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

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

Performance of polarimetric beamformers for phased array radio telescopes

The results of four recently introduced beamforming schemes for phased array systems are discussed, each of which is capable to provide high sensitivity and accurate polarimetric performance of array-based radio telescopes. Ideally, a radio polarimeter should recover the actual polarization state of the celestial source, and thus compensate for unwanted polarization degradation effects which are intrinsic to the instrument. In this paper, we compare the proposed beamforming schemes through an example of a practical phased array system (APERTIF prototype) and demonstrate that the optimal beamformer, the max-SLNR beamformer, the eigenvector beamformer, and the bi-scalar beamformer are sensitivity equivalent but lead to different polarization state solutions, some of which are sub-optimal

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

Estrogen-inducible and liver-specific expression of the chicken very low density apolipoprotein II gene locus in transgenic mice.

We have examined the chicken Very Low Density Apolipoprotein II (apoVLDL II) gene locus in transgenic mice. A DNA fragment composed of the transcribed region, 16 kb of 5' flanking and 400 bp of 3' flanking sequences contained all the information sufficient for estrogen-inducible, liver-specific expression of the apoVLDL II gene. The far-upstream region contains a Negative Regulating Element coinciding with a DNaseI-hypersensitive site at -11 kb. In transgenic mice, the NRE at -11 kb is used for downregulating the expression to a lower maximum level. The NRE might be used for modulating apoVLDL II gene expression, and may be involved in the rapid shut-down of the expression after hormone removal

Modellen en beslisregels voor variabel doseren van gewasbeschermingsmiddelen op basis van variatie in bodem en gewas

Dit rapport bevat kennis en ervaring die nodig is voor brede introductie van variabel doseren van gewasbeschermingsmiddelen in de Nederlandse land- en tuinbouw. De resultaten komen uit Programma Precisielandbouwproject (PPL). Variabel doseren betekent het automatische aanpassen en toedienen van doseringen van middelen aan de plaatselijke behoefte binnen een perceel of gewas. Door enkele recente technische ontwikkelingen (positiebepalingstechnologie (GNSS), sensortechnologie en geavanceerde spuittechniek) komt variabel doseren van gewasbeschermingsmiddelen voor de praktijk binnen handbereik, met voordelen voor (bedrijfs)economie, ecologie en volksgezondheid

Aperture array development for future large radio telescopes

We present the design of a phased array system for future radio telescopes. This includes a system overview and recent results of the designed and implemented system, the Electronic Multi-Beam Radio Astronomy Concept (EMBRACE). Furthermore, simulations with a full-EM antenna simulator, combined with measurements on actual hardware, will provide information for the next design step, the Aperture Array Verification System (AAVS). With AAVS, we will prove design readiness of this novel array technology