Amplitude-only Data Research Articles

Phased array antenna diagnosis from amplitude-only data using parallel deep learning models

There is a need for a reliable diagnostic process that can provide the information required to maintain optimal operating conditions of phased array antennas. The phase and amplitude information can be retrieved from the complex radiated field. However, phase measurement is laborious and costly at high frequencies. To overcome this challenge, it is of great relevance to create diagnostics methods that solely utilize the amplitude data. The shape and characteristics of the radiated field of a phased array antenna are predicated on the number of operating elements in the array. This implies that a failed element in the array will change the nature of the radiated pattern. We can locate a failed element using a learning algorithm to map the location of a failed element to the radiated field. We propose a technique for finding failed elements using only amplitude data by dividing the array into multiple subarrays. Each subarray is trained with dedicated deep convolutional neural networks for faulty element identification. We evaluated the proposed approach using simulated data for 20 × 20 and 30 × 30 array structures. The results demonstrate that the proposed approach can effectively diagnose and locate failed elements in a phased array antenna using amplitude-only data.

Single-Cut Phaseless Near-Field Measurements for Fast Antenna Testing

Single-cut techniques allow for fast antenna characterization by measuring and transforming to far-field individual pattern cuts instead of the full sphere. The cut fields are expanded in a reduced set of cylindrical coefficients, which can be used to accurately compute the far-field in the main pattern cuts for antennas with separable aperture distributions. This communication introduces a fast single-cut near-field to far-field transformation method using amplitude-only data. The technique is based on the measurement of the near-field magnitude in two concentric cuts and starts an iterative propagation process to retrieve their phases. This technique becomes a fast tool for antenna characterization when one is interested in a few cardinal plane cuts of the antenna and only magnitude measurements are available. The proposed single-cut phaseless technique is tested using simulated and measured data, and it shows its potential for fast and more reliable amplitude-only measurements.

Freehand System for Antenna Diagnosis Based on Amplitude-Only Data

This article presents a portable system for freehand antenna diagnosis and characterization based on amplitude-only data. The amplitude-only samples are acquired by moving a handheld probe, which is tracked by a motion capture system, in front of the antenna under test (AUT) aperture. The acquired measurements are processed using the phaseless sources reconstruction method to compute an equivalent current distribution on the AUT aperture. Finally, the radiation pattern of the AUT can be obtained by evaluating the corresponding radiation integrals. Unlike previous work, the use of amplitude-only data avoids the need of a phase reference, paving the way to the diagnosis and characterization of antennas under operational conditions. This fact, together with the handheld capabilities, makes the system very convenient for measurements of already deployed and onboard antennas. Moreover, these amplitude-only acquisitions also simplify the required hardware. The system has been validated through measurements in a wide frequency range from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Ka$ </tex-math></inline-formula> -band up to 300 GHz. Despite that one cannot expect the same degree of accuracy that can be achieved under laboratory conditions (including an anechoic environment and highly accurate positioners), the system shows excellent capabilities to detect malfunctions, such as wrong amplitude/phase distributions, as well as a fair estimation of the far field.

Performance of Phase Retrieval via Phaselift and Quadratic Inversion in Circular Scanning Case

The reconstruction of the field radiated by a source from square amplitude-only data falls into the realm of phase retrieval . In this paper, we tackle the phase retrieval with two different approaches. The first one is based on a convex optimization problem called PhaseLift . The latter exploits the lifting technique to recast phase retrieval as a linear problem with an increased number of unknowns and then, because the linear problem is highly undetermined, it adds further constraints (based on the mathematical properties of the solution) to estimate it. The second approach formulates phase retrieval as a least squares problem and, therefore, it requires to tackle the minimization of a quartic functional which will be carried out by applying a gradient descent method. In the second part of this paper, in order to corroborate the effectiveness of both approaches, we present the numerical results. Afterward, we provide a comparison between the two methods and finally, we emphasize how the ratio between the number of independent data and the number of unknowns impacts on the performance in both approaches.

