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This work presents a flat‐gain antipodal Vivaldi antenna (AVA) for ultra‐wideband (UWB) microwave imaging applications. The proposed antenna demonstrates measured and simulated impedance bandwidth from 2.3 GHz to 20 GHz with a flat gain over the band, with minimum gain variation of 2.9 dBi. The antenna is tested initially for its practical utility by measuring its specific absorption rate (SAR) value. The SAR value is observed in the simulator by modeling a realistic homogeneous as well as a heterogeneous breast phantom. The SAR value obtained complies with both American and European standards when averaged over 1 g as well as 10 g of tissue. The designed antenna is further utilized to detect multiple tumors in a realistic homogeneous and heterogeneous breast phantoms developed in the laboratory environment. The widely popular delay and sum (DAS) algorithm is utilized to reconstruct the tumor images. The imaging is done outside the anechoic chamber with an in‐house monostatic microwave imaging setup just to make it more convincing with real‐world dynamic scenario. The homogeneous phantom with four embedded tumors each of radius 4 mm and the heterogeneous phantom with two tumors, one of radius 3.5 mm and another of radius 5 mm are imaged in this work. The imaging results demonstrate that tumors of different sizes can be detected accurately in the case of both homogeneous and heterogeneous breast phantoms. AVA with an oil paper layer for bandwidth enhancement as well for achieving flat‐gain response is first of its kind to be reported in this work Also, the proposed work utilizes different phantoms for imaging several tumors of differing sizes, which has not before been described. [ABSTRACT FROM AUTHOR]

With the improvement of design capability and manufacture technology, the integration as well as the complexity of chips have also increased dramatically, and it becomes critical to realize noninvasive electromagnetic inspection of chips at higher resolutions. In this paper, we propose a submicron‐resolution microwave near‐field imaging system based on the nitrogen‐vacancy center (NV) in diamond, which mainly consists of an optical hardware system and a software control system, and can be used for nondestructive imaging of microwave fields on the surface of integrated circuits with submicron resolution. We applied this technique to scan the surface microwave field distribution of a GaAs single pole double throw switch chip. The scanning imaging system can image the near‐field magnetic distribution for the chip, which provides a new solution for analyzing the current distribution on the chip surface and thus addressing problems such as excessive current or crosstalk. It is worth mentioning that the diamond NV color center‐based sensor has a very high‐spatial resolution and magnetic field detection sensitivity at room temperature, while the dielectric constant of natural diamond is 5.5, which also provides the possibility of diamond as a sensor in complex radiation environments and is an ideal detector material for near‐field imaging of electromagnetic fields. [ABSTRACT FROM AUTHOR]

In this article, a circular‐polarized microwave correlation imaging antenna which is based on rotating elements is presented. The proposed reflect array consists of the circular‐polarized patches, the filling substrates, and the grounded metal plates. It is known that the key to microwave correlation imaging is the construction of a two‐dimensional random radiation field in space. Reflect array antennas use element rotation to achieve phase compensation, different compensation phases can be achieved through different rotation angles of elements, the rotation angle of the element is more random, and a more stray pattern for microwave correlation imaging can be obtained. In the reflector array, every single patch is randomized the rotation angle within a certain range, then a 25 × 25 unit random array is obtained by using HFSS‐MATLAB co‐simulation. The effective area of the feed source irradiated on the reflector is certain each time, so the use of different units to compensate for the difference in phase can make the pattern group obtained with high randomness. For verification, a sample with the overall size of 450 mm × 450 mm is designed, fabricated, and measured. When the feed source and the array move relative to each other, the correlation between the directional patterns is mainly concentrated between 0.5 and 0.8. The radiation field is sufficiently random, and the gain of the pattern can be reached 15 dB when the operating frequency is 12.5 GHz, which can effectively obtain high‐quality imaging results. [ABSTRACT FROM AUTHOR]

The inherent advantages of microwave imagining (MWI) being noninvasive and nonionizing for cancer detection, has attracted the attention of researchers in this area. Therefore, the present research article proposes an UWB (ultra‐wideband) stacked MSA (Micro‐strip antenna) for MWI of skin cancer. The proposed research work precisely focuses on the diagnosis of skin cancer in human forearm using the designed MSA in mono‐static radar based configuration to collect S parameter data and process it to form an image of the scanned body area. The proposed UWB SAMPA (stacked aperture coupled micro‐strip patch antenna) has three layered stacked aperture coupled geometry with a defected ground structure with dimensions 36 × 30 × 4.85 mm3. A simulated and measured bandwidth of 6.23 GHz (6.14 to 12.37 GHz) and 5.28 GHz (5.72 to 11 GHz) are observed and a gain of 6.3 and 6.4 dB is reported respectively. The in vitro detection of skin cancer is done by fabricating a bio mimic of human forearm with same electrical properties as that of human skin. The proposed SAMPA is used to collect S parameter data from the scan of forearm phantom. This data is then processed in beamforming algorithms to create a 2D dielectric profile of the scanned forearm area. The multiply Delay and Sum with Coherent factor (CF‐DMAS) beam forming algorithm is utilized for reconstruction of the 2D image of the cancerous effected area along with its locations and size in MATLAB. The proposed MWI procedure is validated experimentally with the prototype of proposed SAMPA and a human forearm phantom after simulations in CST MWS. The proposed SAMPA is also safe for the human exposure, as it has simulated specific absorption rate (SAR) on the forearm phantom of 1.013 and 1.09 at frequency 7.04 and 8.67 GHz respectively. [ABSTRACT FROM AUTHOR]

An electrically adjustable multibeam lens antenna with high gain and directivity is proposed for imaging applications. The proposed antenna adopts two methods to narrow beamwidth and enhance gain. The 9‐element planar Yagi antenna array is used as the feed source. Compared with single antenna, the proposed antenna array has narrower beamwidth in H‐plane, and the beam scanning can be realized in the H‐plane through electrical adjustment. In addition, five‐layers planar lens is designed to enhance radiation gain and directivity in the E‐plane. The multibeam lens antenna can achieve a beam scanning range of ±31° with a gain tolerance of 3 dB and the maximum gain of 21.2 dBi with 2.8 ~ 3.2 GHz. The proposed antenna features the merits of higher gain, lower cost, small size, which is suitable for radar and microwave imaging field. [ABSTRACT FROM AUTHOR]

