radial combiner design is proposed. The design incorporates low-loss partially filled radial combiners with a passive peripheral isolation network, where coaxial termination loads are used as isolation resistors. To confirm real-world performance, a 16-way combiner operating at 4 GHz was manufactured and tested. The minimum isolation measured among peripheral ports is better than 27 dB over more than 30% fractional bandwidth (BW), and the average asymmetric peripheral port input reflection is below −15 dB over 50% BW. Simulation analyses show that such excellent peripheral port isolation and matching has a significant value in improving the graceful degradation of a system utilizing such combiners. The prototype also achieves a center port reflection coefficient magnitude below −18 dB over 45% BW, and the measured signal balances are ±0.2 dB and ±2°. The structure that forms the radial cavity and PCB housing was manufactured exclusively on manually controlled machinery, so the measured 1-dB insertion loss BW is approximately 25%.

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.

A computationally efficient method is presented to model the scan-dependent active reflection coefficient ( $\Gamma _{act}$ ) of broadband antenna elements in sparse-regular phased arrays. The method, designed to accurately model the rapid response variations caused by the incursion of grating lobes into the visible region, constructs an adaptively sampled global surrogate model of the desired response with a presampling technique based on array factor theory. The experimental results for narrowband dipole antenna elements show that the proposed method yields a lower maximum model error than the standard space-filling techniques or pure adaptive sampling, with little or no compromise in the mean model error; similar results are attained for a broadband example with a tilted bowtie antenna element. It is also shown that for optimization procedures where the maximum $|\Gamma _{act}|$ is sought across scan angle and frequency, the proposed method consistently provides more accurate and efficient estimates than the other considered techniques.

The 50-meter Large Millimeter Telescope (LMT) operating on the Sierra Negra in Mexico is the largest single- dish millimeter-wave telescope in the world. Although designed to work in the 3 mm and 1 mm bands, there is significant potential for LMT observations at centimeter wavelengths. Here, we summarize the scientific case and operational arguments for a K-band receiver system on the LMT, describe several of the unique technical challenges that the proposed installation would entail, and mention some possible solutions to these challenges.

Water vapour radiometers (WVRs) are critical to both site surveying and site management in microwave and mm-wave very long baseline interferometry (VLBI). We report on the first two years of progress made towards improving the state of water vapour radiometry at HartRAO, South Africa, and the LMT in Mexico, under a SAMexico bilateral programme. We report on progress in the development of low-cost site surveying instruments, multi-purpose cooled receivers, as well as refurbishment and upgrades to existing 22/31 GHz and 215 GHz tipping radiometers.

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.

Traditionally, regularly spaced antenna arrays follow the spatial Nyquist criterion to guarantee an unambiguous analysis. We present a novel technique that makes use of two sparse non-Nyquist regularly spaced antenna arrays, where one of the arrays is just a shifted version of the other. The method offers several advantages over the use of traditional dense Nyquist-spaced arrays, while maintaining a comparable algorithmic complexity for the analysis. Among the advantages we mention: an improved resolution for the same number of receivers and reduced mutual coupling effects between the receivers, both due to the increased separation between the antennas. 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 co-prime arrays. In addition, an easy validation step allows to automatically detect the precise number of incoming signals, which is usually considered a difficult issue. At the same time, the validation step improves the accuracy of the retrieved results and eliminates unreliable results in the case of noisy data. The performance of the proposed method is illustrated with respect to the influence of noise as well as to the effect of mutual coupling.

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.

Designing shaped offset Gregorian reflector systems to operate with several interchangeable feed horns, over frequency bandwidths of more than a decade, with multiple, often conflicting, performance figures of merit such as aperture efficiency, receiving sensitivity, sidelobe levels, and cross polarization isolation is a difficult optimization problem. An additional complication may be that the radiation patterns of all the feeds to be used in the system are not known at the time of the dish designs, as upgrades to the feeds may happen throughout the lifetime of large reflector systems. This paper presents a systematic parametric study to quantify the effects of the main causes of performance degradation in such a system, i.e. reflector diffraction and feed pattern variations. First, ideal Gaussian feed patterns are used in order to isolate the diffraction effects, and then the ideal patterns are varied to model the effect of using wideband feeds exhibiting radiation pattern variations over frequency. It is shown that the peak position in the shaping parameter space of the receiving sensitivity is not strongly influenced by diffraction - although the peak value is, as expected, reduced at lower frequencies. This allows similar feed patterns to be used in different frequency bands to still produce systems operating near the maximum sensitivity. When using non-ideal feed patterns it is shown that, for most performance metrics, diffraction effects dominate the feed variation performance degradation in smaller dishes. This allows possibly relaxed requirements on the radiation patterns of feeds used to illuminate electrically small reflector systems.