A Novel Method for Source Reconstruction Based on Spherical Wave Expansion and NF-FF Transformation Using Amplitude-Only Data

In this paper, we present a novel method that utilizes amplitude-only data of the electric field over a completely closed surface for the reconstruction of the spherical electric field, which represents a radiation source or an antenna under test (AUT) in the near-field (NF) region. The electric field exploiting genetic algorithm (GA) for global optimization is originally derived from a random initial estimate of field distribution and will gradually approximate to the exact value during iterations. When GA reaches the convergence criterion, we can calculate the far field (FF) by spherical NF-to-FF (NF-FF) transformation. The simulated and numerical results are in good agreement with each other, which shows that this method is effective over one or two closed surfaces. Furthermore, the related discussions corresponding to GA are given, confirming that the improved GA is feasible and efficient.

Scalar Calibration for Broadband Synthetic Aperture Radar Operating with Amplitude-Only Data

Off-axis holography techniques, which are commonly used to perform phase retrieval from amplitude-only data, require a full-characterization of the reference signal in terms of amplitude and phase. In this letter, it is demonstrated that the recently proposed version of the off-axis holography, working with a broadband synthetic aperture radar, can bypass the need for this calibration. The strategy is based on employing a calibration object to adjust the system. Furthermore, certain properties of the reflectivity calculation are exploited to correct potential misalignment errors. The performance of the approach is demonstrated by reconstructing the profile of several objects at ${K}_{a}$ -band.

On the Uniqueness of the Phase Retrieval Problem From Far Field Amplitude-Only Data

The phase retrieval problem has gained considerable attention since the wave phase is lost or impractical to measure in several applications in Electromagnetic Engineering. Quite a few approaches have been proposed recently for the extraction of phase from amplitude-only data; however few of them investigate the uniqueness of the problem prior to the solution. In this communication, the problem of the uniqueness of the phase retrieval problem from far field amplitude-only data is examined in detail. It is shown that uniqueness is guaranteed if two experiments can be performed that differ by a quadratic term in the phase. Conversely, experiments creating only linear phase differences in the data do not guarantee uniqueness. This result is demonstrated by considering the problems of a scattering target illuminated in two experiments differing by a quadratic phase shift, corresponding to two different values of defocusing of the incident wave.

Using Millimeter and Terahertz Frequencies for Complex Permittivity Retrieval of Low-Loss Materials

A free-space measurement method has been proposed for complex permittivity determination of low-loss materials at millimeter and terahertz frequencies. The method relies on the Fabry-Perot interference effect between two identical samples with different lengths separated by a varying air region. A metric function has been derived for fast and accurate complex permittivity determination of low-loss materials using amplitude-only transmission power data and then we performed its functional analysis to evaluate its dependence on unknown parameters. Next, we employed a graphical method for testing the number of measurements required to extract a unique permittivity. We conclude that 1) at least three independent measurements are required for one permittivity determination and 2) using samples with smaller lengths increases the robustness of the proposed method.

Two-Dimensional Contrast Source Inversion Method With Phaseless Data: TM Case

In this paper, two new approaches are presented for the solution of electromagnetic inverse scattering problems when amplitude-only data are available. The proposed techniques are based on a customized version, which are the so-called contrast source inversion (CSI) and multiplicative regularized CSI (MRCSI) methods. In the proposed approaches, denoted as the phaseless-data (PD)-CSI and the PD-MRCSI, only the term of the cost functional concerning the mismatch between the measured and estimated field data (i.e., the data equation) has been properly redefined. Moreover, the back-projection algorithm has been modified to provide an initial solution ensuring the rapid convergence of the optimization procedures and avoid the reconstruction of false solutions. A set of representative results concerning numerical as well as experimental tests is reported to show the accuracy of the proposed amplitude-only reconstruction approaches.

Prediction of channel parameters from amplitude-only data using Prony algorithm

Proposed is a new technique based on the Prony algorithm that utilises amplitude-only data to estimate relative delays and amplitudes of the multipath rays. The use of amplitude-only data instead of complex data in the estimation process reduces the complexity and cost of the data measurement and acquisition systems. The theoretical analysis is validated by computer simulation and experimental measurement. Prediction of the different parameters showed very good agreement with actual values.