This article presents printed log periodic antennas with metamaterials for use in microwave imaging. A single layer of epsilon negative (ENG) metamaterial (MTM) array (1 × 6) of the unit cell is on the radiating patch. Adding a single negative metamaterial structure enhances the properties of far-field antennas, such as radiation pattern and gain, both of which are vital for breast imaging. Two frequency bands exhibit negative permittivity: 3–3.3 GHz and 3.6–4.5 GHz. In the operating band, the proposed antennas have achieved a maximum gain of 5.5 dBi and impedance bandwidth of 3 GHz (2–5 GHz) with a reflection coefficient less than −10 dB. At the lowest operating frequency of 2 GHz, the electrical dimensions of this designed antenna are 0.34λ × 0.26λ × 0.01λ. All 16 transceiver antennas are arranged vertically in a circular pattern around the phantom, each acting as a transmitter and the rest as receivers. The system design is carried out with the electromagnetic simulators CST and HFSS. After receiving the extracted data, the data are postprocessed using the MATLAB software and the delay multiply and sum (DMAS) imaging algorithm. Based on the reconstructed image, it is evident that the MTM-loaded antenna-based imaging system can detect many undesired tumors inside the breast phantom. [ABSTRACT FROM AUTHOR]

A high‐gain antenna with multi‐layer phase‐gradient metasurface (PGMS) lens is proposed. It consists of three parts, feed antenna, PGMS lens and back cavity. The proposed PGMS can transform quasi‐spherical electromagnetic wave to plane wave within the frequency band 5.5–6.5 GHz. The half‐power beamwidth of the feed antenna is reduced by up to 83.1°, and the gain is increased by up to 8.7 dB. Because the loading of the lens increases the back lobe, this article also introduces an antenna back cavity, which reflects the energy radiated from the antenna back to the front, thereby improving the radiation performance of the feed antenna. The measured results are in good agreement with the simulation results, verifying that the loading of the lens and the back cavity can achieve the purpose of increasing the gain and reducing the back lobe. The cavity‐backed lens antenna provides potential application for microwave imaging and wireless communication. [ABSTRACT FROM AUTHOR]

This article presents a novel CPW fed ultrawideband (UWB) antenna for detection of unwanted tumor cells in the human breast. Here, a compact and miniaturized UWB antenna employed with dodecagonal edges is proposed for broadening the bandwidth from 2.26 to 13.71 GHz (measured fractional bandwidth of about 143.39%). The antenna is designed on a durable RT 5880 substrate having 0.254 mm thickness, efficiency of 85% and gain of 3.84 dBi. For its flexibility characterization, the prototype of the fabricated antenna is analyzed in both the flat and bending configurations. The simulated testing is carried out by permitting the antenna to emit radiation in conjunction to the human breast phantom to obtain desired antenna characteristics for the microwave imaging (MI) applications. The changes in reflection coefficient with the variation of dielectric content of the breast phantom structure are analyzed. The suggested antenna for MI technique is apt for human exposure because the antenna bids the highest SAR value of 0.932 W/Kg at 10.6 GHz. Moreover, the proposed antenna is flexible, compact and low‐profile with low SAR values that can be useful for breast cancer detection. [ABSTRACT FROM AUTHOR]

In the field of materials research, scanning microwave microscopy imaging has already become a vital research tool due to its high sensitivity and nondestructive testing of samples. In this article, we review the main theoretical and fundamental components of microwave imaging, in addition to the wide range of applications of microwave imaging. Rather than the indirect determination of material properties by measuring dielectric constants and conductivity, microwave microscopy now permits the direct investigation of semiconductor devices, electromagnetic fields, and ferroelectric domains. This paper reviews recent advances in scanning microwave microscopy in the areas of resolution and operating frequency and presents a discussion of possible future industrial and academic applications. [ABSTRACT FROM AUTHOR]

In this article, an ultra‐wideband (UWB) monostatic radar based microwave imaging (MWI) approach is proposed to detect the presence and localize the position of breast tumor in women (usually greater than 45 years of age). For this purpose, a "C" shaped elliptically polarized DRA (dielectric resonator antenna) with an impedance bandwidth of 7.3 GHz (i.e., 3.4–10.7 GHz) covering the UWB has been proposed as a sensor to record the backscattered S parameter signals from the breast tissues. This is done by rotating the proposed DRA in monostatic radar configuration around the breast phantom with an interval of 30° in elevation (0–180) and azimuthal planes (0–360), respectively. The recorded "S" parameter data is then processed using beam‐forming algorithms namely delay and sum (DAS), improved delay and sum (IDAS), delay multiply and sum (DMAS) to form 2D images of the breast tumor and locate its position (using MATLAB R 2013a). The design and simulation of the entire set up that includes the C shaped UWB DRA & Bio media (breast phantom) is done using CST MWS V'17. The fabrication of the proposed DRA is carried out using wet etching and water jet cutting process. The physical model of the Bio media is prepared using gelatin (skin), petroleum‐jelly (fat), and wheat‐flour (tumor). The validation of antenna radiation parameters is done using a VNA (vector network analyzer) model no. E5063A and good agreement between the simulated and measured results is observed. The proposed C shaped DRA is used in a monostatic radar based configuration to collect "S" parameters and the data is processed in a GPU to reconstruct a 2D image of the scanned breast phantom area. The size and detected location of tumor using beam‐forming is also close to the actual physical location of tumor with an error of 6.2% from the actual tumor location. [ABSTRACT FROM AUTHOR]