A Kriging interpolation-based method for the rapid multi-objective optimisation (MO) of the radiation patterns of pencil beam reflector antenna systems is presented. MO provides a designer with quantitative trade-off information about conflicting design goals, but typically requires a large number of time-consuming function evaluations. The smoothly varying response surfaces of pencil beam reflector systems (gain, receiving sensitivity, sidelobe levels, etc.), as functions of the design parameters, are well suited to an interpolation-based response surface approximation optimisation strategy. By constructing Kriging interpolants of all the design goals and performing the MO on these fast models the optimisation can be significantly accelerated. This study presents an optimisation strategy where an iterative method is used to improve the accuracy of the response surface interpolants along the perceived Pareto front. Several examples are provided to illustrate the algorithmic flow, as well as confirm the accuracy and acceleration of the optimisation strategy achievable for typical reflector problems.

This paper presents a general coaxial-conical-coaxial transmission line transition with a built-in impedance taper that can explicitly be chosen or optimized. The presented transition has a smooth profile, which is advantageous for peak power handling, and provides more degrees of freedom and greater flexibility to optimize devices compared with other transitions between conical and coaxial lines or coaxial lines of different sizes. The method is validated by designing and fabricating an eight-way coaxial line combiner, 0.55 $\lambda $ long and $0.26~\lambda $ in diameter, with a measured reflection coefficient of better than −22 dB over a 115% bandwidth (BW), and better than −12 dB over a 130% BW.

The Galactic Centre contains a supermassive black hole with a mass of 4 million suns within an environment that differs markedly from that of the Galactic disk. While the black hole is essentially quiescent in the broader context of active galactic nuclei, X-ray observations have provided evidence for energetic outbursts from its surroundings. Also, while the levels of star formation in the Galactic Centre have been approximately constant over the last few hundred Myr, there is evidence of elevated short-duration bursts, strongly influenced by interaction of the black hole with the enhanced gas density present within the ring-like Central Molecular Zone at Galactic longitude |l| < 0.7 degrees and latitude |b| < 0.2 degrees. The inner 200 pc region is characterized by large amounts of warm molecular gas, a high cosmic ray ionization rate, unusual gas chemistry, enhanced synchrotron emission, and a multitude of radio-emitting magnetised filaments, the origin of which has not been established. Here we report radio imaging that reveals bipolar bubbles spanning 1 degree x 3 degrees (140 parsecs x 430 parsecs), extending above and below the Galactic plane and apparently associated with the Galactic Centre. The structure is edge-brightened and bounded, with symmetry implying creation by an energetic event in the Galactic Centre. We estimate the age of the bubbles to be a few million years, with a total energy of 7 x 10^52 ergs. We postulate that the progenitor event was a major contributor to the increased cosmic-ray density in the Galactic Centre, and is in turn the principal source of the relativistic particles required to power the synchrotron emission of the radio filaments within and in the vicinity of the bubble cavities., 25 pages, 3 figures, 3 tables, authors' version of a Letter published in Nature on 11 September 2019

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.

The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is a new project to perform radiometric measurements in the 50–200 MHz frequency range, with the purpose of realising a confident detection of the red-shifted 21 cm line signature pertaining to the Epoch of Reionisation (EoR) and Cosmic Dawn (CD). The brightness temperature of this signal is several orders of magnitude lower than those of the galactic and extra-galactic foregrounds, necessitating a radiometer design that is capable of extracting the 21 cm signal with minimal distortion due to systematic effects in the receiver instrumentation. The REACH project is an international collaboration between numerous research institutions, with prototype deployment for late 2019 planned in the Karoo region of South Africa.

