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Array antennas are increasingly used for transmission and reception in applications such as telecommunication, medical imaging, radar, or radio astronomy. Low-loss matching networks are inserted in each channel between the antenna and amplifiers in order to optimize power transfer in transmission and/or signal-to-noise ratio (SNR) in reception. The adjustment of these matching components becomes challenging if residual coupling between antenna elements significantly influences the results. The most general, so-called, array noise matching then leads to the optimum SNR taking into account the noise-coupling effects also. This paper describes that concept completely in the scattering matrix domain using waves for signal and noise rather than voltages and currents. [ABSTRACT FROM AUTHOR]

Performance of digitally beamformed phased arrays relies on accurate calibration of the array by obtaining gains of each antenna in the array. The stations of the Square Kilometer Array‐Low (SKA‐Low) are such digital arrays, where the station calibration is currently performed using conventional interferometric techniques. An alternative calibration technique similar to holography of dish based telescopes has been suggested in the past. In this paper, we develop a novel mathematical framework for holography employing tensors, which are multi‐way data structures. Self‐holography using a reference beam formed with the station under test itself and cross‐holography using a different station to obtain the reference beam are unified under the same formalism. Besides, the relation between the two apparently distinct holographic approaches in the literature for phased arrays is shown, and we show that under certain conditions the two methods yield the same results. We test the various holographic techniques on an SKA‐Low prototype station Aperture Array Verification System 2 (AAVS2) with the Sun as the calibrator. We perform self‐holography of AAVS2 and cross‐holography with simultaneous observations carried out with another station Engineering Development Array 2. We find the results from the holographic techniques to be consistent among themselves as well as with a more conventional calibration technique. Key Points: A framework for phased array holography using tensors is presentedMultiple holographic techniques are unifiedSelf‐holography and cross‐holography of Aperture Array Verification System 2 are performed and the results compared [ABSTRACT FROM AUTHOR]

In this paper, a high-gain THz antenna array is presented. The array uses a polyimide substrate with a thickness of 10 μ m, a relative permittivity of 3.5, and an overall volume of 2920 μ m × 1055 μ m × 10 μ m, which can be employed for THz band space communication and other interesting applications. The dual-band single-element antenna is designed in four steps, while operating at 0.714 and 0.7412 THz with −10 dB bandwidths of 4.71 and 3.13 GHz, providing gain of 5.14 and 5 dB, respectively. In order to achieve a high gain, multiple order antenna arrays are designed such as the 2 × 1 antenna array and the 4 × 1 antenna array, named type B and C, respectively. The gain and directivity of the proposed type C THz antenna array are 12.5 and 11.23 dB, and 12.532 and 11.625 dBi at 0.714 and 0.7412 THz, with 99.76 and 96.6% radiation efficiency, respectively. For justification purposes, the simulations of the type B antenna are carried out in two simulators such as the CST microwave studio (CSTMWS) and the advance design system (ADS), and the performance of the type B antenna is compared with an equivalent circuit model on the bases of return loss, resulting in strong agreement. Furthermore, the parametric analysis for the type C antenna is done on the basis of separation among the radiating elements in the range 513 to 553 μ m. A 64 × 1 antenna array is used to achieve possible gains of 23.8 and 24.1 dB, and directivity of 24.2 and 24.5 dBi with good efficiencies of about 91.66 and 90.35% at 0.7085 and 0.75225 THz, respectively, while the 128 × 1 antenna array provides a gain of 26.8 and 27.2 dB, and directivity of 27.2 and 27.7 dBi with good efficiency of 91.66 and 90.35% at 0.7085 and 0.75225 THz, respectively. All the results achieved in this manuscript ensure the proposed design is a feasible candidate for high-speed and free space wireless communication systems. [ABSTRACT FROM AUTHOR]

The recently developed new generation, low-frequency Giant Ukrainian Radio Telescope (GURT) is built nearby the well-known Ukrainian T-shaped Radio Telescope. The new facility employs a phased antenna array composed of many subarrays of $5 \times 5$ active antenna elements. In this paper, the parameters of the active antenna used as array element are studied, with special attention paid to sensitivity. The electrical and noise parameters are calculated using computer simulation and wave techniques for noise modeling of two-port networks. The results of numerical calculations of the sensitivity are given in terms of the sky noise dominance (SND) and system equivalent flux density of the GURT element within 10–80 MHz. The calculated results are compared with in situ measurements. [ABSTRACT FROM AUTHOR]

Low frequency Radio Astronomy instruments like LOFAR and SKA-LOW use arrays of dipole antennas for the collection of radio signals from the sky. Due to the large number of antennas involved, the total data rate produced by all the antennas is enormous. Storage of the antenna data is both economically and technologically infeasible using the current state of the art storage technology. Therefore, real-time processing of the antenna voltage data using beam forming and correlation is applied to achieve a data reduction throughout the signal chain. However, most science could equally well be performed using an archive of raw antenna voltage data coming straight from the A/D converters instead of capturing and processing the antenna data in real time over and over again. Trends on storage and computing technology make such an approach feasible on a time scale of approximately 10 years. The benefits of such a system approach are more science output and a higher flexibility with respect to the science operations. In this paper we present a radically new system concept for a radio telescope based on storage of raw antenna data. LOFAR is used as an example for such a future instrument. [ABSTRACT FROM AUTHOR]

