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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Physical Sensors".

Deadline for manuscript submissions: closed (30 November 2021) | Viewed by 46529

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Special Issue Editors

Prof. Dr. Andrea Randazzo

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Guest Editor

Department of Electrical, Electronic, Telecommunications Engineering, and Naval Architecture, University of Genoa, Genoa, Italy
Interests: microwave imaging; electromagnetic propagation; inverse scattering; inverse problems; antennas
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Dr. Cristina Ponti

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Guest Editor

Department of Engineering, “Roma Tre” University, 00154 Roma, Italy
Interests: electromagnetic scattering, electromagnetic propagation, inverse scattering, antennas

Dr. Alessandro Fedeli

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Guest Editor

Department of Electrical, Electronic, Telecommunications Engineering, and Naval Architecture, University of Genoa, 16145 Genoa, Italy
Interests: forward and inverse electromagnetic scattering; computational electromagnetics; microwave imaging
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

In the last years, microwave sensing and imaging have acquired an ever-growing importance in several applicative fields, such as non-destructive evaluation in industry and civil engineering, subsurface prospection, security, and biomedical imaging.

In principle, microwave techniques can be used to retrieve information about some physical parameters of the inspected targets (e.g., dielectric properties, shape, etc.) by using safe electromagnetic radiations and cost-effective systems, since the frequency band of interest is the same as in several other commercial apparatuses. Despite the great technological advances attained in the last years in this field, there are still some topics that could be addressed to further improve imaging systems. First, even more efficient and reliable measurement systems need to be designed and validated on a case-by-case basis, especially in realistic scenarios. Second, great attention should be paid to the development of effective data processing algorithms, able to solve the underlying electromagnetic inverse scattering problem (which is generally nonlinear and ill-posed) in order to retrieve the required information about the inspected targets from the measured scattered-field samples. Finally, efficient forward solvers are also fundamental for modeling the electromagnetic interactions between the interrogating fields and the targets in a suitable way.

In such a framework, this Special Issue aims at providing some insights into recent microwave sensing and imaging systems and techniques. Topics of interest include, but are not limited to: computational methods for electromagnetic imaging and inverse scattering, analytical and numerical forward modeling techniques in complex scenarios, sensors and antenna design, as well as innovative applications of microwave sensing and imaging.

Prof. Andrea Randazzo
Dr. Cristina Ponti
Dr. Alessandro Fedeli
Guest Editors

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Keywords

Microwave imaging
Electromagnetic scattering
Inverse scattering
Inverse problems
Electromagnetic modelling and simulation
Microwave sensors

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Published Papers (13 papers)

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20 pages, 1456 KiB

Open AccessArticle

A Multi-Frequency Tomographic Inverse Scattering Using Beam Basis Functions

by Ram Tuvi

Sensors 2022, 22(4), 1684; https://doi.org/10.3390/s22041684 - 21 Feb 2022

Viewed by 1737

Abstract

We present an overview of a beam-based approach to ultra-wide band (UWB) tomographic inverse scattering, where beam-waves are used for local data-processing and local imaging, as an alternative to the conventional plane-wave and Green’s function approaches. Specifically, the method utilizes a phase–space set of iso-diffracting beam-waves that emerge from a discrete set of points and directions in the source domain. It is shown that with a proper choice of parameters, this set constitutes a frame (an overcomplete generalization of a basis), termed “beam frame”, over the entire propagation domain. An important feature of these beam frames is that they need to be calculated once and then used for all frequencies, hence the method can be implemented either in the multi-frequency domain (FD), or directly in the time domain (TD). The algorithm consists of two phases: in the processing phase, the scattering data is transformed to the beam domain using windowed phase–space transformations, while in the imaging phase, the beams are backpropagated to the target domain to form the image. The beam-domain data is not only localized and compressed, but it is also physically related to the local Radon transform (RT) of the scatterer via a local Snell’s reflection of the beam-waves. This expresses the imaging as an inverse local RT that can be applied to any local domain of interest (DoI). In previous publications, the emphasis has been set on TD data processing using a special class of localized space–time beam-waves (wave-packets). The goal of the present paper is to present the imaging scheme in the UWB FD, utilizing simpler Fourier-based data-processing tools in the space and time domains. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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15 pages, 1007 KiB