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.

A comparison of measured and simulated primary beam patterns of the MeerKAT radio telescope is presented. It is shown that the measured data, after removing noise through aperture projection, displays some asymmetry across the apparent symmetry plane of the offset Gregorian reflector structure. This is absent in the simulated results, but the residual between the simulations and measurements is slowly varying, and mainly due to systematic errors in the geometry of the antenna. This effect will be exploited to estimate the beam patterns of the different reflectors in frequency channels where radio frequency interference has corrupted the measured data.

A new N-way partially-filled radial combiner design is proposed. The design integrates the advantages of printed circuit board (PCB) technology and those of air cavities into a single reactive power combiner. Minimal transmission losses are achieved using an air cavity, and the PCB allows flexibility with regard to port terminations. Importantly, simple and accurate PCB etching is exploited to achieve a wide-band central port match, without resorting to the typical high accuracy machining of the central feed pin. A 16-way prototype was manufactured and tested. When symmetrically excited, the measured input reflection coefficient is less than –15 dB over 20 % fractional bandwidth at 4 GHz. Peripheral port signal amplitude and phase imbalances are within ± 0.3 dB and 2.5 degrees, respectively. The prototype incurs less than 0.45± dB transmission loss, yielding a combining efficiency better than 90% in the operating band. Additionally, the cavity housing as tested was made without the use of CNC machines.

After a decade of design and construction, South Africa's SKA-MID precursor MeerKAT has begun its science operations. To make full use of the widefield capability of the array, it is imperative that we have an accurate model of the primary beam of its antennas. We have taken available L-band full-polarization 'astro-holographic' observations of three antennas and a generic electromagnetic simulation and created sparse representations of the beams using principal components and Zernike polynomials. The spectral behaviour of the spatial coefficients has been modelled using discrete cosine transform. We have provided the Zernike-based model over a diameter of 10 deg averaged over the beams of three antennas in an associated software tool (EIDOS) that can be useful in direction-dependent calibration and imaging. The model is more accurate for the diagonal elements of the beam Jones matrix and at lower frequencies. As we get more accurate beam measurements and simulations in the future, especially for the cross-polarization patterns, our pipeline can be used to create more accurate sparse representations of MeerKAT beams., 16 pages, 18 figures. This is a pre-copyedited, author-produced PDF of an article accepted for publication in MNRAS following peer review. The version of record [K. M. B. Asad et al., 2021] is available online at: https://doi.org/10.1093/mnras/stab104

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.

This paper describes the design of sinuous antenna reflector feeds using blended rational interpolation. The blended rational interpolation method is developed to interpolate a sparse set of high‐fidelity (HF) data while following the trends of a denser set of low‐fidelity (LF) data. The HF data are obtained by full‐wave computational electromagnetics simulations of the input impedance of a pyramidal sinuous antenna above a ground plane, while the LF data are obtained, at a significantly reduced computational cost, through simulations of a truncated version of the same antenna. Comparisons with other interpolation schemes, both for HF as well as for multifidelity data sets, are presented. It is shown that the blended rational interpolation scheme presented herein yields improved accuracy in most cases. A design example is also presented where a global model of the maximum input reflection coefficient over frequency is built over the design space and thus used to identify the region of acceptable performance. Comparisons with a validation set of HF data resulted in similar results, with the blended rational interpolation model requiring significantly shorter computer simulation time.