Front-end amplifiers with high input impedance are exploited to build active array elements for modern low-frequency radio telescopes. The combination of such loading and mutual coupling between adjacent elements is responsible for the deep nulls that are present in some embedded element patterns (EEPs) of the Low-Frequency Array. This paper quantifies this phenomenon with both models and experimental data. The full-wave array simulations and the measured amplifier scattering parameters are combined to obtain the EEP models. These models are in good agreement with the in situ measurements that have been performed with a micro unmanned aerial vehicle carrying a field source. [ABSTRACT FROM PUBLISHER]

In radio astronomy, holography is a commonly used technique to create an image of the electric field distribution in the aperture of a dish antenna. The image is used to detect imperfections in the reflector surface. Similarly, holography can be applied to phased array telescopes, in order to measure the complex gains of the receive paths of individual antennas. In this paper, a holographic technique is suggested to calibrate the digital beamformer of a phased array telescope. The effectiveness of the technique was demonstrated by applying it on data from the Engineering Development Array 2, one of the prototype stations of the low frequency component of the Square Kilometre Array. The calibration method is very quick and requires few resources. In contrast to holography for dish antennas, it works without a reference antenna. We demonstrate the utility of this technique for initial station commissioning and verification as well as for routine station calibration. Key Points: Holographic aperture imaging for phased array telescopes is presentedSuggested technique requires few resources and can be used for calibrationTechnique was successfully tested on prototype station of SKA‐Low [ABSTRACT FROM AUTHOR]

The development of new phased-array systems in radio astronomy, as the low-frequency array (LOFAR) and the square kilometre array (SKA), formed of a large number of small and flexible elementary antennas has led to significant challenges. Among them, model calibration is a crucial step in order to provide accurate and, thus, meaningful images and requires the estimation of all the perturbation effects introduced along the signal propagation path for a specific source direction and antenna position. Usually, it is common to perform model calibration using the a priori knowledge regarding a small number of known strong calibrator sources but under the assumption of Gaussianity of the noise. Nevertheless, observations in the context of radio astronomy are known to be affected by the presence of outliers, which are due to several causes, e.g., weak non-calibrator sources or man-made radio frequency interferences. Consequently, the classical Gaussian noise assumption is violated leading to severe degradation in performances. In order to take into account the outlier effects, we assume that the noise follows a spherically invariant random distribution. Based on this modeling, a robust calibration algorithm is presented in this paper. More precisely, this new scheme is based on the design of an iterative relaxed concentrated maximum likelihood estimation procedure that allows us to obtain closed-form expressions for the unknown parameters with a reasonable computational cost. Numerical simulations reveal that the proposed algorithm outperforms the state-of-the-art calibration techniques. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

Many aspects of the Atacama Large Millimeter Array (ALMA) instrument are still unknown due to its young age. One such aspect is the true nature of the primary beam of each baseline, and how changes to the individual primary beams affect astronomical observations when said changes are ignored during imaging. This paper aims to create a more thorough understanding of the strengths and weaknesses of ALMA through realistic modeling of the primary beams and simulated observations, which in turn can inform the user of the necessity of implementing more computationally costly algorithms, such as A-Projection, and when simpler, quicker algorithms will suffice. We quantify our results by examining the dynamic range of each observation, along with the ability to reconstruct the Stokes I amplitude of the test sources. These tests conclude that for the dynamic ranges of less than 1000, for point sources and sources much smaller than the main lobe of the primary beam, the accuracy of the primary beam model beyond the physical size of the aperture simply does not matter. In the observations of large extended sources, deconvolution errors dominate the reconstructed images and the individual primary beam errors were indistinguishable from each other. [ABSTRACT FROM PUBLISHER]

Radio astronomical observations are increasingly contaminated by RF interference. Assuming an array of telescopes, a previous technique considered spatial filtering based on projecting out the interferer array signature vector. A disadvantage is that this effectively reduces the array by one (expensive) telescope. In this paper, we consider extending the astronomical array with a reference antenna array, and develop spatial filtering algorithms for this situation. The information from the reference antennas improves the quality of the interferer signature vector estimation, hence more of the interference can be projected out. Moreover, since only the covariance data of the astronomical array has to be reconstructed, the conditioning of the problem improves as well. The algorithms are tested both on simulated and experimental data. [ABSTRACT FROM AUTHOR]

The very demanding requirements of the SKA-low instrument call for a challenging antenna design capable of delivering excellent performance in radiation patterns, impedance matching, polarization purity, cost, longevity, etc. This paper is devoted to the development (design and test of the first prototypes) of an active ultra-wideband antenna element for the low-frequency instrument of the SKA radio telescope. The antenna element and differential low noise amplifier described here were originally designed to cover the former SKA-low band (70-450 MHz) but it is now aimed to cover the re-defined SKA-low band (50-350 MHz) and furthermore the antenna is capable of performing up to 650 MHz with the current design. The design is focused on maximum sensitivity in a wide field of view (+/− 45° from zenith) and low cross-polarization ratios. Furthermore, the size and cost of the element has to be kept to a minimum as millions of these antennas will need to be deployed for the full SKA in very compact configurations. The primary focus of this paper is therefore to discuss various design implications for the SKA-low telescope. [ABSTRACT FROM AUTHOR]