MICROWAVE IMAGING FROM AMPLITUDE-ONLY DATA - ADVANTAGES AND OPEN PROBLEMS OF A TWO-STEP MULTI-RESOLUTION STRATEGY

In this report, a two step strategy for the inversion of amplitude-only data in microwave imaging applications is analyzed. At the first step of the proposed method, the illuminating source is synthesized according to a line sources model in order to compute the incident field in the investigation domain starting from the values available in the measurement domain. The second step is aimed at reconstructing the profile of the objects under test thanks to the iterative multi-scaling approach integrated with the Particle Swarm Optimizer, an effective evolutionary minimization technique. The reconstruction accuracy of the proposed phaseless retrieval strategy is analyzed using synthetic data concerned with a multiple scatterer configuration and successively further assessed inverting experimental data

Characterization of Real-World Steered-Beam Antennas from Amplitude-Only Near-Field Data

Phase-less near-field techniques are becoming increasingly more important for antenna characterization, due to the growing interest in millimeter-and sub-millimeter-wave applications, where the near-field phase is difficult or even impossible to measure. In this framework, the routine application of phase-less near-field/far-field (NFFF) transformations to real-world operational antennas is a challenging problem, recently questioned in the literature, requiring algorithms capable of providing reliable and accurate results over a large class of radiators. In the present paper, the possibility of applying phase-less near-field techniques for routine testing of antennas is discussed. We point out how -following the recent developments in the field, and by a formulation of the problem based on proper representations of unknowns and data - it is possible to gain the reliability and the accuracy required for this. Experimental tests were carried out on steered-beam antennas, which have lately been pointed out as "difficult" workbenches, to test the feasibility of operational phase-less near-field/far-field transformations. The experimental results refer to a reflectarray radiating a tilted beam, and to a phased array of large electrical dimensions, radiating a scanned beam and actually employed in real-world applications.

Interpolation/Extrapolation of Radar Cross-Section (RCS) Data in the Frequency Domain Using the Cauchy Method

We apply the Cauchy method to interpolate/extrapolate the radar cross-section (RCS) data which is amplitude-only data over a given frequency band. This is accomplished by approximating the amplitude-only data by a ratio of two polynomials, the coefficients of which are calculated by using the total least squares (TLS) implementation of a singular value decomposition (SVD) technique so as to properly estimate the dimension of the null space. By applying the Cauchy method, the power spectrum of an electromagnetic system is represented by a set of symmetric pole and zero pairs in the -plane. Once these coefficients in the numerator and the denominator polynomials in the Cauchy method are computed using the amplitude-only data, the response can be interpolated/extrapolated over other frequencies of interest. Numerical examples are presented to illustrate the applicability of the Cauchy method in interpolating/extrapolating RCS data over a frequency band, including a method of generating the phase response from the amplitude-only data.

Phaseless characterisation of compact antenna test ranges

Antenna characterisation and radar cross section measurements can be conveniently and successfully carried out by means of a compact antenna test range (CATR), whose performance is primarily assessed in terms of quiet zone size and field uniformity. CATR characterisation and diagnostics are usually performed by means of (amplitude and phase) field measurements in the quiet zone and are of timely interest either on CATR installation or on its periodic reassessments performed by manufacturers and/or users. This paper addresses CATR characterisation from amplitude-only data, an attractive topic because of the application of CATRs to the millimetre or sub-millimetre wave ranges. The approach is based on an appropriate strategy retrieving the missing quiet zone phase and facing the issue of the large electrical dimensions of the CATR. To take into account the characteristics of the collimating system as well as to keep as low as possible the number of unknowns to be sought for, it exploits the Jacobi-Bessel expansion of the aperture field. The performance of the algorithm has been tested through a wide numerical analysis against synthetic data generated by GRASP8-SE whose full version is routinely employed for CATR design. The main results related to a Gregorian dual offset reflector CATR, modelled on that proposed in the literature by Pistorius et al., and to a single offset reflector CATR, inspired by the one available at the Queen Mary & Westfield College, London (UK), are reported.

Genetic algorithm applied to determine the spherical wave expansion from amplitude-only far-field data

A method is presented for solving the spherical wave expansion (SWE) from far-field amplitude-only data. The idea is to minimize the least-squares difference between the amplitude of the measured field and that of a calculated SWE using a combination of a genetic algorithm (GA) and Newton's method. The uniqueness aspects of the problem are addressed. A measured antenna radiation pattern is analyzed with and without phase data and the results are compared. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 402–406, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21000

Microwave detection and dielectric characterization of cylindrical objects from amplitude-only data by means of neural networks

In this paper, we propose a new microwave technique for the localization and the dielectric characterization of physically inaccessible cylindrical objects from amplitude-only data. By means of a neural network used to solve the inverse scattering problem; this technique allows to directly achieve the object retrieval, avoiding the drawbacks related to the measurement of the phase distribution of the field that generally represent a critical point, especially at high frequency. The efficiency of the proposed technique in the reconstruction of both the position and the dielectric properties of a circular cylindrical body from amplitude-only information is illustrated and compared with the reconstruction performances of a neural network imaging technique that makes use of both amplitude and phase of the scattered field. The presence of noisy data is also taken into account, showing the dependence of the reconstruction accuracy on the signal-to-noise-ratio.

