Subreflector shaping for antenna distortion compensation: an efficient Fourier-Jacobi expansion with GO/PO analysis

Technological demands have brought a renewed interest in the application of large reflector antennas with steadily increasing operating frequencies and antenna dimensions. The high surface accuracy of the main reflector required by these antennas can often not be achieved with available manufacturing technologies. The utilization of a shaped subreflector for main reflector-distortion compensation is considered an effective measure to enhance the overall radiation performance of an antenna system. In the process of evaluating the suitability of the subreflector shaping, however, it is crucial to accurately assess the most suitable subreflector shape within a reasonable amount of computational time. This is especially true for electrically large reflectors, where simple analysis of the radiation characteristics already creates a serious computational burden, moreover, since reflector shaping is a synthesis process that necessitates repeated computation of the radiation characteristics. In this paper, the development of an efficient computational tool for subreflector shaping is presented. The subreflector shaping is performed through a combination of geometrical optics (GO) and physical optics (PO) on the subreflector and the main reflector, respectively. To significantly limit the number of parameters subject to optimization, the subreflector surface is parameterized by the coefficients of a global, orthogonal Fourier-Jacobi set (related to Zernike polynomials), which allows us to accurately represent a surface with only a small number of coefficients. The incorporation of this surface expansion into a GO/PO synthesis technique is detailed, representative results are given for a computationally challenging reflector configuration, and the tolerances for the shaped subreflector surface are studied.     

The collection efficiency of a particle-loaded single filter fiber

The collection efficiency η(M) of single fibers, both isolated and within a parallel array, was determined as a function of accumulated particle mass for Stokes numbers St=0.3–3 and interception parameters In=0.04–0.32. This regime is dominated by inertia, interception and particle bounce. Measurements were made with an optical in-situ particle counting technique. Monodisperse polystyrene aerosols (1.3, 2.6, 3.6 and 5.2 μm) were deposited onto steel wires of 8 and 30 μm, respectively. Particle accumulation reached values on the order of M=0.5–1 μg/mm fiber, corresponding to 10⁴–10⁶ particles per mm, depending on size.η(M) was found to increase for all operating conditions between about 2 and nearly 50 fold, most strongly however for those conditions were the bare fiber is ineffective as a particle collector, i.e. for very low or very high values of St number, corresponding to low collision or low adhesion probabilities, respectively. The absolute efficiency increased to values approaching or exceeding unity, i.e. the effective fiber collision cross-section exceeded the diameter of the bare fiber. For fibers within an array of parallel fibers (equivalent packing density ∼0.004), the initial efficiency η₀ was higher than for an isolated fiber by almost an order of magnitude. However the increase with loading was substantially smaller, typically by an order of magnitude.The efficiency increase can be described by a power law of the type where b and c are empirical fit coefficients. Within an array, the exponent c is on the order of 0.7±0.05, but lower than reported in earlier work on fiber arrays which suggest a value near unity. For isolated fibers, c→1 as interception becomes dominant (at very low flow velocities) while at high impact velocities and significant particle bounce c≈0.5. The coefficient c correlates with fiber Reynolds number, but not with other parameters. The coefficient b is inversely proportional to η₀ (consistent with earlier work, however with significantly lower values than previously published) and a function of (St/R)².The experimental section of the paper is preceded by a literature review on single-fiber efficiency data and models for the inertia-interception regime, including both information on bare fibers and dust loaded fibers. An improved, general fit function with physically meaningful limits for St→0 and St→∞ is proposed for the efficiency of bare fibers.     

Characterization of nanostructured surfaces generated by reconstitution of the porin MspA from Mycobacterium smegmatis

Nanostructures with long-term stability at the surface of gold electrodes are generated by reconstituting the porin MspA from Mycobacterium smegmatis into a specially designed monolayer of long-chain lipid surfactant on gold. Tailored surface coverage of gold electrodes with long-chain surfactants is achieved by electrochemically assisted deposition of organic thiosulfates (Bunte salts). The subsequent reconstitution of the octameric-pore MspA is guided by its extraordinary self-assembling properties. Importantly, electrochemical reduction of copper(II) yields copper nanoparticles within the MspA nanopores. Electrochemical impedance spectroscopy, reflection electron microscopy, and atomic force microscopy (AFM) show that: 1) the MspA pores within the self-assembled monolayer (SAM) are monodisperse and electrochemically active, 2) MspA reconstitutes in SAMs and with a 10-nm thickness, 3) AFM is a suitable method to detect pores within SAMs, and 4) the electrochemical reduction of Cu2+ to Cu0 under overpotential conditions starts within the MspA pores.     