The Ultralong‐Wavelength (ULW) regime of longer than 10 m (corresponding to frequencies below 30 MHz) remains as the last virtually unexplored window in radio astronomy and is presently attracting considerable attention as an area of potentially rewarding studies. However, the opaqueness of the Earth's ionosphere makes the ULW celestial radio emission very difficult to detect with ground‐based instrumentation. The impact of the ionosphere on ULW radio emission depends on the solar cycle activities and varies with time. In addition, the ULW spectrum region is densely populated by intensive artificial radio frequency interference. An obvious solution of these problems is to place an ULW radio telescope in space. However, this solution is expensive and poses nonnegligible technological challenges. An alternative approach is triggered by recent studies showing that the period of post 2020 will be most suitable for exploratory ground‐based ULW radio observations due to the expected "calm" state of the ionosphere; the ionospheric cutoff frequency could be well below 10 MHz, even in the day time. In anticipation of this upcoming opportunity, we propose and present in this paper a concept of an experimental ULW radio array, with the intention of setting it up in Inner Mongolia, China. This ULW facility will use the infrastructure of the currently operational Mingantu spectral radio heliograph. The proposed ULW array covers the frequency range from 1 to 72 MHz. This experimental array will be used for exploratory studies of celestial radio emission in the ULW range of the spectrum. Key Points: We propose an experimental Ultralong‐Wavelength (ULW) radio array operating in the frequency range of 1–72 MHzThe proposed ULW radio array is of low cost with easy implementability and will be useful post 2020 during the solar cycle minimum [ABSTRACT FROM AUTHOR]

Context. Modern radio astronomical arrays have (or will have) more than one order of magnitude more receivers than classical synthesis arrays, such as the VLA and the WSRT. This makes gain calibration a computationally demanding task. Several alternating direction implicit (ADI) approaches have therefore been proposed that reduce numerical complexity for this task from O(P3) to O(P2), where P is the number of receive paths to be calibrated Aims. We present an ADI method, show that it converges to the optimal solution, and assess its numerical, computational and statistical performance. We also discuss its suitability for application in self-calibration and report on its successful application in LOFAR standard pipelines. Methods. Convergence is proved by rigorous mathematical analysis using a contraction mapping. Its numerical, algorithmic, and statistical performance, as well as its suitability for application in self-calibration, are assessed using simulations. Results. Our simulations confirm the O(P2) complexity and excellent numerical and computational properties of the algorithm. They also confirm that the algorithm performs at or close to the Cramer-Rao bound (CRB, lower bound on the variance of estimated parameters). We find that the algorithm is suitable for application in self-calibration and discuss how it can be included. We demonstrate an order-of-magnitude speed improvement in calibration over traditional methods on actual LOFAR data. Conclusions. In this paper, we demonstrate that ADI methods are a valid and computationally more efficient alternative to traditional gain calibration methods and we report on its successful application in a number of actual data reduction pipelines. [ABSTRACT FROM AUTHOR]

With the ongoing growth in radio communications, there is an increased contamination of radio astronomical source data, which hinders the study of celestial radio sources. In many cases, fast mitigation of strong radio frequency interference (RFI) is valuable for studying short lived radio transients so that the astronomers can perform detailed observations of celestial radio sources. The standard method to manually excise contaminated blocks in time and frequency makes the removed data useless for radio astronomy analyses. This motivates the need for better RFI mitigation techniques for array of size M antennas. Although many solutions for mitigating strong RFI improves the quality of the final celestial source signal, many standard approaches require all the eigenvalues of the spatial covariance matrix (R ∈ ℂ M × M ) of the received signal, which has O (M 3) computation complexity for removing RFI of size d where d ≪ M. In this work, we investigate two approaches for RFI mitigation, (1) the computationally efficient Lanczos method based on the Quadratic Mean to Arithmetic Mean (QMAM) approach using information from previously-collected data under similar radio-sky-conditions, and (2) an approach using a celestial source as a reference for RFI mitigation. QMAM uses the Lanczos method for finding the Rayleigh–Ritz values of the covariance matrix R , thus, reducing the computational complexity of the overall approach to O (d M 2). Our numerical results, using data from the radio observatory Long Wavelength Array (LWA-1), demonstrate the effectiveness of both proposed approaches to remove strong RFI, with the QMAM-based approach still being computationally efficient. [ABSTRACT FROM AUTHOR]

This paper presents a 324-element 2-D broadside array for radio astronomy instrumentation which is sensitive to two mutually orthogonal polarizations. The array is composed of cruciform units consisting of a group of four Vivaldi antennas arranged in a cross-shaped structure. The Vivaldi antenna used in this array exhibits a radiation intensity characteristic with a symmetrical main beam of 87.5^\circ at 3 GHz and 44.2^\circ at 6 GHz. The measured maximum side/backlobe level is 10.3 dB below the main beam level. The array can operate at a high frequency of 5.4 GHz without the formation of grating lobes. [ABSTRACT FROM PUBLISHER]