Open AccessArticle

Experimental Validation of a Microwave Imaging Method for Shallow Buried Target Detection by Under-Sampled Data and a Non-Cooperative Source

by Adriana Brancaccio, Giovanni Leone, Rocco Pierri and Raffaele Solimene

Sensors 2021, 21(15), 5148; https://doi.org/10.3390/s21155148 - 29 Jul 2021

Cited by 3 | Viewed by 1832

Abstract

In microwave imaging, it is often of interest to inspect electrically large spatial regions. In these cases, data must be collected over a great deal of measurement points which entails long measurement time and/or costly, and often unfeasible, measurement configurations. In order to counteract such drawbacks, we have recently introduced a microwave imaging algorithm that looks for the scattering targets in terms of equivalent surface currents supported over a given reference plane. While this method is suited to detect shallowly buried targets, it allows one to independently process all frequency data, and hence the source and the receivers do not need to be synchronized. Moreover, spatial data can be reduced to a large extent, without any aliasing artifacts, by properly combining single-frequency reconstructions. In this paper, we validate such an approach by experimental measurements. In particular, the experimental test site consists of a sand box in open air where metallic plate targets are shallowly buried a (few

c m

) under the air/soil interface. The investigated region is illuminated by a fixed transmitting horn antenna, whereas the scattered field is collected over a planar measurement aperture at a fixed height from the air-sand interface. The transmitter and the receiver share only the working frequency information. Experimental results confirm the feasibility of the method. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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18 pages, 1655 KiB

Open AccessArticle

Field Representation Microwave Thermography Utilizing Lossy Microwave Design Materials

by Christoph Baer, Kerstin Orend, Birk Hattenhorst and Thomas Musch

Sensors 2021, 21(14), 4830; https://doi.org/10.3390/s21144830 - 15 Jul 2021

Cited by 4 | Viewed by 2343

Abstract

In this contribution, we are investigating a technique for the representation of electromagnetic fields by recording their thermal footprints on an indicator material using a thermal camera. Fundamentals regarding the interaction of electromagnetic heating, thermodynamics, and fluid dynamics are derived which allow for a precise design of the field illustration method. The synthesis and description of high-loss dielectric materials is discussed and a technique for a simple estimation of the broadband material’s imaginary permittivity part is introduced. Finally, exemplifying investigations, comparing simulations and measurements on the fundamental TE10-mode in an X-band waveguide are presented, which prove the above introduced sensing theory. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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18 pages, 2186 KiB

Open AccessArticle

Model-Based Systems Engineering Applied to Trade-Off Analysis of Wireless Power Transfer Technologies for Implanted Biomedical Microdevices

by Juan A. Martínez Rojas, José L. Fernández, Rocío Sánchez Montero, Pablo Luis López Espí and Efren Diez-Jimenez

Sensors 2021, 21(9), 3201; https://doi.org/10.3390/s21093201 - 5 May 2021

Cited by 16 | Viewed by 3875

Abstract

Decision-making is an important part of human life and particularly in any engineering process related to a complex product. New sensors and actuators based on MEMS technologies are increasingly complex and quickly evolving into products. New biomedical implanted devices may benefit from system engineering approaches, previously reserved to very large projects, and it is expected that this need will increase in the future. Here, we propose the application of Model Based Systems Engineering (MBSE) to systematize and optimize the trade-off analysis process. The criteria, their utility functions and the weighting factors are applied in a systematic way for the selection of the best alternative. Combining trade-off with MBSE allow us to identify the more suitable technology to be implemented to transfer energy to an implanted biomedical micro device. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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20 pages, 7477 KiB

Open AccessArticle

Three-Dimensional Microwave Imaging: Fast and Accurate Computations with Block Resolution Algorithms

by Corentin Friedrich, Sébastien Bourguignon, Jérôme Idier and Yves Goussard

Sensors 2020, 20(21), 6282; https://doi.org/10.3390/s20216282 - 4 Nov 2020

Cited by 7 | Viewed by 2777

Abstract

This paper considers the microwave imaging reconstruction problem, based on additive penalization and gradient-based optimization. Each evaluation of the cost function and of its gradient requires the resolution of as many high-dimensional linear systems as the number of incident fields, which represents a large amount of computations. Since all such systems involve the same matrix, we propose a block inversion strategy, based on the block-biconjugate gradient stabilized (BiCGStab) algorithm, with efficient implementations specific to the microwave imaging context. Numerical experiments performed on synthetic data and on real measurements show that savings in computing time can reach a factor of two compared to the standard, sequential, BiCGStab implementation. Improvements brought by the block approach are even more important for the most difficult reconstruction problems, that is, with high-frequency illuminations and/or highly contrasted objects. The proposed reconstruction strategy is shown to achieve satisfactory estimates for objects of the Fresnel database, even on the most contrasted ones. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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19 pages, 5126 KiB

Open AccessArticle

Low Cost, High Performance, 16-Channel Microwave Measurement System for Tomographic Applications

by Paul Meaney, Alexander Hartov, Timothy Raynolds, Cynthia Davis, Sebastian Richter, Florian Schoenberger, Shireen Geimer and Keith Paulsen