A 1‐bit reconfigurable metasurface antenna for Ground penetrating radar (GPR) application is proposed. The element is loaded with one PIN diode, which can be electronically controlled to generate two states with 200° reflection phase difference at 2.0 GHz. The two states are regarded as "1" and "0." Based on the proposed element, a 10 × 10 reconfigurable metasurface is designed. Besides, a digital control board is used to control the state of element. A reconfigurable metasurface antenna is simulated and fabricated. The simulated and experimented results indicate that the proposed antenna can achieve ±40° beam scanning with a gain tolerance of 3 dB and the maximum gain of 16.7 dBi. In addition, the beam‐forming performance can be achieved, such as multibeam. These properties ensure the proposed antenna has great potential in GPR and microwave imaging systems. [ABSTRACT FROM AUTHOR]

A microwave imaging matrix algorithm based on the contrast source is proposed. It is known that the contrast source and the total field change by shifting the position of the incident field. To ensure the constancy of the contrast source, this paper first proposes a novel inversion model based on the contrast source. The excitation source is located at a single fixed position, and the scattering data is obtained by measuring points evenly distributed on multiple circular layers. Secondly, a matrix algorithm based on the contrast source is proposed, which transforms the nonlinear imaging problem into a matrix operation step-by-step without iteration. The numerical simulation results show that the matrix algorithm proposed in this paper has high precision and good performance with complex imaging problems. In addition, the limitation of the proposed algorithm when inversing data with noise is also analyzed. [ABSTRACT FROM AUTHOR]

Researchers are continuously exploring the potential use of microwave imaging in the early detection of breast cancer. The technique offers a promising alternative to mammography, a standard clinical imaging procedure today. The contrast in dielectric properties between normal and cancerous tissues makes microwave imaging a viable technique for detecting breast cancer. Experimental results are presented in this paper that demonstrate the detection of breast cancer using microwaves operating at 2.4 GHz. The procedure involves antenna fabrication, phantom tissue development, and image reconstruction. Design and fabrication of patch antenna are used in the study, described in detail. The patch antenna pair is used for transmitting and receiving source waves. Tissue mimicking models were developed from paraffin wax and glycerin for the dielectric constants of 9 and 47, respectively, representing the tissue and tumor. Further, AI-based tomographic images were obtained by implementing a filtered back-projection algorithm in the computer. In the results, the presence of the tumor is quantitatively analyzed. [ABSTRACT FROM AUTHOR]

In this article, an ultra‐wideband (UWB) topology optimized frequency selective surface (FSS) is introduced as a reflecting layer, to maximize the gain and overall performances of an UWB monopole antenna. The single Rogers RO4350B‐based FSS layer is synthesized using an automated system, based on an interface bridged between CST Microwave studio and Matlab, and optimized using a binary genetic algorithm. First, the FSS unit cell foot print needs to be as small as possible, while covering a wider frequency range, and to achieve these performances, the proposed genetic algorithm synthesizing system achieved an FSS unit cell with only 0.1 λ × 0.1 λ at the lower‐end frequency, covering a bandwidth of 2.9–14.5 GHz. Polarization independence is achieved also, due to the four‐folded symmetry imposed on the FSS unit cell. The proposed antenna is designed on a Rogers RO4350B substrate, and backed at a distance of 18.74 mm by an FSS structure. The fabricated prototype shows a bandwidth of 3.1–13.9 GHz, and an excellent maximum peak gain of 9.7 dBi, with an improvement of 3.41 dBi cross the UWB spectrum (from 3.7 to 7.11 dBi), and in good agreement with the simulation results, which made the proposed design a promising candidate for UWB applications requiring high gain, such as ground‐penetrating radar, and microwave radiology imaging (MRI) systems. [ABSTRACT FROM AUTHOR]

With the complexity and integration of integrated chips increasing, existing integrated circuits electromagnetic interference techniques using metallic probes cannot meet the emerging demand at single die level or wafer level, which requires micron spatial resolution and ultraweak invasiveness. In this article, we developed a non‐invasive near field probe in the fiber format using nitrogen‐vacancy center in diamond, which was attached to the tip of fiber as microwave B‐field sensing element. Subsequently, we applied the fiber diamond probe to near field scanning of a low noise amplifier chip and a power amplifier chip. Through numerical simulation, we find that the surface field strength is proportional to current density at chip surface, thus this probe can be used as a current imaging technique at microwave frequency. [ABSTRACT FROM AUTHOR]

This paper presents a novel method for three-dimensional microwave imaging based on sparse processing. To enforce the sparsity of the unknown function, we take advantage of the fact that arbitrary three-dimensional electromagnetic fields can be decomposed into two components with respect to the radial direction: one with transverse-magnetic polarization and the other with transverse-electric polarization. Each component can be further expressed as a sum of spherical harmonics, which provide the dictionary exploited by the sparse processing algorithm. Our measurement model relates the data and the parameters of the spherical harmonics' sources, which are uniformly distributed on a grid sampling the imaging domain. By relying on the theory of degrees of freedom of electromagnetic fields, it can be shown that only a few harmonics are sufficient to accurately represent the measured scattered field from objects whose diameter is of the order of the wavelength, thus allowing reducing the dimension of the adopted dictionary. We analyze several imaging scenarios to assess the algorithm's performance, including different object shapes, sensor orientations, and signal-to-noise ratios. Moreover, we compare the obtained results with other state-of-the-art linear imaging techniques. Notably, thanks to the adopted dictionary, the proposed algorithm can yield accurate images of both convex and concave objects. [ABSTRACT FROM AUTHOR]

The near‐field convergent lens has aroused wide attention in the field of microwave passive imaging, medicine, and beam shaping in recent years as it can converge electromagnetic energy with high efficiency. The main objective of the paper is to exhibit the design process of a multiple‐conductor‐layers convergent lens using all‐metallic elements. A quad‐layer metal cross‐Jerusalem‐slot structure is proposed to obtain compact unit cells without dielectric substrate, and then an all‐metallic near‐field convergent lens based on the cells is constructed. To validate the proposed method, an L‐band lens sample with 20 × 20 cells is designed and numerically simulated. Results show that the waves radiated from a horn antenna to the lens are focused in the near‐field area, 500 mm away from the lens, with the full width at half maximum of the focal spot of less than 200 mm. In addition, it is observed that the overall size of the lens has a significant effect on the near‐field convergent performance. Therefore, when the array is small, how to adjust the type of array elements to achieve a better convergence effect is analyzed in detail. [ABSTRACT FROM AUTHOR]