We present the confusion-limited 1.28 GHz MeerKAT DEEP2 image covering one $\approx 68'$ FWHM primary beam area with $7.6''$ FWHM resolution and $0.55 \pm 0.01$ $\mu$Jy/beam rms noise. Its J2000 center position $\alpha=04^h 13^m 26.4^s$, $\delta=-80^\circ 00' 00''$ was selected to minimize artifacts caused by bright sources. We introduce the new 64-element MeerKAT array and describe commissioning observations to measure the primary beam attenuation pattern, estimate telescope pointing errors, and pinpoint $(u,v)$ coordinate errors caused by offsets in frequency or time. We constructed a 1.4 GHz differential source count by combining a power-law count fit to the DEEP2 confusion $P(D)$ distribution from $0.25$ to $10$ $\mu$Jy with counts of individual DEEP2 sources between $10$ $\mu$Jy and $2.5$ mJy. Most sources fainter than $S \sim 100$ $\mu$Jy are distant star-forming galaxies obeying the FIR/radio correlation, and sources stronger than $0.25$ $\mu$Jy account for $\sim93\%$ of the radio background produced by star-forming galaxies. For the first time, the DEEP2 source count has reached the depth needed to reveal the majority of the star formation history of the universe. A pure luminosity evolution of the 1.4 GHz local luminosity function consistent with the Madau & Dickinson (2014) model for the evolution of star-forming galaxies based on UV and infrared data underpredicts our 1.4 GHz source count in the range $-5 \lesssim \log[S(\mathrm{Jy})] \lesssim -4$., Comment: 20 pages, 18 figures. Accepted for publication in ApJ

This paper describes a data-driven method to model the radiation patterns (over a large angular region) and scattering parameters of antennas as a function of the geometry of the antenna. The radiation pattern model consists of a linear combination of characteristic basis function patterns (CBFPs), where the expansion coefficients of the CBFPs are the functions of geometrical features of the antenna. Scattering parameters are modeled by means of parameterized state-space matrices. The obtained models are quick to evaluate and are thus suitable for design activities where multiple simulations are required. The proposed method is validated through illustrative examples.

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.

New radio (MeerKAT and Parkes) and X-ray (XMM-Newton, Swift, Chandra, and NuSTAR) observations of PSR J1622-4950 indicate that the magnetar, in a quiescent state since at least early 2015, reactivated between 2017 March 19 and April 5. The radio flux density, while variable, is approximately 100x larger than during its dormant state. The X-ray flux one month after reactivation was at least 800x larger than during quiescence, and has been decaying exponentially on a 111+/-19 day timescale. This high-flux state, together with a radio-derived rotational ephemeris, enabled for the first time the detection of X-ray pulsations for this magnetar. At 5%, the 0.3-6 keV pulsed fraction is comparable to the smallest observed for magnetars. The overall pulsar geometry inferred from polarized radio emission appears to be broadly consistent with that determined 6-8 years earlier. However, rotating vector model fits suggest that we are now seeing radio emission from a different location in the magnetosphere than previously. This indicates a novel way in which radio emission from magnetars can differ from that of ordinary pulsars. The torque on the neutron star is varying rapidly and unsteadily, as is common for magnetars following outburst, having changed by a factor of 7 within six months of reactivation., Comment: Published in ApJ (2018 April 5); 13 pages, 4 figures

A simple method is proposed that is capable of predicting antenna array embedded element patterns based on the radiation and scattering characteristics of a single isolated antenna element. The effect of mutual coupling, which causes embedded element patterns to differ, is accounted for through the multi-scattering of electromagnetic fields between array elements. This is a post-processing step and therefore obviates the need to run time-consuming full-wave electromagnetic field simulations, which is particularly important when numerous antenna array configurations need to be analyzed rapidly. The accuracy of three single-scattering versions of the pattern prediction method are evaluated on a range of two-element dipole arrays.

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 reflector’s 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.

This communication shows that, given the correct terminal voltages on each element, a log-periodic dipole array (LPDA) can achieve an omnidirectional radiation pattern over a wide frequency bandwidth. This cannot be achieved by analytical means, but through a global search optimization strategy. An optimization method, which achieves omnidirectional radiation of the array with maximum gain deviations of less than 0.06 dB over more than 30% bandwidth, is discussed and the results are presented.

Antenna noise temperature calculations of reflector systems is often a slow process and makes direct optimization of the sensitivity of these systems a difficult and time-consuming task. This paper presents an improvement to a recently proposed method to speed up these calculations by several orders of magnitude for large dish systems. The accuracy of the improved method is tested for several types of offset Gregorian systems, and errors are shown to be in the order of a few percent. Comparisons of several layers of simplification to a standard brightness temperature model are also presented to aid the designer in the choice of model complexity to use.