Marianna Ivashina

An Optimal Beamforming Strategy for Wide-Field Surveys With Phased-Array-Fed Reflector Antennas

An optimal beamforming strategy is proposed for performing large-field surveys with dual-polarized phased-array-fed reflector antennas. This strategy uses signal-processing algorithms that maximize the beam sensitivity and the continuity of a field of view (FOV) that is formed by multiple closely overlapping beams. A mathematical framework and a newly developed numerical approach are described to analyze and optimize a phased array feed (PAF) system. The modeling approach has been applied to an experimental PAF system (APERTIF prototype) that is installed on the Westerbork Synthesis Radio Telescope. The resulting beam shapes, sensitivity, and polarization diversity characteristics (such as the beam orthogonality and the intrinsic cross-polarization ratio) are examined over a large FOV and frequency bandwidth. We consider weighting schemes to achieve a conjugate-field matched situation (max. received power), maximum signal-to-noise ratio (SNR), and a reduced SNR scenario but with constraints on the beam shape. The latter improves the rotational symmetry of the beam and reduces the sensitivity ripple, at a modest maximum sensitivity penalty. The obtained numerical results demonstrate a very good agreement with the measurements performed at the telescope.

Decoupling Efficiency of a Wideband Vivaldi Focal Plane Array Feeding a Reflector Antenna

A focal plane array (FPA) feeding a reflector can be used to achieve a large field of view (FOV) with overlapping simultaneous beams. In order to provide a continuous FOV over more than an octave bandwidth, the inter-element spacing in the FPA has to be electrically small over large parts of the band. This will inevitably result in strong mutual coupling effects between the array elements. On transmit, the total lost power due to mutual coupling can be quantified by the decoupling efficiency, a term recently introduced for antenna arrays. This paper presents measured decoupling efficiencies of a Vivaldi element FPA operating between 2.3 and 7 GHz. The radiation patterns of the FPA are calculated for two beam excitations by using measured embedded element patterns, and the corresponding decoupling efficiencies are evaluated by using measured S -parameters between all element ports. The FPA is assumed to illuminate a deep reflector with F/D=0.35 , and the overall reflector aperture efficiencies are computed. The decoupling efficiencies are also determined through the measurements of the total radiation efficiencies in a reverberation chamber, which includes material absorption losses.

Equivalent System Representation to Model the Beam Sensitivity of Receiving Antenna Arrays

In this letter, it is demonstrated that the beam sensitivity of an antenna array receiving system can be analyzed by using an equivalent single-channel receiver model. In this model, the antenna array is represented by an equivalent single-port antenna and the multiport active beamforming network is replaced by an equivalent two-port amplifier. Herein, the beam sensitivity is defined at the input of the receiving system and is a function of the equivalent antenna model parameters. Such a simplified representation helps us to identify the predominant factors that affect the receiver sensitivity of complex antenna array systems, without having to consider the entire system in full detail. The receiver noise is computed with the proposed model and compared to the one computed by a direct numerical method to validate its consistency. For this purpose, we consider a four-element actively beamformed dipole array with strongly coupled antenna elements causing significant noise coupling effects.

Apertif, a focal plane array for the WSRT

In this paper we describe a focal plane array (FPA) prototype, based on Vivaldi elements, developed for the Westerbork Synthesis Radio Telescope (WSRT) to increase its instantaneous field of view by a factor 25 and double its current bandwidth. This prototype is the first step in a project that has the ambition to equip most of the WSRT antennas with FPAs to improve the survey speed of the telescope. Examples of scientific applications are surveys of the northern sky in polarised continuum and HI emission, and efficient searches for pulsars and transients.

Unified Definitions of Efficiencies and System Noise Temperature for Receiving Antenna Arrays

Two methods for defining the efficiencies and system noise temperature of a receiving antenna array have recently been developed, one based on the isotropic noise response of the array and the other on an equivalent system representation. This letter demonstrates the equivalence of the two formulations and proposes a new set of standard definitions of antenna figures of merit for beamforming arrays that accounts for the effect of interactions between antenna element mutual coupling and receiver noise on system performance.

Analysis of Large Microstrip-Fed Tapered Slot Antenna Arrays by Combining Electrodynamic and Quasi-Static Field Models

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.