A number of new and planned radio telescopes will consist of large arrays of low-gain antennas operating at frequencies below 300 MHz. In this frequency regime, Galactic noise can be a significant or dominant contribution to the total noise. This, combined with mutual coupling between antennas, makes it difficult to predict the sensitivity of these instruments. This paper describes a system model and procedure for estimating the system equivalent flux density (SEFD)-a useful and meaningful metric of the sensitivity of a radio telescope-that accounts for these issues. The method is applied to LWA-1, the first “station” of the Long Wavelength Array (LWA) interferometer. LWA-1 consists of 512 bowtie-type antennas within a 110 × 100 m elliptical footprint, and is designed to operate between 10 MHz and 88 MHz using receivers having noise temperature of about 250 K. It is shown that the correlation of Galactic noise between antennas significantly desensitizes the array for beam pointings which are not close to the zenith. It is also shown that considerable improvement is possible using beamforming coefficients which are designed to optimize signal-to-noise ratio under these conditions. Mutual coupling is found to play a significant role, but does not have a consistently positive or negative influence. In particular, we demonstrate that pattern multiplication (assuming the behavior of single antennas embedded in the array is the same as those same antennas by themselves) does not generate reliable estimates of SEFD. [ABSTRACT FROM AUTHOR]

In this paper, differential low-noise amplifiers are presented as a very powerful solution for radio astronomy applications. A fully differential amplifier topology has been analyzed and implemented in microstrip technology with discrete surface mount components. The amplifier design is made for an active receiving dense antenna array. Thus, the differential amplifier source impedance is no longer 50 \Omega, but 150 \Omega from the proposed bunny-ear antennas. A full characterization in terms of gain and noise has been undertaken. Source–pull measurements have been included in order to evaluate the performance of the amplifiers operating with variable source impedances. Noise temperatures below 55 K have been obtained for the differential design in the 300–1000-MHz band for the 150-\Omega impedance. In addition, the results for different scanning angles are also presented. [ABSTRACT FROM AUTHOR]

The Long Wavelength Array (LWA) will be a new multipurpose radio telescope operating in the frequency range 10-88 MHz. Upon completion, the LWA will consist of 53 phased array "stations" distributed over a region about 400 km in diameter in the state of New Mexico. Each station will consist of 256 pairs of dipole-type antennas whose signals are formed into beams, with outputs transported to a central location for high-resolution aperture synthesis imaging. The resulting image sensitivity is estimated to be a few millijanskys (5σ, 8 MHz, two polarizations, 1 h, zenith) in 20-80 MHz; with resolution and field of view of (8", 8°) and (2", 2°) at 20 and 80 MHz, respectively. Notable engineering features of the instrument, demonstrated in this paper, include Galactic-noise limited active antennas and direct sampling digitization of the entire tuning range. This paper also summarizes the LWA science goals, specifications, and analysis leading to top-level design decisions. [ABSTRACT FROM AUTHOR]

In this paper, a rigorous procedure for calculating the antenna noise temperature is described, and applied to the antenna of BEST-i (Basic Element for SKA Training - version i), which represents one of the SKA (Square Kilometer Array) demonstrators. The SKA will be a new-generation radio telescope, with a collecting area 50 times larger than the area of today's largest radio telescope. BEST is the Italian reduced-scale SKA demonstrator, based on the re-instrumentation of about 8000 m² of the Northern Cross radio telescope, built with cylindrical parabolic antennas. In order to perform the antenna-temperature analysis, an electromagnetic tool to accurately evaluate the antenna pattern in the whole space surrounding the antenna itself is required. We used the commercial software GRASP8, developed by TICRA, to characterize reflector antennas. The antenna temperature was evaluated using the guideline adopted by the Antenna Task Force of the SKA world consortium. For BEST-1 at 408 MHz, we found an antenna temperature equal to 30 K in the zenith direction and 60 K at the horizon. The numerical results have been verified through several celestial calibration radio sources. [ABSTRACT FROM AUTHOR]

This paper describes a microstrip array antenna designed to operate at 35 GHz, and used as a position detector of moving targets. To obtain a wide detection range, the array was arranged to have a relatively narrower beamwidth in the elevation plane compared to the beamwidth in the azimuthal plane. This was achieved by aligning the electric-field plane of each element to the array axis. Employing a 3 dB tapered feed network, the array has a sidelobe level of less than -20 dB, while retaining a wide azimuthal beamwidth. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 276–280, 2001. [ABSTRACT FROM AUTHOR]

