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The clinical use of microwave tomography (MT) requires addressing the significant mismatch between simulated environment, which is used in the forward solver, and real-life system. To alleviate this mismatch, a calibrated tomography, which uses two homogeneous calibration phantoms and a modified distorted Born iterative method (DBIM), is presented. The two phantoms are used to derive a linear model that matches the forward solver to real-life measurements. Moreover, experimental observations indicate that signal quality at different frequencies varies between different antennas due to inevitably inconsistent manufacturing tolerance and variances in radio-frequency chains. An optimum frequency, at which the simulated and measured signals of the antenna present maximum similarity when irradiating the calibrated phantoms, is thus calculated for each antenna. A frequency-division DBIM (FD-DBIM), in which different antennas in the array transmit their corresponding optimum frequencies, is subsequently developed. A clinical brain scanner is then used to assess performance of the algorithm in lab and healthy volunteers' tests. The linear calibration model is first used to calibrate the measured data. After that FD-DBIM is used to solve the problem and map the dielectric properties of the imaged domain. The simulated and experimental results confirm validity of the presented approach and its superiority to other tomographic method.

A planar dual-band dual-mode antenna with omni-/unidirectional radiation is proposed for operating 2.45/5.8 GHz bands. The lower-band antenna consists of 4 modified quarter-mode substrate-integrated waveugide (QMSIW) elements, which are excited in-phase and with same magnitude to obtain vertically polarized omni-directional radiation. The upper-band antenna is based on a four-port square patch with differential feed to achieve highly-isolated dual-polarization with broadside radiation. In addition, the square patch is loaded with four overlapped-corner-mushroom structures to broaden the upper bandwidth. The final design with overall size of of 90 90 3.66 mm (0.735 0.735 0.03 at 2.45 GHz) was fabricated and measured. Measurements show a 10-dB return loss bandwidth of 160/400 MHz and a gain of 3.4/8.5 dBi at 2.45/5.8 GHz bands, respectively. Furthermore, the antenna prototype demonstrates an isolation of more than 30 dB among all ports. Especially, the isolation among the two ports of the upper-band elements is better than 40 dB.

A thin and broadband frequency selective surface (FSS)-based magnetic absorber is presented for low-microwave frequency applications. The design consists of a double-layers of FSS with the combination of two different magnetic substrates, which provides a much broader bandwidth compared to existing single-layered designs. The magnetic materials are in-house made by mixing different ratios of carbonyl-iron powder with silicone elastomer polydimethylsiloxane (PDMS). The simple single-square loop (SSL) structure is used for both FSSs which are printed on a standard FR4 material using lossy resistive sheet. This configuration allows the use of a closed-form equivalent-circuit model for a fast and efficient bandwidth optimization. A genetic algorithm (GA) is utilized to obtain the design parameters for broadband absorption with minimum overall thickness. The measured results confirm that the proposed absorber operates from 0.78 to 4.8 GHz with -10 dB reflectivity. The design achieves 144% fractional-bandwidth with a 10.12 bandwidth-to-thickness ratio.

A single-layered slot antenna system working at 5.8 GHz Industrial, Scientific and Medical (ISM) band is proposed for in-band full duplex (IBFD) operation applications without the use of a coupler. First, high isolation is achieved by strong separation of even- and odd-mode feeds. The microstrip-coupled coplanar waveguide (CPW) is used at Port 1 (TX port) to excite a stepped-slot antenna in the CPW odd mode. On the opposite side, a microstrip T-junction power divider is employed at Port 2 (RX port) to feed two offset-fed stepped-slot antennas in even mode. Second, isolation is further improved by 30 dB by using a lumped capacitor at the termination of the CPW. The measured isolation between the two ports is about 50 dB across the bandwidth. The measured −10 dB bandwidth of Port 1 is 0.49 GHz (8.5%), while that of Port 2 is 1.06 GHz (18.3%). The gains of TX and RX antennas are 5.4 and 5.8 dBi at 5.8 GHz. The proposed antenna can also be deployed as a dual-polarized antenna. Mathematical analysis and equivalent transmission line circuit models are provided to give physical insight into the working principals of the antenna with validation from ANSYS HFSS simulation.

