1,721,064 research outputs found
The role of diversity on linear scattering operator: The case of strip scatterers observed under the fresnel approximation
No full textThe aim of this paper is to investigate the role of multiple views and multiple frequencies in linear inverse scattering problems. The study was performed assuming the Fresnel-zone approximation on the scattering operator. Due to the crucial role played by singular values into analysing the linear inverse scattering problems, the impact of view and frequency diversities on singular values behaviour was established. In fact, the singular values were related to the most common metrics used to quantify the achievable performances in inverse scattering problems, such as the number of degrees of freedom (NDF), the information content and the resolution
On-axis resolution of a circular aperture
No full textIn this paper, the problem to estimate the on-axis resolution in creating a desired field profile by radiation of an aperture A is addressed. The latter applies in both diffractive optics and antenna synthesis. This is because the ’aperture theory’, A can schematize a source, for example, an antenna or a lens illuminated by an incident field radiating a significant field only on the same aperture. The analysis refers to a customary axicon geometry consisting of a circle aperture transverse to the observation domain. The aim was to find a resolution formula allowing to highlight the impact of aperture geometrical parameters for configurations that are below the Fresnel approximation. The results show that the aperture cannot approximate the target field with the same level of accuracy along with the observation domain. In particular, near the aperture, smaller details can be retrieved and as the distance increases this ability degrades
Near-field warping sampling scheme for broad-side antenna characterization
No full textIn this paper the problem of sampling the field radiated by a planar source observed over a finite planar aperture located in the near-field is addressed. The problem is cast as the determination of the spatial measurement positions which allow us to discretize the radiation problem so that the singular values of the radiation operator are well-approximated. More in detail, thanks to a suitably warping transformation of the observation variables, the kernel function of the relevant operator is approximated by a band-limited function and hence the sampling theorem applied to achieved discretization. It results in the sampling points having to be non-linearity arranged across the measurement aperture and their number can be considerably lowered as compared to more standard sampling approach. It is shown that the proposed sampling scheme works well for measurement apertures that are not too large as compared to the source’s size. As a consequence, the method appears better suited for broad-side large antenna whose radiated field is mainly concentrated in front of the antenna. A numerical analysis is included to check the theoretical findings and to study the trade-off between the field accuracy representation (over the measurement aperture) and the truncation error in the estimated far-field radiation pattern
Spatial Sampling in Monostatic Radar Imaging
No full textThis letter deals with microwave imaging via a migration-like inversion method. The aim is the determination of a spatial sampling scheme which allows to collect as low as possible spatial measurements and to preserve the point-spread function features and hence, the achievable performance in the reconstructions. To keep math simple and to more easily illustrate the theoretical arguments, the study is developed for a 2-D monostatic scalar configuration and a homogeneous background medium. It is shown that the new sampling scheme requires much fewer samples than the standard literature results and that the samples must be nonuniformly deployed across the measurement domain.This letter deals with microwave imaging via a migration-like inversion method. The aim is the determination of a spatial sampling scheme which allows to collect as low as possible spatial measurements and to preserve the point-spread function features and hence, the achievable performance in the reconstructions. To keep math simple and to more easily illustrate the theoretical arguments, the study is developed for a 2-D monostatic scalar configuration and a homogeneous background medium. It is shown that the new sampling scheme requires much fewer samples than the standard literature results and that the samples must be nonuniformly deployed across the measurement domain
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Solving 1D Inverse Scattering Problem for Media and Materials Profiling
No full textThe inverse problem at hand tackles the reconstruction of the dielectric profile of a slab. To this end, single-view multi-frequency configuration is exploited, wherein the scattered field can be collected in reflection (single antenna) and in transmission (two antennas) mode. As it is well known, besides being ill-posed, the problem is also non-linear, thus requiring local or global optimization schemes which could be trapped into false solutions. This drawback is completely avoided if inversion methods based on some classical linear scattering models are employed. However, by doing so, the cost to pay is that the class of retrievable profiles for which they work is extremely narrow. In this contribution we analyse a possible effective strategy to enlarge the class of retrievable profiles
Warping-Driven Greedy Method for Data Reduction in Planar Near-Field Antenna Measurements
No full textNear-field techniques for antenna testing can require the collection of a very large amount of data when the working frequency increases and/or the antenna is large in terms of wavelength. To mitigate such a drawback, a greedy method, which we call the warping-driven maximum noise propagation error (MNPE) algorithm, is introduced. Antenna under test (AUT) diagnostics is cast as the reconstruction of an equivalent surface current. Starting from a densely populated initial grid, the algorithm selects the sampling points one by one till the noise factor (NF), a figure that controls the propagation of noise from data to the source reconstruction and does not meet a slowly varying region. Hence, the procedure is arrested since no relevant further NF reduction would be obtained. It is shown that MNPE allows for a significant data reduction compared to other literature methods, especially when the starting grid is chosen according to the recently introduced warping sampling theory. Moreover, the initial grid selected according to the warping sampling, in turn, allows to dramatically reduce the computational cost of the selection procedure. A distinctive feature of the MNPE is that a priori information about the AUT can be easily included by projecting the unknown current onto properly selected base functions. As an example of that, we exploit the a priori information concerning the size of the source and the measurement aperture that leads to the radiation pattern being reliable within a bounded part of the visible spectrum (i.e., the so-called valid angle region). Extensive numerical analysis and a few experimental results pertaining to planar scanning confirm the effectiveness of the proposed method in dramatically reducing the amount of data without incurring performance degradation while estimating the source spatial spectrum
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