Binary-constrained inversion of a buried cylindricalobstacle from complete and phaseless magneticfields

The investigation herein is two-fold: specialization to a homogeneousobstacle of recent work (Lambert M et al 1998 Inverse Problems 14 1265-83) carried on the retrievalof an inhomogeneous cylindrical obstacle buried in a half-space in thetransverse electric (or H) polarization case; extension from the usualcase of complete wavefield data (known amplitude and phase) to the moresevere case of amplitude-only data (absent phase). The developed inversionmethod belongs to the class of modified gradient methods. The fielddistribution (here, the magnetic field) and the distribution of theobstacle parameters (here, the permittivity and conductivity) aresimultaneously sought in a search domain \U0001d49f. This is done by minimizinga two-component objective function, one of which is characterizing thesatisfaction of the wave equations within \U0001d49f, the other thedata fit. But now the electrical parameters of the sought obstacle areprescribed beforehand; this allows one to equate an appropriate complex-valuedcontrast function either to 0 (outside the obstacle) or to a known constant (inside). Two variants of abinary-constrained modified gradient algorithm are developed accordingly,tailored to either complete or phaseless data. Numericalexperimentation illustrates how they behave in a variety of obstacleconfigurations, for both exact and erroneous prescribed contrasts.

An effective near-field far-field transformation technique from truncated and inaccurate amplitude-only data

A general approach to the near-field far-field transformation from amplitude only near-field data is presented. The estimation of the far field is stated as an intersection finding problem and is solved by the minimization of a suitable functional. The difficulties related to the possible trapping of the algorithm by a false solution (common to any nonlinear inverse problem) are mitigated by setting the problem in the space of the squared field amplitudes (as already done in a number of existing papers) and by incorporating all the a priori knowledge concerning the system under test in the formulation of the problem. Accordingly, the a priori information concerning the far field, the near field outside the measurement region and the accuracy of the measurement setup and its dynamic range are properly taken into account in the objective functional. The intrinsic ill conditioning of the problem is managed by adopting a general, flexible, and nonredundant sampling representation of the field, which takes into account the geometrical characteristics of the source. As a consequence, the number of unknowns is minimized and a technique is obtained, which easily matches the available knowledge concerning the behavior of the field. The effectiveness of the approach is shown by reporting the main results of an extensive numerical analysis, as well as an experimental validation performed by using a very low cost near field facility available at the Electronic Engineering Department, University of Napoli, Italy.

Generation of nonminimum phase from amplitude-only data

A method is presented for the generation of nonminimum phase from amplitude-only data. The nonminimum phase is generated utilizing the principles of causality and the Hilbert transform. The application of the theory has been applied to some antenna radiation-power patterns and to measured transfer functions of microwave filters to illustrate the applicability of this approach.

Born-type schemes for the acoustic probing of 1-D fluid media from time-harmonic planar reflection coefficients at two incidences

The probing of a 1-D (depth-) inhomogeneous fluid half-space is carried out from a limited knowledge of plane-wave reflection coefficients with respect to frequency at two precritical incidences. Born-type inversion algorithms are investigated. First, index profiles as a function of a density-dependent pseudodepth are retrieved at two incidences; then density and speed-of-sound profiles versus depth are extracted. A Born-iterative scheme builds up the solution as the limit of a sequence of solutions of approximate, constrained linear problems. A Born-extended scheme directly yields a linearized solution. Two variants of the Born-iterative scheme are developed, the first one when data are known in both amplitude and phase, the second one with amplitude-only data. Possible—but not limiting—applications are in underwater acoustics, once the planar reflection coefficient of a seabed has been found, e.g., from the depth-dependent Green’s function in the spectral domain. Numerical simulations illustrate the schemes. Good index profiles are obtained with reasonable behavior versus noise or other causes of limitations. As is expected, separate probing of speed and density profiles remains difficult. The computationally costly exact scheme is not better than the cheaper approximate one when good data are available, but handles more complicated situations.