Radio interferometry most commonly involves antennas or antenna arrays of identical design. The identical antenna assumption leads to a convenient and useful mathematical simplification resulting in a scalar problem. An interesting variant to this is a “hybrid” interferometer involving two designs. We encounter this in the characterization of low-frequency antenna/array prototypes using a homogenous low-frequency array telescope such as the Murchison Widefield Array. In this paper, we present an interferometry equation that applies to hybrid antennas. The resulting equation involves vector inner products rather than scalar multiplications. We discuss physical interpretation and useful applications of this concept in the areas of sensitivity measurement and calibration of an antenna/array under test using a compact calibrator source. [ABSTRACT FROM PUBLISHER]

This paper presents the design and characterization process of an active array demonstrator for the mid-frequency range (i.e., 300 MHz?1000 MHz) of the future Square Kilometre Array (SKA) radio telescope. This demonstrator, called FIDA3 (FG-IGN: Fundaci?n General Instituto Geogr?fico Nacional ? Differential Active Antenna Array), is part of the Spanish contribution for the SKA project. The main advantages provided by this design include the use of a dielectric-free structure, and the use of a fully-differential receiver in which differential low-noise amplifiers (LNAs) are directly connected to the balanced tapered-slot antennas (TSAs). First, the radiating structure and the differential low-noise amplifiers were separately designed and measured, obtaining good results (antenna elements with low voltage standing-wave ratios, array scanning capabilities up to 45?, and noise temperatures better than 52 K with low-noise amplifiers at room temperature). The potential problems due to the differential nature of the proposed solution are discussed, so some effective methods to overcome such limitations are proposed. Second, the complete active antenna array receiving system was assembled, and a 1 m2 active antenna array tile was characterized. [ABSTRACT FROM AUTHOR]

A general approach to determination of geometrical parameters of planar array antennas designed for forming of orthogonal beams steered in the nodes of a Cartesian periodic lattice specified in the space of direction cosines is proposed. The proof of orthogonality of the complex amplitude distributions and appropriate array factors for the arrays constructed according to the proposed approach is given. The role of the array element patterns in providing the beam pattern orthogonality is underlined. The issue of the number of the array elements necessary for forming of the specified number of beams is discussed. A few demonstrative examples of the beam clusters and appropriate configurations of the arrays where the number of the elements is equal to the specified number of the beams are considered. [ABSTRACT FROM PUBLISHER]

Phased-array antenna systems are playing an increasingly significant role in passive, i.e., receive-only, radio-astronomy applications. They provide attractive features, such as a large instantaneous field-of-view, multi-beaming capability, and rapid response time. In this paper, we present an overview of current developments in phased-array antenna systems for radio astronomy, with an emphasis on the design challenges posed by this application. [ABSTRACT FROM AUTHOR]

A 5:1 frequency bandwidth aperture array antenna based on a new planar structure is reported. Dual polarization is implemented with high cross-polarization discrimination over ±45° scan angle from zenith. It is a low-loss cost-effective approach using minimum dielectric materials. In the proposed planar antenna array design, a mutual coupled interconnected ring structure is used which extends the operational frequency band to the region where the element separation can be considerably greater than half a wavelength, unlike the previously reported interconnected array structures. Hence, the total number of elements can be reduced for a specified aperture size. The dual polarization has coincident phase centers and in the implementation reported here two low-noise amplifiers on the same board are integrated with a pair of dual-polarized elements to form an active array. A 10 by 10 element array prototype of the design has been manufactured and the measured results confirm it as a low cost, high-performance front-end solution. [ABSTRACT FROM AUTHOR]

21st century Radio Astronomy at frequencies below 1 GHz will be dominated by arrays of thousands or millions of elements in configurations which are dense or sparse, regular or random, delivering data to powerful digital back-ends. High performance, specified by parameters such as the effective aperture and system noise temperature, will be crucial to achieve the ambitious survey speeds and sensitivities demanded by astronomers. In this paper we analyze the different trade-offs and difficulties in the design of ultra-wideband array antenna elements for these radio telescopes at frequencies below 500 MHz. Bandwidths of 7:1 are desired while using low-profile elements to keep the overall cost within the budget. [ABSTRACT FROM AUTHOR]

We present two methods for measuring the noise temperature of a differential input single-ended output low-noise amplifier (DISO LNA) connected to an antenna. The first method is the direct measurement of the DISO LNA and antenna in an anechoic chamber at ambient temperature. The second is a simple and low-cost noise parameter extraction of the DISO device using a coaxial long cable. The reconstruction of the DISO noise parameter from the noise wave measurements of the DISO LNA with one terminated input port is discussed in detail. We successfully applied these methods to the Murchison widefield array LNA and antenna. [ABSTRACT FROM AUTHOR]