Microwave tomography using the contrast source inversion (CSI) method has been intensively researched especially in medical applications, where a coupling medium (CM) is usually used to improve imaging. Previous works noted that better results can be achieved when the CM is lossy. Nevertheless, lossy CM weakens the target response, especially when complicated scenario is imaged such as the head. In this letter, CSI is analyzed to indicate that the dielectric properties of CM are related to the condition number of a part of the objective function in each of CSI iteration. It is found that the value of this condition number is proportional to the ratio of maximum and minimum electric field intensity in the computational domain. Therefore, to reduce that ratio, a random multiple-input–multiple-output (RMIMO) scheme, which is implemented by applying a transformation matrix on the measured S -parameter matrix, is proposed to enable the use of loss-less CM and improves the imaging results. The proposed analysis and RMIMO-CSI method are validated using numerical head models.

In this letter, a broadband dual-polarized metasurface-based antenna is proposed for in-band full-duplex applications. The design concept exploits the wideband and low-profile features of a metasurface, as well as its structural symmetry to implement the differential-fed scheme for high isolation. An antenna design operating at 3.5 GHz is optimized, fabricated, and tested. The prototype with dimensions of 0.97 $\lambda _{\text{min}}$ × 0.97 $\lambda _{\text{min}}$ × 0.078 $\lambda _{\text{min}}$ ( $\lambda _{\text{min}}$ is the free-space wavelength referring to the lowest operational frequency) achieves a 10 dB return loss bandwidth of 28.4% (2.93–3.90 GHz) and an interport isolation of $>$ 43 dB. The far-field measurements indicate that the antenna exhibits broadside radiation with a realized gain of $>$ 8.0 dBi and low cross-polarization level of ${ 30 dB within the wide operational bandwidth.

This paper investigates the effects of parasitic elements on the performance of MIMO patch antenna. Multiple square parasitic elements are added in close proximity to each of the rectangular patch elements. These parasitic elements affect the electromagnetic field distribution and consequently reduce the mutual coupling. In addition, wider bandwidth is also achieved. Two MIMO antennas coupled in H-plane and E-plane are designed and measured to demonstrate the proposed concept. The measured results show that both designs have a wide impedance bandwidth of 14% with isolation of more than 20 dB across the whole band (with a small antenna profile of $0.05\lambda _{0}$ ). For 40-dB isolation, the H-coupled and E-coupled design achieve a bandwidth of 2.7% and 1.5%, respectively. Compared to other methods, the proposed method has much simpler structure, wider bandwidth and comparable improvement in isolation, with the tradeoff in larger antenna element size.

In this article, the results of the research on organic pollutant treatment in the wastewater of printing processes on fabric by H2O2 under the catalytic role of the complex between ion Ni2+ and Citric acid (H4L) were presented. The condition of pH, H4L/Ni2+, H2O2, Ni2+ concentration has been explored to get the optimal conditions for improving COD efficient treatment. The results provide the solutions of the homogeneous complex catalysts in the industrial wastewater treatment at room temperature and atmosphere.

An accurate equivalent circuit (EC) design approach for wideband nonmagnetic absorbers operating at the low microwave frequency (1–10 GHz) is presented. Following the impedance matching approach, this communication introduces an EC model based on the simulated data and synthetic asymptotes for single- and double-layer frequency-selective surface (FSS)-based nonmagnetic absorbers. Two simple and commonly used resistive FSSs, i.e., square patch and single square loop, are considered in this communication. Compared to the full-wave simulations, the proposed EC model shows more than 95% accuracy. By employing the proposed model and genetic algorithm-based optimization, several designs of broadband absorbers are demonstrated. The presented single- and double-layer FSSs show 126% and 161% fractional bandwidth, respectively, with the total thickness close to the Rozanov limit. The results confirm that the proposed method is a simple and efficient way of designing thin wideband absorbers using single- or double-layer FSS configurations.

This letter presents a simple single-layered dual-band circularly polarized microstrip patch leaky-wave antenna (LWA). The proposed design consists of two linear circular patch arrays that are coupled to a 50 $\Omega$ microstrip line. While having high gain, the rotating sense in each band is opposite to each other, which makes the antenna concept well-suited for the uplink and downlink satellite communication. For demonstration, an antenna has been designed to operate at 5.0 and 8.0 GHz. It is noted that the concept can be easily adapted to other frequencies. With a total length of 2.89 $\lambda _0$ ( $\lambda _0$ is the free-space wavelength at 5.0 GHz), the antenna prototype achieves a satisfactory measured axial ratio (AR) with peak realized gains of approximately 12 and 13.6 dBi at 5.0 and 8.0 GHz, respectively.