This article presented a novel compact multi‐input‐multi‐output (MIMO) hexagonal monopole antenna with a uniplanar compact frequency‐selective‐surface (FSS) array for microwave imaging (MWI). The ultra‐wideband (UWB) dual‐element linear MIMO antenna was designed on the FR4 substrate with 50 Ω coplanar waveguide feed, T‐strip isolation, novel numerical calculation and equivalent circuit analyses. The main issues of realising high‐resolution images based on planer UWB antenna for MWI are the low gain, omnidirectional pattern, design size and mutual coupling of MIMO design. A novel technique was proposed to solve a hybrid issue (mutual coupling) of the MIMO reflected‐waves from the FSS array and direct‐wave. The uniplanar UWB‐FSS unit cell was compacted by combining a square‐loop and cross‐dipole with a size of 0.095λ × 0.095λ. The novel‐isolated UWB‐MIMO antenna and UWB‐FSS array (IMAF) were integrated, after investigating the distance between the antenna and FSS. The fabricated IMAF with a stable gain improvement of 4.5 dBi higher than the antenna without FSS, directional radiation pattern, size of 30 × 73.8 × 21.6 mm3 observed that a low mutual coupling of −27 dB, and operation bandwidth of 3.0–11.7 GHz. Moreover, a handbag was scanned experimentally via the bi‐static approach to detect a small concealed object. The MWI system based on the MIMO antenna with FSS was displayed image resolution of 55% higher than that of MIMO antenna without FSS. The new baggage‐scanner approach confirmed that the proposed MIMO antenna with FSS array can lead the humanity for healthy MWI applications. [ABSTRACT FROM AUTHOR]

Introduction. Microwave imaging can obtain 360° anatomical and functional images of the colon representing the existing contrast in dielectric properties between different tissues. Microwaves are safe (nonionizing) and have the potential of reducing the visualization problems of conventional colonoscopy. This study assessed the efficacy of a microwave-based colonoscopy device to detect neoplastic lesions in an ex vivo human colon model. Methods. Fresh surgically excised colorectal specimens containing cancer or polyps were fixed to a 3D positioning system, and the accessory device was introduced horizontally inside the ex vivo colon lumen and moved along it simulating a real colonoscopy exploration. Measurements of the colon were taken every 4 mm with the microwave-based colonoscopy device and processed with a microwave imaging algorithm. Results. 14 ex vivo human colorectal specimens with carcinomas (n = 11) or adenomas with high grade dysplasia (n = 3) were examined with a microwave-based device. Using a detection threshold of 2.79 for the dielectric property contrast, all lesions were detected without false positives or false negatives. Conclusions. This study demonstrates the use of a microwave-based device to be used as an accessory of a standard colonoscope to detect neoplastic lesions in surgically excised colorectal specimens. [ABSTRACT FROM AUTHOR]

In order to obtain a narrower half power beamwidth (HPBW) and a higher antenna gain in a wide frequency band, metamaterial layers and constant refracting‐index lens are used to optimize the antenna performance in this article. The proposed metamaterial covered modified antipodal Vivaldi antenna with lens (M‐MAVA with lens) has an operating frequency band of 1–8 GHz, and the maximize gain here is 18.81 dBi at 8 GHz, which increase about 9 dB. The HPBW in the entire frequency range is decreased significantly. Beyond this, the simulation results are in good accordance with the measurements. Based on these characteristics, the proposed M‐MAVA with lens has an application in optimizing the structure of ground penetrating radar, microwave imaging, UAV countermeasures system and other system which need the good radiation gain and directivity. [ABSTRACT FROM AUTHOR]

Holographic imaging is a fast and robust technique that has been adapted for near‐field applications of microwave imaging. In this work, the experimental development of a miniaturized microwave holographic setup is proposed. This miniaturized setup includes power divider, phase tuner, DC block, and hybrid ring on a single microstrip circuit board. The designed phase tuner component has the ability to tune the phase electronically up to 360°, which makes it suitable to use in the phase‐shifting methodology. Moreover, the experimental results for the imaging of metallic objects are shown and compared with the conventional waveguide setup to validate the system's performance. The qualitative results exhibit the effectiveness of utilizing the miniaturized microstrip circuit, making the setup compact and economical. Hence, the proposed structure gives the possibility to make a portable phase‐shifting holographic system. [ABSTRACT FROM AUTHOR]

A high‐gain bow‐tie antenna with double‐layer phase‐gradient metasurface (PGMS) lens is proposed. A modified bow‐tie antenna is designed as a feed, which has end‐fire characteristics. The proposed PGMS can transform quasi‐spherical electromagnetic wave to plane wave within the frequency band from 1.7 to 2.1 GHz, thereby the proposed lens antenna has a good beam focusing effect. The measured gain is from 9.5 to 12.8 dBi and a −3 dB gain bandwidth of around 20% is achieved. At the same time, the gain increases 6.5 dB on average in the whole working frequency band. Simulated and measured results show that the designed modified bow‐tie antenna with PGMS lens has high gain and high directivity, which can well meet the requirement of microwave imaging system. [ABSTRACT FROM AUTHOR]