The Atacama Large Millimeter/submillimeter (ALMA) Array Front End (FE) system is the first element in a complex chain of signal receiving, conversion, processing and recording. 70 Front Ends will be required for the project. The Front End is designed to receive signals in ten different frequency bands. In the initial phase of operations, the antennas will be fully equipped with six bands. These are Band 3 (84-116 GHz), Band 4 (125-163 GHz), Band 6 (211-275 GHz), Band 7 (275-373 GHz), Band 8 (385-500 GHz) and Band 9 (602-720 GHz). It is planned to equip the antennas with the missing bands at a later stage of ALMA operations, with a few Band 5 (163-211 GHz) and Band 10 (787-950 GHz) receivers in use before the end of the construction project. The ALMA Front End is far superior to any existing receiver systems; spin-offs of the ALMA prototypes are leading to improved sensitivities in existing millimeter and submillimeter observatories. The Front End units are comprised of numerous elements, produced at different locations in Europe, North America and East Asia and are integrated at several Front End integration centers (FEIC) to insure timely delivery of all the units to Chile. The North American FEIC (NA FEIC) is at the National Radio Astronomy Observatory facility in Charlottesville, Virginia, USA. This paper describes the design and performance of the test set used at the NA FEIC to check the performance of the Front Ends, following integration and prior to shipment to Chile. [ABSTRACT FROM AUTHOR]

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The Atacama Large Millimeter/Submillimeter Array (ALMA) is an international radio telescope under construction in the Atacama Desert of northern Chile. ALMA is situated on a dry site at 5000 m elevation, allowing excellent atmospheric transmission over the instrument wavelength range of O.3-10 mm. ALMA will consist of two arrays of high-precision antennas. One, of up to 64 12-m-diameter antennas, is reconfigurable in multiple patterns ranging in size from 150 m up to ~15 km. A second array is composed of a set of four 12-m and 12 7-m antennas operating in one of two closely packed configurations ~50 m in diameter. The instrument will provide both interferometric and total-power astronomical information on atomic, molecular, and ionized gas and dust in the solar system, our galaxy, and the nearby to high-redshift universe. In this paper, we outline the scientific drivers, technical challenges, and planned progress of ALMA. [ABSTRACT FROM AUTHOR]

The Square Kilometre Array (SKA) will be an ultrasensitive radio telescope, built to further the understanding of the most important phenomena in the Universe, including some pertaining to the birth and eventual death of the Universe itself. Over the next few years, the SKA will make the transition from an early formative to a well-defined design. This paper outlines how the scientific challenges are translated into technical challenges, how the application of recent technology offers the potential of affordably meeting these challenges, and how the choices of technology will ultimately be made. The SKA will be an array of coherently connected antennas spread over an area about 3000 km in extent, with an aggregate antenna collecting area of up to 106 m² at centimeter and meter wavelengths. A key scientific requirement is the ability to carry out sensitive observations of the sky over large areas (surveys). The "survey speed" of the SKA will be enabled by the application of the most up-to-date signal-processing technology available. The SKA science impact will be widely felt in astroparticle physics and cosmology, fundamental physics, galactic and extragalactic astronomy, solar system science, and astrobiology. [ABSTRACT FROM AUTHOR]

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/ performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed. [ABSTRACT FROM AUTHOR]

The Low Frequency Array (LOFAR) is a large distributed radio telescope, consisting of phased array antenna stations that are combined in an aperture synthesis array. Antenna stations consist of many simple, omnidirectional antennas. Flexible station-based signal processing allows for trading bandwidth against instantaneous sky coverage. Central processing implements a software correlator, which can be reconfigured as a full tied array beamformer, and online calibration functions to handle the large data streams produced by the system. The key science programs for LOFAR challenge the technical specifications in several directions, which resulted in a highly reconfigurable architecture. This paper describes the LOFAR system design, the configuration, and the signal-processing chain. LOFAR has been developed by ASTRON and a consortium of universities and industrial partners. The instrument is currently being deployed in The Netherlands. Additional stations are being built in several other European countries. The telescope is considered an important pathfinder for the Square Kilometer Array (SKA) in demonstrating the potential of (sparse) aperture arrays, in developing solutions to major calibration issues that are directly applicable to the SKA, and in paving the way for the mass-production and operations of such large distributed radio telescope systems. [ABSTRACT FROM AUTHOR]

Argus is an experimental antenna array system designed to demonstrate all-sky monitoring for transient signals in the frequency range 1200-1700 MHz. It currently consists of 22 broadband spiral antennas (expandable to 32) which are individually instrumented, digitized, and analyzed in an attempt to detect and localize both broadband and narrowband astronomical transients. In this paper, we describe the design of the instrument. Notable features include a novel array data aggregation architecture, a detection algorithm which does not require accurate calibration or detailed knowledge of the array manifold, and very low per-element cost of about US$lk/element. A sensitivity of at least 6.6 x 10-22 Wm-2 Hz-1 = 66 kJy (zenith at 1700 MHz for a 209 ms observation with 60 kHz bandwidth) is achieved for the system as implemented. Performance is demonstrated in an experiment in which the Sun is detected, localized, and tracked as it moves across the sky. Other experiments confirming the functionality of Argus as an all-sky surveillance system are summarized. [ABSTRACT FROM AUTHOR]

A field-trial system is presented in this paper to demonstrate that radio-over-fiber (RoF) techniques can be successfully applied to the remoting of a large antenna array for radio astronomical applications. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 48–54, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20898 [ABSTRACT FROM AUTHOR]