Sensors 2020, 20(18), 5436; https://doi.org/10.3390/s20185436 - 22 Sep 2020

Cited by 9 | Viewed by 3453

Abstract

We have developed a multichannel software defined radio-based transceiver measurement system for use in general microwave tomographic applications. The unit is compact enough to fit conveniently underneath the current illumination tank of the Dartmouth microwave breast imaging system. The system includes 16 channels that can both transmit and receive and it operates from 500 MHz to 2.5 GHz while measuring signals down to −140 dBm. As is the case with multichannel systems, cross-channel leakage is an important specification and must be lower than the noise floors for each receiver. This design exploits the isolation inherent when the individual receivers for each channel are physically separate; however, these challenging specifications require more involved signal isolation techniques at both the system design level and the individual, shielded component level. We describe the isolation design techniques for the critical system elements and demonstrate specification compliance at both the component and system level. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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12 pages, 3790 KiB

Open AccessArticle

Through-the-Wall Microwave Imaging: Forward and Inverse Scattering Modeling

by Alessandro Fedeli, Matteo Pastorino, Cristina Ponti, Andrea Randazzo and Giuseppe Schettini

Sensors 2020, 20(10), 2865; https://doi.org/10.3390/s20102865 - 18 May 2020

Cited by 22 | Viewed by 3632

Abstract

The imaging of dielectric targets hidden behind a wall is addressed in this paper. An analytical solver for a fast and accurate computation of the forward scattered field by the targets is proposed, which takes into account all the interactions of the electromagnetic field with the interfaces of the wall. Furthermore, an inversion procedure able to address the full underlying non-linear inverse scattering problem is introduced. This technique exploits a regularizing scheme in Lebesgue spaces in order to reconstruct an image of the hidden targets. Preliminary numerical results are provided in order to initially assess the capabilities of the developed solvers. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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16 pages, 26657 KiB

Open AccessArticle

A Prototype Microwave System for 3D Brain Stroke Imaging

by Jorge A. Tobon Vasquez, Rosa Scapaticci, Giovanna Turvani, Gennaro Bellizzi, David O. Rodriguez-Duarte, Nadine Joachimowicz, Bernard Duchêne, Enrico Tedeschi, Mario R. Casu, Lorenzo Crocco and Francesca Vipiana

Sensors 2020, 20(9), 2607; https://doi.org/10.3390/s20092607 - 3 May 2020

Cited by 107 | Viewed by 6694

Abstract

This work focuses on brain stroke imaging via microwave technology. In particular, the open issue of monitoring patients after stroke onset is addressed here in order to provide clinicians with a tool to control the effectiveness of administered therapies during the follow-up period. In this paper, a novel prototype is presented and characterized. The device is based on a low-complexity architecture which makes use of a minimum number of properly positioned and designed antennas placed on a helmet. It exploits a differential imaging approach and provides 3D images of the stroke. Preliminary experiments involving a 3D phantom filled with brain tissue-mimicking liquid confirm the potential of the technology in imaging a spherical target mimicking a stroke of a radius equal to 1.25 cm. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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10 pages, 1030 KiB

Open AccessArticle

On the Orbital Angular Momentum Incident Fields in Linearized Microwave Imaging

by Santi Concetto Pavone, Gino Sorbello and Loreto Di Donato

Sensors 2020, 20(7), 1905; https://doi.org/10.3390/s20071905 - 30 Mar 2020

Cited by 15 | Viewed by 2952

Abstract

Orbital angular momentum (OAM) is gaining great attention in the physics and electromagnetic community owing to an intriguing debate concerning its suitability for widening channel capacity in next-generation wireless communications. While such a debate is still a matter of controversy, we exploit OAM generation for microwave imaging within the classical first order linearized models, i.e., Born and Rytov approximation. Physical insights into different fields carrying ℓ-order OAM are conveniently exploited to propose possible alternative imaging approaches and paradigms in microwave imaging. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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16 pages, 4406 KiB

Open AccessArticle

Experimental Validation of Microwave Tomography with the DBIM-TwIST Algorithm for Brain Stroke Detection and Classification

by Olympia Karadima, Mohammed Rahman, Ioannis Sotiriou, Navid Ghavami, Pan Lu, Syed Ahsan and Panagiotis Kosmas

