Claudio Curcio

Phaseless Antenna Characterization by Effective Aperture Field and Data Representations

The problem of antenna characterization from phaseless near-field data is addressed by appropriate use of the available information on the Antenna Under Test (AUT) and on the scanning geometry to provide efficient representations for both the unknowns and the data. Such a strategy allows improving the reliability and the accuracy of the proposed characterization algorithm and, at the same time, shortens the overall measurement process. An extensive numerical and experimental analysis, together with a comparison with existing approaches, endorses the algorithm reliability and accuracy and confirms its usefulness for antennas having a general radiating (vector) behavior, i.e., either focusing or non-focusing.

Photonic Probes and Advanced (Also Phaseless) Near-Field Far-Field Techniques

We present innovative near-field test ranges, named compact-near-field (CNF) and very-near-field (VNF). These use photonic probes, and advanced near-field far-field (NFFF) transformations from amplitude and phase (complex) or phaseless measurements. The photonic probe allows AUT-probe distances of less than one wavelength. This drastically reduces test-range and scanner dimensions, improves the signal-to-clutter ratio and the signal-to-noise ratio, and reduces the scanning area and time. In both the cases of complex and phaseless measurements, the neat-field-to-far-field transformation problem is properly formulated to further improve the rejection of clutter, noise, and truncation error. The advantages of the compact-near-field and very-near-field test ranges are discussed and numerically analyzed. Experimental results are presented for both planar and cylindrical scanning geometries.

Dielectric Field Probes for Very-Near-Field and Compact-Near-Field Antenna Characterization [Measurements Corner]

A novel setup, based on the use of non-invasive dielectric field probes, and employing accurate and reliable algorithms for antenna characterization, is described. Experimental results show how the proposed system can provide even more accurate characterizations than standard near-field systems, enabling considerable reduction of indoor test-range dimensions (compact near-field). Also, the potential for very-near-field acquisition, as well as the sampling strategy in the reactive zone of the radiator, are pointed out.

Singular-Value Optimization in Plane-Polar Near-Field Antenna Characterization

We develop a near-field/far-field (NFFF) transformation approach for characterizing planar aperture antennas from plane-polar scanning data. The radial and azimuthal field-sampling spacings are chosen to provide the minimum number of near-field samples, optimizing the singular-value dynamics of the relevant linear operator connecting unknowns and data. A numerical and experimental analysis is provided, showing how the approach is capable of significantly reducing both the number of required samples and the number of measurement rings, as compared to the advanced sampling techniques available in the literature.

GPU-BASED Ï-K TOMOGRAPHIC PROCESSING BY 1D NON-UNIFORM FFTS

We present an ω-k approach based on the use of a 1D NonUniform FFT (NUFFT) routine, of NER (Non-Equispaced Results) type, programmed on a GPU in CUDA language, amenable to realtime applications. A Matlab main program links, via mex files, a compiled parallel (CUDA) routine implementing the NUFFT. The approach is shown to be an extension of an already developed parallel algorithm based on standard backprojection processing to account also for near-field data. The implementation of the GPU-based, parallel NUFFT routine is detailed and the computational advantages of the developed approach are highlighted against other confronted sequential or parallel (on multi-core CPU) procedures. Furthermore, the benefits of the ω-k, NUFFT-based processing are pointed out by both comparing its accuracy and computational convenience against other interpolators, and by providing numerical results. By comparing the computational performance of the algorithm against a multi-core, Matlab implementation, the speedup has been about 20 for a medium size image. The performance of the approach has been pointed out in the applicative case of vegetation imaging against experimental data of a boxtree (Buxus tree), also under a source of temporal decorrelation (wind).

Introduction [Singular-value optimization in plane-polar near-field antenna characterization]

We develop a near-field/far-field (NFFF) transformation approach for characterizing planar aperture antennas from plane-polar scanning data. The radial and azimuthal field-sampling spacings are chosen to provide the minimum number of near-field samples, optimizing the singular-value dynamics of the relevant linear operator connecting unknowns and data. A numerical and experimental analysis is provided, showing how the approach is capable of significantly reducing both the number of required samples and the number of measurement rings, as compared to the advanced sampling techniques available in the literature.

Millimeter-Wave Phaseless Antenna Characterization

Phaseless antenna characterization at millimeter-wave frequencies is an awkward task for a number of reasons. First, suitable network analyzers appropriately working at a high frequency, as well as a complex hardware ensuring high positioning and alignment accuracies, are required. Furthermore, the characterization algorithm should be reliable and accurate and should require a reasonable measurement time, which is a key issue in the millimeter and submillimeter ranges. In this paper, we present an efficient algorithm that faces this problem by exploiting the available a priori information on the shape and size of the antenna. The reliability, accuracy, and stability of the approach reside on ldquoeffectiverdquo expansions of the unknown aperture field distribution, reducing the ldquoeffectiverdquo number of unknowns. ldquoFastrdquo acquisitions are enabled by nonuniformly distributed near-field measurements. Rectangular and circular apertures are dealt with by introducing prolate spheroidal wave functions and a Jacobi-Bessel series representation of the aperture field, respectively. The performances of the approach and its effectiveness in the millimeter frequency range are proven by the experimental results at 40, 94, and 100 GHz, made at the Antenna Laboratory of the University of Napoli Federico II, on horn antennas and a Cassegrain reflector.

NUFFT-ACCELERATED PLANE-POLAR (ALSO PHASELESS) NEAR-FIELD/FAR-FIELD TRANSFORMATION

The paper introduces the use of Non-Uniform Fast Fourier Transform (NUFFT) routines in “complex” (i.e., amplitude and phase) and phaseless Near-Field/Far-Field transformations. The use of those routines results computationally very convenient when non-regular field sampling prevents the use of standard FFTs. The attention is focused on a plane-polar acquisition geometry. Numerical and experimental results show the effectiveness of the developed algorithms.

A Probe-Compensated Helicoidal NF-FF Transformation for Aperture Antennas Using a Prolate Spheroidal Expansion

A new probe-compensated near-field-far-field (NF-FF) transformation for aperture antennas in a cylindrical scanning geometry is presented. Such a technique takes the advantage of the NF data acquisition made according to a very efficient sampling strategy along a helix and exploits a proper aperture field expansion based on the use of the prolate spheroidal wave functions (PSWFs), accounting for the a priori information on shape and size of the antenna under test. The unknown aperture field expansion coefficients of the PSWFs are evaluated from the acquired voltage samples by an inversion process using a regularized version of the singular value decomposition method. Experimental results on connected and disconnected radiating aperture antennas, including sum and difference patterns, show the effectiveness of the approach and, in particular, how it enables a serious reduction of the measurement points without impairing the FF estimation accuracy.

Time-Harmonic Echo Generation

We propose an approach to the design of array-based, 2D echo generators. The radiating elements are located on non-uniform grids and the quiet zone (QZ) design specifications are enforced at non-uniformly spaced sampling locations. The approach, based on a singular values optimization process, allows dimensioning the size of the echo generator to meet the QZ specifications, defining the number and locations of the QZ sampling points and of the radiators to control the ill-conditioning when searching for the excitation coefficients, dramatically reducing the number of radiating elements, and finding the excitations by a Singular Value Decomposition approach. The performance of the method in terms of QZ field behavior and robustness against realization errors is numerically assessed. An experimental validation of the technique is also presented.