Accurate Beam Prediction Through Characteristic Basis Function Patterns for the MeerKAT/SKA Radio Telescope Antenna

A novel beam expansion method is presented that requires employing only a few Characteristic Basis Function Patterns (CBFPs) for the accurate prediction of antenna beam patterns. The method is applied to a proposed design of the MeerKAT/SKA radio telescope, whose antenna geometry is subject to small deformations caused by mechanical or gravitational forces. The resulting deformed pattern, which is affected in a nonlinear fashion by these deformations is then sampled in a few directions only after which the interpolatory CBFPs accurately predict the entire beam shape (beam calibration). The procedure for generating a set of CBFPs—and determining their expansion coefficients using a few reference point sources in the sky—is explained, and the error of the final predicted pattern relative to the actual pattern is examined. The proposed method shows excellent beam prediction capabilities, which is an important step forward towards the development of efficient beam calibration methods for future imaging antenna systems.

Polarimetry With Phased Array Antennas: Sensitivity and Polarimetric Performance Using Unpolarized Sources for Calibration

Polarimetric phased arrays require a calibration method that allows the system to measure the polarization state of the received signals. In this paper, we assess the polarimetric performance of two commonly used calibration methods that exploit unpolarized calibration sources. The first method obtains a polarimetrically calibrated beamforming solution from the two dominant eigenvectors of the measured signal covariance matrix. We demonstrate that this method is sensitivity equivalent to the theoretical optimal method, but suffers from an ambiguity that has to be resolved by additional measurements on (partially) polarized sources or by exploiting the intrinsic polarimetric quality of the antenna system. The easy-to-implement bi-scalar approach assumes that the feed system consists of two sets of orthogonally oriented antenna elements, each associated with one polarization. We assess its sensitivity and polarimetric performance over a wide field-of-view (FoV) using simulations of a phased array feed system for the Westerbork Synthesis Radio Telescope. Our results indicate that the sensitivity loss can be limited to 4.5% and that the polarimetric performance over the FoV is close to the best achievable performance. The latter implies that the intrinsic polarimetric quality of the antennas remains a crucial factor despite the development of novel polarimetric calibration methods.

Polarimetry With Phased Array Antennas: Theoretical Framework and Definitions

For phased array receivers, the accuracy with which the polarization state of a received signal can be measured depends on the antenna configuration, array calibration process, and beamforming algorithms. A signal and noise model for a dual-polarized array is developed and related to standard polarimetric antenna figures of merit, and the ideal polarimetrically calibrated, maximum-sensitivity beamforming solution for a dual-polarized phased array feed is derived. A practical polarimetric beamformer solution that does not require exact knowledge of the array polarimetric response is shown to be equivalent to the optimal solution in the sense that when the practical beamformers are calibrated, the optimal solution is obtained. To provide a rough initial polarimetric calibration for the practical beamformer solution, an approximate single-source polarimetric calibration method is developed. The modeled instrumental polarization error for a dipole phased array feed with the practical beamformer solution and single-source polarimetric calibration was -10 dB or lower over the array field of view for elements with alignments perturbed by random rotations with 5 degree standard deviation.

Efficient Prediction of Array Element Patterns Using Physics-Based Expansions and a Single Far-Field Measurement

A method is proposed to predict the antenna array beam through employing a relatively small set of physics-based basis functions-called characteristic basis function patterns (CBFPs)-for modeling the embedded element patterns. The primary CBFP can be measured or extracted from numerical simulations, while additional (secondary) CBFPs are derived from the primary one. Furthermore, each numerically generated CBFP, which is typically simulated/measured for discrete directions only, can in turn be approximated by analytical basis functions with fixed expansion coefficients to evaluate the resulting array pattern at any angle through interpolation. This hierarchical basis reduces the number of unknown expansion coefficients significantly. Accordingly, the CBFP expansion coefficients can be determined through a single far-field measurement of only a few reference sources in the field of view. This is particularly important for multibeam array applications where only a limited number of reference sources are available for predicting the beam shape. Furthermore, this instantaneous beam calibration is fast, i.e., potentially capable to speed up the array calibration by one or two orders of magnitude, which is particularly important if the antenna radiation characteristics are subject to drifts.