A novel near field subsurface microwave imaging scheme is proposed by using a high gain planar metasurface (MS) loaded slot antenna integrated with the synthetic aperture radar (SAR) algorithm. The antenna structure here comprises a slot antenna integrated with the proposed MS designed using the epsilon very large (EVL) unit cell. The proposed MS‐based antenna structure has a very low profile of 0.033λ, with a measured peak gain of 5.5 dB at 6.47 GHz. After adding two more additional MS layers, the peak gain of the resultant antenna is 12.1 dB. The broadside gain and efficiency are significantly enhanced with the incorporation of the EVL MS. Afterward, the antenna structure, along with the vector network analyzer and the 2‐D automated scanner is employed for the near field imaging of test media by recording the scattering data along the target area. A spatial subwavelength resolution with a typical value of about 0.43λ is obtained at 1λ standoff distance after applying the single frequency SAR algorithm to the measured raw scattering data. The overall applicability of the proposed low‐profile MS loaded antenna integrated with the SAR algorithm is demonstrated for the near field subsurface microwave imaging of various metallic and dielectric targets with subwavelength spatial resolution in free space and lossy media. [ABSTRACT FROM AUTHOR]

The contrast source inversion (CSI) is an effective method for solving microwave imaging problems which is widely utilized. The core of the CSI is to change the conventional inverse scattering problem into an optimization problem. The two items in the objective function describe the state error and data error, respectively. As it is all known, there is almost no complete performance comparison based on Fresnel data for the CSI and its related improved algorithms. In addition, the performance of the algorithm under different weights was not analyzed before and the convergence speed of original CSI is slow. Firstly, this paper compares the performance of traditional CSI and its improved algorithms from three aspects of qualitative imaging effect, convergence speed, and objective function value based on Fresnel data. Secondly, the influence of the state error and the data error under different weights on the convergence rate and the objective function value are studied. For the limitation of a slower convergence rate, the CSI with weights (W-CSI), the CSI with dynamic reduction factor (CSI-DRF), and its related algorithms, which can get better convergence rate compared with their relative original algorithms, are proposed. Eventually, the future research work is prospected. [ABSTRACT FROM AUTHOR]

Microwave holography utilizes an antenna to record the hologram. In a phase shifting microwave holographic method, the antenna has to perform the 2D scanning over an area corresponding to each phase shift in reference wave. Since this methodology requires phase shifting in each scan, this makes the process lengthy and laborious. In this work, a numerical 4‐step phase shifting method is presented for generating the microwave hologram, in which a single (0° phase) hologram is recorded experimentally and the other three holograms are generated numerically by shifting the phase in reference wave. The amplitude and phase information of object is extracted using angular spectrum diffraction method. An error introduced by various components in experiments is calculated and corrected to enhance the accuracy of reconstructed phase values. The calculated phase step error for the experiment is 0.0501 rad. The quantitative and qualitative results along with the experimental phase shifting phase results are shown to envision the applications of numerical phase shifting microwave holography in imaging of metallic objects and highly scattered biological phantom. [ABSTRACT FROM AUTHOR]

With the rapid development of materials science and medical imaging technology, traditional optimization algorithms cannot solve the problem of inverse scattering of complex scatterers well. Therefore, more and more imaging algorithms for solving complex scatterers were proposed. In this paper, a novel hybrid algorithm is put forward for the microwave imaging problem. First, the proposed algorithm improves the search path of the traditional sine cosine algorithm, which obtains better global search capability. Second, the least square is introduced to form judging and contrasting mechanisms, which forms the parallel algorithm simultaneously, in order to make the proposed algorithm more suitable for the diverse microwave imaging problem. To prove the efficiency of the proposed algorithm, several examples, including ten benchmark functions, two engineering design problems, and three different microwave imaging problem tests are adopted. As expected, the results show that the proposed algorithm achieves not only superior optimal value but also the reconstruction of the complicated permittivity of the scatterer compared with several traditional optimization algorithms. [ABSTRACT FROM AUTHOR]

In this paper, a low‐profile aperture‐stacked patch (LP‐ASP) antenna for breast cancer detection applications is proposed. The antenna is embodied by three substrate layers, in which one layer is applied to feed all stacks and the other two are adjoined substrates with rectangular patches, mounted on the top of the feeding layer. The feeding layer consists of a fork‐shaped strip on one side, and an H‐shaped slot attached to a cross‐shaped slot on the other side. By using the proposed feeding structure and a designed defected ground plane, an additional current path is provided which generates extra resonances and widens the impedance bandwidth from 2.5 to 15 GHz. The overall size of the antenna is 10 × 10 × 3.475 mm3, which is approximately 0.083λ × 0.083λ × 0.03λ at the antenna's lowest operating frequency. To evaluate the performance of the designed antenna in an imaging system, first, a breast model with an embedded spherical tumor (with a radius of 4 mm) is constructed. Then, a conformal array of the designed antenna with 17 elements on a cross‐arranged lattice is placed around the breast model. The quantitative metrics of the reconstructed images show that the proposed antenna can be a good candidate for the breast cancer detection applications. [ABSTRACT FROM AUTHOR]

In this report, a free space frequency‐time‐domain technique is presented for characterizing the electrical properties and thickness of the sample using multiple reflections and fabry‐perot resonance phenomenon. The retrieval of constitutive electromagnetic parameters of the sample has been carried out by comparing the measured reflection coefficient data from the sample at two different incident angles. The relative permittivity as well as relative permeability along with the thickness of different samples viz., beryllia, silicon, and plexiglass have been evaluated with high accuracy in the frequency range 1 to 15 GHz. The method is also experimentally validated by successfully reconstructing the unknown material properties of two different samples. The unique advantage of this method lies in non‐requirement of any prior knowledge of the sample's thickness for measuring the complex relative dielectric constant as well as relative permeability of the sample. To determine the electromagnetic properties of the sample, the sole knowledge of reflection coefficient data are needed. Moreover, the method does not involve any additional measurement for the reference calibration. The simple, cost‐effective proposed scheme is quite useful in many applications like accurate determination of signal strength in indoor wireless communication, through wall imaging, food industry, and so on. [ABSTRACT FROM AUTHOR]