Calculation of residual stresses in case-hardened SAE5115 steel cylinders

The development of stresses for five differently case-hardened SAE5115 steel cylinders with variable surface carbon mass contents c_S and carburization depths CD during single hardening are calculated. The carbon profiles and the retained austenite distributions in the near-surface material areas are approximated in a stepwise manner. Exemplarily, in the middle plane of cylinders with c_S = 0.5 % C (CD = 0.07 mm) and 1.1 % C (CD = 0.2 mm), the local axial stresses at the surface, at four near-surface points and in the core are described during the hardening process up to temperature balance. The influence of c_S and CD on the surface tangential residual stress values along the casing of the cylinders is outlined. Also the depths distributions of the tangential residual stresses in the middle plane are presented for differently carburized cylinders. At c_S = 1.1 % C = const. the distance of maximum compressive residual stresses beneath the surface as well as the thickness of the compressed surface and the distances with the largest tensile residual stresses increase with increasing CD.     

The mixing state of fine powders measured by magnetic resonance imaging

The usefulness of magnetic resonance imaging (MRI) for the spatially resolved, quantitative characterization of mixtures of fine powders in the size range of about 10 μm is investigated. The sources of errors inherent in the signal translation process between raw MRI data and local concentrations are analyzed in detail. Possibilities for their correction are pointed out in deriving a global characteristic such as the standard deviation.The analysis of the mixing state of powders by MRI and the benefits of error correction are then illustrated by simple examples covering a wide range of compositions and mixing states. These experiments were made by mixing spherical, oil-filled (hence “visible” to MRI) melamine microcapsules in the size range of about 1-20 μm with solid melamine spheres of similar size and density. The mixture volume of typically 1 cm³ was imaged with an isotropic resolution of 235 μm, providing over 60,000 contiguous data points per measurement. The benefit of error correction is shown to be significant, provided the mixtures are homogeneous to a large extent.     

Scalable Numerical Tools for Flow and Pressure Drop Computation in Fibrous Filter Media

The prediction of flow behavior and pressure drop in fibrous filter media is challenging due to the complexity of the nonuniform fiber structure. Numerical calculation tools can considerably contribute to pressure drop determination for inhomogeneous filter structures. A numerical solution approach based on the finite element method to simulate 2D and 3D filter structures is considered. As numerical examples, computer designed homogeneous and inhomogeneous 2D cases where the numerical approach is validated by analytical models are investigated. Furthermore, the capability of the numerical method to simulate real 3D structures corresponding to more than 25 million degrees of freedom of the related algebraic system is demonstrated. The large systems involved require the use of dedicated techniques related to high performance computing.     

RF characterization of an inflatable parabolic torus reflectorantenna for space-borne applications

Space-borne satellite applications provide a vast array of services extending from global connectivity to Earth observation systems. The soil moisture radiation mission is a proposed space-borne passive microwave system complementary to the existing Earth observing system operating at low microwave frequencies and requiring an antenna with multibeam, high-beam efficiency, and dual polarization capabilities. To achieve both the large reflector size and the multibeam pattern at the operational frequencies an innovative multibeam reflector antenna design was needed. The advances in inflatable antenna technology has been proposed to overcome the launch vehicle size and weight restrictions. This paper describes a novel offset parabolic torus reflector antenna design that produces the desired multibeam pattern and is compatible with the inflatable antenna technology. Using the system requirements of this mission as an example, the design process for an inflatable parabolic torus reflector antenna is outlined, the development of suitable distortion models is given, and representative RF characteristics are presented. These RF characteristics include far-field patterns, beam contour patterns, beam efficiency, and other key performance parameters. The development of an advanced analytical modeling/numerical tool in support of the design effort is also detailed     

Highly Resolved Determination of Structure and Particle Deposition in Fibrous Filters by MRI

A new method for the quantitative three‐dimensional structure determination of a coarse filter medium as well as the particle deposition inside this medium is described. The filter structure was imaged at a voxel resolution of 59 μm which is close to the mean fiber diameter, therefore allowing individual fibers and their position to be determined. The data processing method for determining the filter structure as well as the quantification of the mass deposited in the filter are explained in detail. For particle deposition investigations the spatial resolution used was 235 μm. Hereby, a linear correlation between deposited mass and signal intensity was observed.     

Beam efficiency of reflector antennas: the simple formula

Beam efficiency has proven to be an important measure for characterizing the performance of reflector antennas. In this paper, a simple formula is presented to allow for a quick estimate of the beam efficiency of reflector antennas. This estimate includes the effects of edge tapers, feed spillover, random surface errors, and central blockage. The application of this formula is detailed in a case study, and its accuracy is demonstrated in comparison with a numerical simulation using diffraction analysis