Element gain drifts as an imaging dynamic range limitation in PAF-based interferometers

Interferometry with phased-array feeds (PAFs) presents new calibration challenges in comparison with single-pixel feeds. In particular, temporal instability of the compound beam patterns due to element gain drifts (EGDs) can produce calibration artefacts in interferometric images. To translate imaging dynamic range requirements into PAF hardware and calibration requirements, we must learn to relate EGD levels to imaging artefact levels. We present a MeqTrees-based simulations framework that addresses this problem, and apply it to the APERTIF prototype currently in development for the WSRT.Comment: 4 pages, 3 figures, poster presentation at the XXX URSI General Assembly and Scientific Symposium (Istanbul, Turkey, August 13-20, 2011

Joint Design and Co-integration of Antenna-IC Systems

An overview of design challenges for beamforming active antenna arrays, which are needed to meet high-performance demands of future emerging applications, is presented. The critical role of antenna element mutual coupling on the receiving system sensitivity of array receivers, and effective radiated power of MIMO-type array transmitters is discussed. Trade-offs, common misconceptions, and practical examples are shown and discussed. Techniques towards strong integration between antennas and LNAs/PAs that blurs the geometrical boundaries between them are presented. This will cover mm-wave antenna design examples, where direct matching of active devices to their optimal source/load impedances eliminates the losses of 50-Ohm impedance matching networks. An antenna-integrated high-efficiency (Doherty) PA, operating at the sub-6 GHz band and utilizing active load modulation, will be taken as an on-antenna power combining example, including optimization aspects and over-the-air characterization

Reaching for Yield in the Bond Market

Reaching for yield—the propensity to buy riskier assets in order to achieve higher yields—is believed to be an important factor contributing to the credit cycle. This paper analyses this phenomenon in the corporate bond market. Specifically, we show evidence for reaching for yield among insurance companies, the largest institutional holders of corporate bonds. Insurance companies have capital requirements tied to the credit ratings of their investments. Conditional on ratings, insurance portfolios are systematically biased toward higher yield, higher CDS bonds. This behavior appears to be related to the business cycle, being most pronounced during economic expansions. It is also more pronounced for the insurance firms for which regulatory capital requirements are more binding. The results hold both at issuance and for trading in the secondary market and are robust to a series of bond and issuer controls, including issuer fixed effects as well as liquidity and duration. Comparison of the ex-post performance of bonds acquired by insurance companies does not show outperformance but higher volatility of realized returns

Cyclicality of Credit Supply: Firm Level Evidence

Theory predicts that there is a close link between bank credit supply and the evolution of the business cycle. Yet fluctuations in bank-loan supply have been hard to quantify in the time series. While loan issuance falls in recessions, it is not clear if this is due to demand or supply. We address this question by studying firms' substitution between bank debt and non-bank debt (public bonds) using firm-level data. Any firm that raises new debt must have a positive demand for external funds. Conditional on issuance of new debt, we interpret firm's switching from loans to bonds as a contraction in bank credit supply. We find strong evidence of substitution from loans to bonds at times characterized by tight lending standards, high levels of non-performing loans and loan allowances, low bank share prices, and tight monetary policy. The bank-to-bond substitution can only be measured for firms with access to bond markets. However, we show that this substitution behavior has strong predictive power for bank borrowing and investments by small, out-of-sample firms. We consider and reject several alternative explanations of our findings

Towards a uniform evaluation of the science quality of SKA technology options: Polarimetrie aspects

We discuss how to evaluate SKA technology options with regard to science output quality. In this work we will focus on polarimetry. We review the SKA specification for polarimetry and assess these requirements. In particular we will use as a illustrative case study a comparison of two dish types combined with two different feeds. The dish types we consider are optimized axi-symmetric prime-focus and offset Gregorian reflector systems; and the two feeds are the Eleven-feed (wideband) and a choked horn (octave band). To evaluate the imaging performance we employ end-to-end simulations in which given sky models are, in software, passed through a model of the telescope design according to its corresponding radio interferometrical measurement equation to produce simulated visibilities. The simulated visibilities are then used to generate simulated sky images. These simulated sky images are then compared to the input sky models and various figures-of-merit for the imaging performance are computed. A difficulty is the vast parameter space for observing modes and configurations that exists even when the technology is fixed. However one can fixed certain standard benchmark observation modes that can be applied across the board to the various technology options. The importance of standardized, end-to-end simulations, such as the one presented here, is that they address the high-level science output from SKA as a whole rather than low-level specifications of its individual parts

Numerical Results for the System Noise Temperature of an Aperture Array Tile and Comparison with Measurements

The purpose of this report is to document the noise performance of a complex beamforming array antenna system and to characterize the recently developed noise measurement facility called THACO, which was developed at ASTRON. The receiver system includes the array antenna of strongly coupled 144 TSA elements, 144 Low Noise Amplifiers (LNAs) (Tmin =35-40K) and the data recording/storing facilities of the initial test station that allow for off-line digital beamforming. The primary goal of this study is to compare the measured receiver noise temperatures with the simulated values for several practical beamformers, and to predict the associated receiver noise coupling contribution, antenna thermal noise and ground noise pick-up (due to the back radiation).Comment: ASTRON repor

Wideband Open-Ended Ridge Gap Waveguide Antenna Elements for 1-D and 2-D Wide-Angle Scanning Phased Arrays at 100 GHz

A new antenna element type based on the open-ended ridge gap waveguide (RGW) is proposed for beam-steering phased array applications. This element type is of a particular interest at high mm-wave frequencies (≥ 100 GHz) owing to a contactless design alleviating active beam-steering electronics integration. The key challenge addressed here is a realization of a wide fractional bandwidth and scan range with high radiation efficiency. We demonstrate a relatively simple wideband impedance matching network comprised of an aperture stepped ridge segment and a single-pin RGW section. Furthermore, the E- and H-plane grooves are added that effectively suppress antenna elements mutual coupling. Results demonstrate a wide-angle beam steering (≥ 50\ub0) over ≥ 20% fractional bandwidth at W-band with ≥ 89% radiation efficiency that significantly outperforms existing solutions at these frequencies. An experimental prototype of a 1 719 W-band array validates the proposed design concept through the embedded element pattern measurements

Characteristic Basis Function Analysis of Large Aperture-Fed Antenna Arrays

The Characteristic Basis Function Method (CBFM) is applied to rapidly compute the impedance and radiation characteristics of electrically large aperture-fed antenna arrays. A stationary formula for the antenna input admittance matrix is expressed in terms of a product of matrix blocks that are readily available from a method of moment formulation. Numerical results are shown for large arrays of waveguide antennas requiring more than 2 million basis functions, which is reduced by a factor of 9000, so that the solution for the currents are still obtainable in-core on a single desktop computer, while being orders faster than commercial software codes or a standard MoM approach, provided that sufficient memory is available for the Gaussian elimination

Synthesis of circular isophoric sparse arrays by using compressive-sensing

A design approach for large-scale sparse arrays based on Compressive Sensing has been recently introduced in the literature and extended to include complex EM effects and scan performance. However, that approach cannot directly control the number of excitation amplitudes. Here, we apply a two-step procedure that first synthesizes continuous rings with unconstrained amplitudes using an iterative ℓ1-norm minimization approach, and then replaces them with a circular isophoric ring array with a number of elements proportional to the original amplitude of each ring. The procedure is demonstrated for an isotropic array of a 10λ radius, for which a reference solution based on the analytical density-taper approach is available in the literature. Results show the capability of the proposed method to achieve a significant reduction of the array aperture (20%) with 25% less elements or 4dB lower peak side lobe level