The oil palm trees in Southeast Asia face a great challenge due to voids within tree trunks. Besides, both plantations and the palm oil industry suffered considerable losses due to ineffective inspection of defective trees. Several techniques such as ultrasound, X-ray, capacitive volume, gamma computed tomography, and microwave tomography were applied to detect and control the decay of a trunk. However, all the techniques showed substantial drawbacks (such as limited resolution, tedious processing time, and use of bulky equipment with limited mobility) except for microwave tomography which overtook the other techniques by using a method that distinguishes the dielectric properties of healthy and diseased trunks. This work proposes ultra-wide-band (UWB) signal transmission and reception using antenna sensor arrays that record reflections from affected regions in the trunk. Various factors have been considered, such as different cylindrical arrays of 4, 8, 12, and 16 antennas; different positions of hollows; a heterogeneous trunk; multiple targets; and larger trunk samples (16–30 cm). To validate the system's capabilities, two cylindrical wood samples with different diameters of 100 mm and 140 mm and with one hollow and three hollows within are 3D-printed, investigated, and then measured. The authors recommended a robust time-reversal algorithm (RTR) to reconstruct 2D images that successfully identified and localized cavities with the smallest diameter of 3.5 mm. Furthermore, reconstructed images of measured data verified a practical and reliable oil palm trunk imaging and sensing system with a high structural similarity index and resolution. [ABSTRACT FROM AUTHOR]

Microwave breast imaging exploits dielectric contrasts between cancerous and healthy tissues at microwave frequencies to detect breast tumors. The estimation of breast optimal effective dielectric properties has a significant influence on the image quality. Recently, a novel method based on focal quality metrics (FQMs) has been proposed to investigate the patient‐specific effective dielectric properties. Although FQMs can be used to estimate the effective permittivity, the small increment of permittivity during the imaging process may lead to large computational resource consumption. In this article, an optimized microwave breast imaging method is proposed. In this method, a hybrid image quality metric is put forward which mixes the signal‐to‐clutter ratio (SCR) and focal quality metric. In addition, the simulated annealing algorithm is incorporated to facilitate image reconstruction. Experimental results demonstrate that using the proposed method, SCR is improved by 0.2 dB and tumor location error is reduced by 3.5 mm in realistic breast phantom. [ABSTRACT FROM AUTHOR]

An increase in the quantity and density of antenna elements increases the mismatched failure rate and measurement difficulty of the multiple-input multiple-output. To simplify the measurement method of the S 11 parameter utilizing the traditional vector network analyzer, this article proposes a multiple-input multiple-output measurement method based on microwave imaging. The multiple-input multiple-output element was designed, and then the existence of mismatched scattering of the mismatched state through microwave one-dimensional and two-dimensional imaging simulations was verified. A wideband Vivaldi antenna was designed for measurement imaging verification. The research results show that the proposed method is capable of detecting the mismatched scattering of mismatched elements as well as accurately locating the mismatched elements and mismatched position of circuits behind the element, which improves the measurement efficiency. [ABSTRACT FROM AUTHOR]

A miniaturized inset‐fed on‐body meandered bowtie antenna designed for brain microwave imaging systems is presented in this article. The proposed on‐body antenna can contribute to the realization of a wearable and portable brain microwave imaging system. The size of 18 × 18 mm2 is achieved at a frequency range of 0.75 to 4 GHz by the simultaneous use of self‐complementary structures and meandered lines. The frequency band is a trade‐off between penetration depth and spatial resolution. The proposed antenna performance was studied at different positions on the human head voxel model in terms of several parameters such as reflection coefficient, near‐field directivity, and fidelity factor. In addition, the antenna bandwidth was surveyed on several volunteers using a wearable measurement setup. It has been found that the averages of measured reflection coefficients in different scenarios are in good agreement with the corresponding simulation results, and the antenna shows stable performance under different practical situations. The proposed antenna takes advantage of a small footprint and body matching, which make it an eligible choice for compact, portable, and wearable head microwave imaging systems. [ABSTRACT FROM AUTHOR]

In the present work, a phaseless approach for microwave imaging applications is presented. The proposed solution strategy is based on the formulation of the scattering phenomena in terms of contrast source, while no phase-recovery stage is involved into the numerical procedure, thus providing a phaseless single-step resolution method. The image recovering potentialities of the discussed method are numerically validated by successfully distinguishing different tissues of a slice breast model, with a tumor located wherein. The above preliminary assessment encourages the adoption of the proposed solution in the framework of biomedical imaging. [ABSTRACT FROM AUTHOR]

We introduce the discrete dipole approximation (DDA) for efficiently calculating the two-dimensional electric field distribution for our microwave tomographic breast imaging system. For iterative inverse problems such as microwave tomography, the forward field computation is the time limiting step. In this paper, the two-dimensional algorithm is derived and formulated such that the iterative conjugate orthogonal conjugate gradient (COCG) method can be used for efficiently solving the forward problem. We have also optimized the matrix-vector multiplication step by formulating the problem such that the nondiagonal portion of the matrix used to compute the dipole moments is block-Toeplitz. The computation costs for multiplying the block matrices times a vector can be dramatically accelerated by expanding each Toeplitz matrix to a circulant matrix for which the convolution theorem is applied for fast computation utilizing the fast Fourier transform (FFT). The results demonstrate that this formulation is accurate and efficient. In this work, the computation times for the direct solvers, the iterative solver (COCG), and the iterative solver using the fast Fourier transform (COCG-FFT) are compared with the best performance achieved using the iterative solver (COCG-FFT) in C++. Utilizing this formulation provides a computationally efficient building block for developing a low cost and fast breast imaging system to serve under-resourced populations. [ABSTRACT FROM AUTHOR]

An increasing number of practical applications of three-dimensional microwave imaging require accurate and efficient inversion techniques. In this context, a full-wave 3D inverse-scattering method, aimed at characterizing dielectric targets, is described in this paper. In particular, the inversion approach has a Newton-based structure, in which the internal linear solver is a conjugate gradient-like algorithm in lp spaces. The presented results, which include the inversion of both numerical and experimental scattered-field data obtained in the presence of homogeneous and inhomogeneous targets, validate the reconstruction capabilities of the proposed technique. [ABSTRACT FROM AUTHOR]