Sensors 2020, 20(3), 840; https://doi.org/10.3390/s20030840 - 4 Feb 2020

Cited by 67 | Viewed by 5772

Abstract

We present an initial experimental validation of a microwave tomography (MWT) prototype for brain stroke detection and classification using the distorted Born iterative method, two-step iterative shrinkage thresholding (DBIM-TwIST) algorithm. The validation study consists of first preparing and characterizing gel phantoms which mimic the structure and the dielectric properties of a simplified brain model with a haemorrhagic or ischemic stroke target. Then, we measure the S-parameters of the phantoms in our experimental prototype and process the scattered signals from 0.5 to 2.5 GHz using the DBIM-TwIST algorithm to estimate the dielectric properties of the reconstruction domain. Our results demonstrate that we are able to detect the stroke target in scenarios where the initial guess of the inverse problem is only an approximation of the true experimental phantom. Moreover, the prototype can differentiate between haemorrhagic and ischemic strokes based on the estimation of their dielectric properties. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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16 pages, 5671 KiB

Open AccessArticle

An Imaging Plane Calibration Method for MIMO Radar Imaging

by Yuanyue Guo, Bo Yuan, Zhaohui Wang and Rui Xia

Sensors 2019, 19(23), 5261; https://doi.org/10.3390/s19235261 - 29 Nov 2019

Viewed by 2513

Abstract

In two dimensional cross-range multiple-input multiple-output radar imaging for aerial targets, due to the non-cooperative movement of the targets, the estimated imaging plane parameters, namely the center and the posture angles of the imaging plane, may have deviations from true values, which defocus the final image. This problem is called imaging plane mismatch in this paper. Focusing on this problem, firstly the deviations of spatial spectrum fulfilling region caused by imaging plane mismatch is analyzed, as well as the errors of the corresponding spatial spectral values. Thereupon, the calibration operation is deduced when the imaging plane parameters are accurately obtained. Afterwards, an imaging plane calibration algorithm is proposed to utilize particle swarm optimization to search out the imaging plane parameters. Finally, it is demonstrated through simulations that the proposed algorithm can accurately estimate the imaging plane parameters and achieve good image focusing performance. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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9 pages, 2991 KiB

Open AccessArticle

Detecting Axial Ratio of Microwave Field with High Resolution Using NV Centers in Diamond

by Cui-Hong Li, Deng-Feng Li, Yu Zheng, Fang-Wen Sun, A. M. Du and Ya-Song Ge

Sensors 2019, 19(10), 2347; https://doi.org/10.3390/s19102347 - 21 May 2019

Cited by 2 | Viewed by 4105

Abstract

Polarization property characterization of the microwave (MW) field with high speed and resolution is vitally beneficial as the circularly-polarized MW field plays an important role in the development of quantum technologies and satellite communication technologies. In this work, we propose a scheme to detect the axial ratio of the MW field with optical diffraction limit resolution with a nitrogen vacancy (NV) center in diamond. Firstly, the idea of polarization selective detection of the MW magnetic field is carried out using a single NV center implanted in a type-IIa CVD diamond with a confocal microscope system achieving a sensitivity of 1.7

μ

\sqrt{Hz}

. Then, high speed wide-field characterization of the MW magnetic field at the submillimeter scale is realized by combining wide-field microscopy and ensemble NV centers inherent in a general CVD diamond. The precision axial ratio can be detected by measuring the magnitudes of two counter-rotating circularly-polarized MW magnetic fields. The wide-field detection of the axial ratio and strength parameters of microwave fields enables high speed testing of small-scale microwave devices. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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13 pages, 5527 KiB

Open AccessLetter

Terahertz Frequency-Scaled Differential Imaging for Sub-6 GHz Vehicular Antenna Signature Analysis

by Jose Antonio Solano-Perez, María-Teresa Martínez-Inglés, Jose-Maria Molina-Garcia-Pardo, Jordi Romeu, Lluis Jofre-Roca, Christian Ballesteros-Sánchez, José-Víctor Rodríguez and Antonio Mateo-Aroca

Sensors 2020, 20(19), 5636; https://doi.org/10.3390/s20195636 - 2 Oct 2020

Cited by 1 | Viewed by 2279

Abstract

The next generation of connected and autonomous vehicles will be equipped with high numbers of antennas operating in a wide frequency range for communications and environment sensing. The study of 3D spatial angular responses and the radiation patterns modified by vehicular structure will allow for better integration of the associated communication and sensing antennas. The use of near-field monostatic focusing, applied with frequency-dimension scale translation and differential imaging, offers a novel imaging application. The objective of this paper is to theoretically and experimentally study the method of obtaining currents produced by an antenna radiating on top of a vehicular platform using differential imaging. The experimental part of the study focuses on measuring a scaled target using an imaging system operating in a terahertz band—from 220 to 330 GHz—that matches a 5G frequency band according to frequency-dimension scale translation. The results show that the induced currents are properly estimated using this methodology, and that the influence of the bandwidth is assessed. Full article

(This article belongs to the Special Issue Microwave Sensing and Imaging)

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