Many astrophysical phenomena can only be studied in detail for objects in our galaxy, the Milly Way, but we know much more about the structure of thousands of nearby galaxies than we do about our own Galaxy. Accurate distance measurements in the Milky Way underpin our ability to understand a wide range of astrophysical phenomena and this requires observations from both the northern and southern hemisphere. Our ability to measure accurate parallaxes to southern masers has been hampered a range of factors, in particular the absence of a dedicated, homogeneous VLBI array in the south. We have recently made significant advances in astrometric calibration techniques which allow us to achieve trigonometric parallax accuracies of around 10 micro-arcseconds (μ as) for 6.7 GHz methanol masers with a hetrogeneous array of 4 antennas. We outline the details of this new "multiview" technique and present the first trigonometric parallax measurements that utilise this approach. [ABSTRACT FROM AUTHOR]

An almost all‐metal L band multi‐frequency high‐gain GNSS array antenna for an asteroid sampling return mission has been developed for orbit determination. The proposed antenna is designed for reception and is compatible with Beidou and GPS at L‐band (i.e. GPS‐L1: 1575.42 MHz; BDS‐B1I: 1561.098 MHz; BDS‐B2a: 1176.45 MHz). The radiation element is an almost all‐metal dipole with structure improvement, and thereby strong enough to handle vibrations and loads. The array antenna is composed of the 7‐radiation element and a low loss air suspended stripline beam‐forming network. The six elements around the center element have uniform distribution, while the center one is excited twice amplitude than outer ones for appropriate gain and beamwidth. A prototype of the proposed GNSS array antenna is designed in a simulation and measured in an experiment. The simulated and measured results agree very well, showing that the reflection coefficient <−17.5 dB over 32.8% bandwidth from 1.1 to 1.7 GHz is achieved with rotationally symmetric radiation patterns and gains >13.4 dBic, 15.2 dBic, 15.2 dBic at BDS‐B2a, GPS‐L1, BDS‐B1I, respectively. [ABSTRACT FROM AUTHOR]

In this paper, we reported the measured results of the antenna efficiency, sensitivity, and system noise temperature of the TM65m radio telescope. The key parameters describing a radio astronomy receiving system were introduced at first. Then, we discussed the measurement methods and the measuring errors. Finally, the measured results of the antenna efficiency, sensitivity, and system noise temperature performances at the L, S, C, and X bands were given. The results show that the efficiency and SEFD (System Equivalent Flux Density) decrease dramatically at both low and high elevations when the position of the subreflector is fixed. The antenna efficiencies at the C and X bands can be raised to 60% and more in the whole elevation range when the subreflector (servo) model is activated. The system noise temperature is independent to the subreflector model. Among the four wavebands, the C-band sensitivity and system noise temperature are optimal. [ABSTRACT FROM AUTHOR]

We report characterization results for an engineering prototype of a next-generation low-frequency radio astronomy array. This prototype, which we refer to as the Aperture Array Verification System 0.5 (AAVS0.5), is a sparse pseudorandom array of 16 log-periodic antennas designed for 70–450 MHz. It is colocated with the Murchison widefield array (MWA) at the Murchison radioastronomy observatory (MRO) near the Australian square kilometre array (SKA) core site. We characterize the AAVS0.5 using two methods: in situ radio interferometry with astronomical sources and an engineering approach based on detailed full-wave simulation. In situ measurement of the small prototype array is challenging due to the dominance of the Galactic noise and the relatively weaker calibration sources easily accessible in the southern sky. The MWA, with its 128 “tiles” and up to 3 km baselines, enabled in situ measurement via radio interferometry. We present array sensitivity and beam pattern characterization results and compare to detailed full-wave simulation. We discuss areas where differences between the two methods exist and offer possibilities for improvement. Our work demonstrates the value of the dual astronomy–simulation approach in upcoming SKA design work. [ABSTRACT FROM PUBLISHER]

This paper describes the design, fabrication, and test results of a room temperature multi-channel heterodyne receiver operating across the 2.3–8.2 GHz radio frequency (RF) band. Such a "Warm Section" (WS) receiver is part of phased arrays for reflector observing systems 2 (PHAROS2), a C-band phased array feed (PAF) demonstrator with digital beamformer for radio astronomy application. The WS receiver is cascaded to the PHAROS2 cryostat, which includes an array of Vivaldi antennas with low noise pre-amplification stages. The WS can handle up to 32 RF signals and, for each of them, realizes the operations of filtering, RF amplification and down-conversion from the RF to the 375–650 MHz intermediate frequency (IF). Also, the WS incorporates an IF-to-optical signal conversion through analogue wavelength division multiplexing IF over fiber (IFoF) and fiber-optic transmitters (OTXs). The 32-channel WS receiver consists of four eight-channel WS RF/IF modules, one local oscillator (LO) splitter module and one monitoring and control module, all hosted in a standard 6U × 19-inch rack. [ABSTRACT FROM AUTHOR]

The Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) is a dual array of omnidirectional VHF radio antennas located near Kilpisjärvi, Finland. It is operated by the Sodankylä Geophysical Observatory. It makes extensive use of the proven LOFAR antenna and digital signal-processing hardware, and can act as a stand-alone passive receiver, as a receiver for the European Incoherent Scatter (EISCAT) very high frequency (VHF) incoherent scatter radar in Tromsø, or for use in conjunction with other Fenno-Scandinavian VHF experiments. In addition to being a powerful observing instrument in its own right, KAIRA will act as a pathfinder for technologies to be used in the planned EISCAT_3-D phased-array incoherent scatter radar system and participate in very long baseline interferometry experiments. This paper gives an overview of KAIRA, its principal hardware and software components, and its main science objectives. We demonstrate the applicability of the radio astronomy technology to our geoscience application. Furthermore, we present a selection of results from the commissioning phase of this new radio observatory. [ABSTRACT FROM PUBLISHER]

We are at the dawn of a new golden age for radio astronomy, with a new generation of facilities under construction and the global community focused on the Square Kilometre Array as its goal for the next decade. These new facilities offer orders of magnitude improvements in survey speed compared to existing radio telescopes and arrays. Furthermore, the study of transient and variable radio sources, and what they can tell us about the extremes of astrophysics as well as the state of the diffuse intervening media, have been embraced as key science projects for these new facilities. In this paper we review the studies of the populations of radio transients made to date, largely based upon archival surveys. Many of these radio transients and variables have been found in the image plane, and their astrophysical origin remains unclear. We take this population and combine it with sensitivity estimates for the next generation arrays to demonstrate that in the coming decade we may find ourselves detecting l05 image plane radio transients per year, providing a vast and rich field of research and an almost limitless set of targets for multi-wavelength follow up. [ABSTRACT FROM AUTHOR]

A reduced-order model for large arrays of microstrip-fed tapered slot antennas (TSAs) is presented. The currents on the antenna conductors are modeled by a relatively small number of physics-based macro-domain basis functions through a technique which is known as the characteristic basis function method (CBFM). The array is treated as a metal-only structure, while the wideband microstrip feeds are separately modeled using quasi-static circuit models. It is demonstrated that, even though the dielectric-supported feeds are non-shielded and therefore form an integral part of each radiating antenna element, the feeds can be modeled independently from the strongly coupled antenna elements. Validation of the combined antenna-feed model has been carried out through the measurements of several practically realized TSA arrays, among them a 8 × 7 × 2 dual-polarized array. The results demonstrate good agreement over a large scan range, as well as over a wide frequency band. The polarization-discrimination capabilities of the antenna, when operating in phased-array mode, have been analyzed in the context of radio-astronomical applications. [ABSTRACT FROM AUTHOR]

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 conunon-mode currents through a (low loss) balancing-dissipation technique. A fully steerable design up to 45° in both principal planes is achieved. [ABSTRACT FROM AUTHOR]

We present catalogues of faint 1.4-GHz radio sources from extremely deep Very Large Array pointings in the Lockman Hole, the Hubble Deep Field-North (HDF-N) and the European Large Area ISO Survey (ELAIS) N2 field. Our analysis of the HDF-N data has produced maps that are significantly deeper than those previously published, and we have used these to search for counterparts to submillimetre sources. For each of the fields, we have derived normalized differential source counts and in the case of the HDF-N find no evidence for the previously reported underdensity of sources; our counts are entirely consistent with those found for the majority of other fields. The catalogues are available as an online supplement to this paper and the maps are also available for download. [ABSTRACT FROM AUTHOR]

Low-frequency (<300 MHz) aperture array systems are one of the new trends in modern radio astronomy. Among the challenges they pose, the instrumental calibration is a key aspect requiring an accurate and reliable model of each element of such electrically large array. A full-wave electromagnetic analysis has been carried out for the lower frequency (30–80 MHz) array of the low frequency array (LOFAR) radio telescope taking into account the presence of soil ground, the mutual coupling between the antennas and the relevant receiver impedance loading effects. The impact of mutual coupling effects on the embedded element and array patterns is assessed for two subarray configurations with different degrees of sparseness. A simplistic array factor approach has been implemented as well to determine the accuracy in the antenna pattern evaluation with respect to the full-wave approach. Finally, results from an experimental campaign conducted by means of a micro hexacopter system show the reliability of the developed array numerical model. [ABSTRACT FROM AUTHOR]

Signal artifacts due to radio frequency interference (RFI) are a common nuisance in radio astronomy. Conventionally, the RFI‐affected data are tagged by an expert data analyst in order to warrant data quality. In view of the increasing data rates obtained with interferometric radio telescope arrays, automatic data filtering procedures are mandatory. Here, we present the results from the implementation of a RFI‐detecting recurrent neural network (RNN) employing long short‐term memory (LSTM) cells. For the training of the algorithm, a discrete model was used that distinguishes RFI and non‐RFI data, based on the amplitude information from radio interferometric observations with the Giant Metre‐Wave Radio Telescope (GMRT) at 610 MHz. The performance of the RNN is evaluated by analyzing a confusion matrix. The true positive and true negative rates of the network are ∼99.9 and ∼97.9%, respectively. However, the overall efficiency of the network is ∼30% because of the fact that a large amount non‐RFI data is classified as being contaminated by RFI. Matthews correlation coefficient is 0.42, suggesting that a still more refined training model is required. [ABSTRACT FROM AUTHOR]