The aim of this paper is to present and experimentally verify the first prototype of a microwave imaging system specifically designed and realized for the continuous monitoring of patients affected by brain stroke, immediately after its onset and diagnosis. The device is a 2D version of the 3D system, currently under construction, and consists of an array of 12 printed monopole antennas connected to a two-port vector network analyzer through a switching matrix so that each antenna can act as a transmitter or receiver, thereby allowing the acquisition of the entire multistatic multiview scattering matrix required for the imaging. The system has been experimentally tested on 2D phantoms with electric properties mimicking the brain. The presence and the evolution of the stroke have been reproduced by filling a proper cavity in the phantom with a liquid having the electric properties of blood. A differential approach has been adopted by acquiring the scattering matrix before and after the filling of the blood cavity. The so achieved differential dataset has been processed by means of a linear imaging algorithm in order to reconstruct the stroke location and dimension. Moreover, the effect of pre- and postprocessing operations on the measured data is investigated. A good agreement has been obtained between the reconstructions and the actual scenario. As a final remark, it is worth noting that the entire data acquisition and processing are sufficiently fast to allow a real-time monitoring. [ABSTRACT FROM AUTHOR]

In this work, a prototype of a laboratory microwave imaging system suitable to methodically test the ability to image, detect, and classify human brain strokes using microwave technology is presented. It consists of an antenna array holder equipped with ten newly developed slot bowtie antennas, a 2.5 D reconfigurable and replaceable human head phantom, stroke phantoms, and related measuring technology and software. This prototype was designed to allow measurement of a complete S-matrix of the antenna array. The reconfigurable and replaceable phantom has currently 23 different predefined positions for stroke phantom placement. This setting allows repeated measurements for the stroke phantoms of different types, sizes/shapes, and at different positions. It is therefore suitable for large-scale measurements with high variability of measured data for stroke detection and classification based on machine learning methods. In order to verify the functionality of the measuring system, S-parameters were measured for a hemorrhagic phantom sequentially placed on 23 different positions and distributions of dielectric parameters were reconstructed using the Gauss-Newton iterative reconstruction algorithm. The results correlate well with the actual position of the stroke phantom and its type. [ABSTRACT FROM AUTHOR]

Abstract: In this article, the approach based on the inverse circular radon transform technique (ICRT) is proposed to obtain the image of the breast cancer screening based on microwaves. A simplified numerical breast model is constructed and computational results are obtained with the use of synthetic aperture radar principle. A tumor phantom is produced and the experimental results are acquired by using sand, screw and phantoms with the use of an experimental setup prototype that is also presented in this study. The simulation results are verified and validated by experimental results. The resultant images generated using the simulated and experimental data show a good agreement with the designed scenarios. [ABSTRACT FROM AUTHOR]

Abstract: Nanocomposite absorbers based on dielectric/magnetic materials are prepared by in situ incorporation of MWCNTs and NiFe alloy nanopowder into epoxy matrix. SEM micrographs and EDAX studies of these nanocomposites clearly revealed the incorporation of MWCNTs/NiFe nanoparticles in the epoxy matrix. Dielectric measurement revealed that the real part of permittivity and permeability of NiFe‐filled MWCNT composites increased with an increase in NiFe concentration. The consolidated loss tangent values revealed that the total losses were highest for 15% of NiFe alloy nanopowder, thus accounting for the highest absorption loss. The two‐layer RAS composed of NiFe alloy nanopowder‐filled MWCNTs/epoxy composite achieves a minimum reflection loss of −12 dB (95% of absorption) in the range of 8.2–9.8 GHz and a maximum peak of −19 dB at 8.2 GHz with a thickness of 4 mm for 15 wt% NiFe alloy nanopowder MWCNTs/epoxy composites. In addition, −10 dB absorption bandwidths of composite material are <2 GHz, i.e., 8.2–9.8 GHz. Microwave absorption was attributed to both the dielectric and magnetic nature of MWCNTs and NiFe alloy nanopowder. The frequency of microwave absorption complies with the quarter wavelength (λ/4) matching model. [ABSTRACT FROM AUTHOR]

A fast and efficient microwave tomographic algorithm is proposed for 2-D and 3-D real-time intrawall imaging. The exploding reflection model is utilized to simplify the imaging formulation, and the half-space Green’s function is expanded in the spectral domain to facilitate the easy implementation of the imaging algorithm with the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). The linearization of the inversion scheme and employment of FFT/IFFT in the imaging formula make the algorithm suitable for various applications pertaining to the inspection of a large probed region and allow real-time processing. Representative numerical and experimental results are presented to show the effectiveness and efficiency of the proposed algorithm for real-time intrawall characterization. [ABSTRACT FROM AUTHOR]

Microwave thermal ablation is a cancer treatment that exploits local heating caused by a microwave electromagnetic field to induce coagulative necrosis of tumor cells. Recently, such a technique has significantly progressed in the clinical practice. However, its effectiveness would dramatically improve if paired with a noninvasive system for the real-time monitoring of the evolving dimension and shape of the thermally ablated area. In this respect, microwave imaging can be a potential candidate to monitor the overall treatment evolution in a noninvasive way, as it takes direct advantage from the dependence of the electromagnetic properties of biological tissues from temperature. This paper explores such a possibility by presenting a proof of concept validation based on accurate simulated imaging experiments, run with respect to a scenario that mimics an ex vivo experimental setup. In particular, two model-based inversion algorithms are exploited to tackle the imaging task. These methods provide independent results in real-time and their integration improves the quality of the overall tracking of the variations occurring in the target and surrounding regions. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) microwave imaging has been proven to be well suited for concealed weapon detection application. For the 3D image reconstruction under two-dimensional (2D) planar aperture condition, most of current imaging algorithms focus on decomposing the 3D free space Green function by exploiting the stationary phase and consequently, the accuracy of the final imagery is obtained at a sacrifice of computational complexity due to the need of interpolation. In this paper, from an alternative viewpoint, we propose a novel interpolation-free imaging algorithm based on wavefront reconstruction theory. The algorithm is an extension of the 2D range stacking algorithm (RSA) with the advantages of low computational cost and high precision. The algorithm uses different reference signal spectrums at different range bins and then forms the target functions at desired range bin by a concise coherent summation. Several practical issues such as the propagation loss compensation, wavefront reconstruction, and aliasing mitigating are also considered. The sampling criterion and the achievable resolutions for the proposed algorithm are also derived. Finally, the proposed method is validated through extensive computer simulations and real-field experiments. The results show that accurate 3D image can be generated at a very high speed by utilizing the proposed algorithm. [ABSTRACT FROM AUTHOR]

Breast cancer, has been a significant cancer type for women on the society. Early diagnosis and timely medical treatment are important key factors spreading to the other tissues and permitting long-time survival of patients. Since the existing methods have several serious shortfalls, microwave imaging method for the diagnosis of early stage tumors has been interested by different scientific research groups in terms of moderating endogenous the electrical property difference between healthy tissue and malignancies. In this article, both an ultra-wideband bow-tie antenna with enhanced bandwidth and a 3D breast model which has different electrical properties which are permittivity and conductivity is created in simulation tool to solve electromagnetic field values. Return loss, VSWR, and radiation pattern characteristics, which are significant antenna parameters, are simulated and obtained whether the antenna possess an efficient characteristic or not. Electric field values over the breast tissue in which there is a tumor or not tumor are evaluated. In this article, above-mentioned values of frequency bandwidth, dielectric constant of antenna's substrate, electric field, and tumor information were consisted in their dataset. This dataset obtained from the Bow-Tie Antenna was used to detect the breast cancer with one of the data mining method, which is K-Nearest Neighbor Algorithm. [ABSTRACT FROM AUTHOR]

A digital breast cancer detection system using 65 nm technology complementary metal oxide semiconductor (CMOS) integrated circuits with rotating 4 × 4 antenna array is presented. Gaussian monocycle pulses are generated by CMOS logic circuits and transmitted by a 4 × 4 matrix antenna array via two CMOS single-pole-eight-throw (SP8T) switching matrices. Radar signals are received and converted to digital signals by CMOS equivalent time sampling circuits. By rotating the 4 × 4 antenna array, the reference signal is obtained by averaging the waveforms from various positions to extract the breast phantom target response. A signal alignment algorithm is proposed to compensate the phase shift of the signals caused by the system jitter. After extracting the scattered signal from the target, a bandpass filter is applied to reduce the noise caused by imperfect subtraction between original and the reference signals. The confocal imaging algorithm for rotating antennas is utilized to reconstruct the breast image. A 1 cm3 bacon block as a cancer phantom target in a rubber substrate as a breast fat phantom can be detected with reduced artifacts. [ABSTRACT FROM AUTHOR]

Cracks in concrete or cement based materials present a great threat to any civil structures; they are very dangerous and have caused a lot of destruction and damage. Even small cracks that look insignificant can grow and may eventually lead to severe structural failure. Besides manual inspection that is ineffective and time-consuming, several nondestructive evaluation techniques have been used for crack detection such as ultrasonic technique, vibration technique, and strain-based technique; however, some of the sensors used are either too large in size or limited in resolution. A high resolution microwave imaging technique with ultrawideband signal for crack detection in concrete structures is proposed. A combination of the delay-and-sum beamformer with full-view mounted antennas constitutes the image reconstruction algorithm. Various anomaly scenarios in cement bricks were simulated using FDTD, constructed, and measured in the lab. The reconstructed images showed a high similarity between the simulation and the experiment with a resolution of λ/14 which enables a detection of cracks as small as 5 mm in size. [ABSTRACT FROM AUTHOR]

A method for microwave imaging of dielectric targets is proposed. It is based on a tomographic approach in which the field scattered by an unknown target (and collected in a proper observation domain) is inverted by using an inexact-Newton method developed in Lp Banach spaces. In particular, the extension of the approach to multifrequency data processing is reported. The mathematical formulation of the new method is described and the results of numerical simulations are reported and discussed, analyzing the behavior of the multifrequency processing technique combined with the Banach spaces reconstruction method. [ABSTRACT FROM AUTHOR]

The detailed design equations and antenna parameters for a directional antenna for breast imaging are presented in this paper. The antenna was designed so that it could be immersed in canola oil to achieve efficient coupling of the electromagnetic energy to the breast tissue. Ridges were used in the horn antenna to increase the operating bandwidth. The antenna has an exponentially tapered section for impedance matching. The double-ridged horn antenna has a wideband performance from 1.5 GHz to 5 GHz (3.75 GHz or 110% of impedance bandwidth), which is suitable for breast microwave radar imaging. The fabricated antenna was tested and compared with simulated results, and similar bandwidths were obtained. Experiments were conducted on breast phantoms using these antennas, to detect a simulated breast lesion. The reconstructed image from the experiments shows distinguishable tumor responses indicating promising results for successful breast cancer detection. [ABSTRACT FROM AUTHOR]

Three contributions that can improve the performance of a Newton-type iterative quantitative microwave imaging algorithm in a biomedical context are proposed. (i) To speed up the iterative forward problem solution, we extrapolate the initial guess of the field from a few field solutions corresponding to previous source positions for the same complex permittivity (i.e., “marching on in source position”) as well as from a Born-type approximation that is computed from a field solution corresponding to one previous complex permittivity profile for the same source position. (ii) The regularized Gauss-Newton update system can be ill-conditioned; hence we propose to employ a two-level preconditioned iterative solution method. We apply the subspace preconditioned LSQR algorithm from Jacobsen et al. (2003) and we employ a 3D cosine basis. (iii) We propose a new constrained line search path in the Gauss-Newton optimization, which incorporates in a smooth manner lower and upper bounds on the object permittivity, such that these bounds never can be violated along the search path. Single-frequency reconstructions from bipolarized synthetic data are shown for various three-dimensional numerical biological phantoms, including a realistic breast phantom from the University of Wisconsin-Madison (UWCEM) online repository. [ABSTRACT FROM AUTHOR]