There is a significant demand for fast and accurate electromagnetic (EM) imaging of stroke in emergency situations. This article presents a method for encoding the raw S-parameters from the Cartesian matrix to polar grid coordinates with weighting coefficients based on the receiver antenna spatial sensitivity. The polar sensitivity encoding (PSE) scheme is based on the fact that the receiver sensitivity generally has an encoding effect and, in this case, it is applied during the transformation of S-matrices to polar grid, which is geometrically congruent with the shape of the head. The PSE scheme alleviates the need for highly accurate and intricate forward and inverse EM field solvers and mitigates the introduction of numerical errors in addition to the unavoidable experimental uncertainties. The simulation and experimental results demonstrate that the PSE method is robust to head shifts up to about 5 mm and accurate in localizing strokes in less than a second.

This letter proposes a differentially fed $\pm$ $45^{\circ }$ dual-polarized antenna with ultrawideband operation. It consists of two orthogonal stepped-dipoles backed by an inverted pyramidal cavity reflector for improvement in broadside gain. The stepped dipoles built on both sides of the substrate are constructed from small square pads and square loops for multiresonances to enhance the bandwidth. The dipoles are fed from four coaxial lines with a crossed-strip structure to accommodate the differential feed. The final design with overall size of 0.75 $\lambda _{\text{min}}$ × 0.75 $\lambda _{\text{min}}$ × 0.21 $\lambda _{\text{min}}$ ( $\lambda _{\text{min}}$ is the free-space wavelength referring to the lowest operational frequency) has been fabricated and measured. The antenna achieves a measured bandwidth of 74.2% (1.74–3.79 GHz) for return loss $>$ 15 dB and a measured isolation better than 43 dB among the differential ports. Its bandwidth can cover the frequency spectrums of the existing and new wireless communication systems such as 2G/3G/4G (1710–2690 MHz) and sub-6 GHz 5G (3100–3800 MHz). Additionally, the measured peak broadside gain is 10.5 dBi with cross-polarization level of $ 20 dB within the operational bandwidth.

A flexible on-body antenna that is compact, unidirectional, and wideband is presented for electromagnetic (EM) biomedical diagnostic systems. To achieve a low operating-frequency band with compact size, an annular ring antenna is loaded with a capacitive gap, a shorting post at its end, and a shorted circular parasitic element. The antenna is designed and fabricated on a purposely built, highly flexible, low-loss, low-cost room-temperature-vulcanizing silicone (RTV) substrate with a tremendously compact size of $0.055\lambda _{0}\times 0.055\lambda _{0}\times 0.009\lambda _{0}$ , where $\lambda _{0}$ is the free-space wavelength at the lowest frequency of 0.55 GHz. The antenna exhibits a measured bandwidth of 0.55–3 GHz at different locations of the human body. The measured near-field radiation patterns demonstrate that the antenna attains unidirectional near-field radiations with more than 10 dB front-to-back-ratio. The received dominant signals of the antenna also exhibit a low distortion level with more than 90% fidelity factor. The achieved features of the proposed antenna confirm its merits for EM biomedical diagnosis applications.

A broadband measurement technique for the normal-incidence absorptivity based on parallel-plate waveguide (PPW) is proposed. The method is well suited for frequency-selective surface (FSS)-based absorbers. By employing the PPW structure, only a small sample size, i.e., 1-D array of FSS, is required for the measurement. The thru-reflect-line (TRL) calibration is implemented to remove the time-gating ambiguity in the conventional PPW-based method. A measurement system working up to 6 GHz is designed such that its length is adjustable to ensure all three calibration standards (thru, reflect, and line) can be carried out in a single device. Moreover, the height of the PPW is also adjustable to allow the measurement of absorbers with a different periodicity of FSSs. Two single-layer FSS-based absorbers with different periodicity are fabricated and measured. The results agree well with the conventional free-space measurement system, which validates the proposed method.

A technique to measure the complex permittivity and permeability using two different types of calibration is presented in this article. The technique is based on a classical transmission-line analysis, implemented on a strip-line structure. A new calibration technique, namely object-reflect-line (ORL), is proposed to meet the challenge of an extremely low reflection line for accurate measurement. In the proposed calibration, the thru standard of conventional thru-reflect-line (TRL) is replaced by a standard with a known object. Since the ORL calibration uses only a single device, it reduces the fabrication cost, the measurement complexity, and the potential error in the thru measurement needed in the TRL method, which uses an additional device. The analysis is extended with consideration of the impact on material loss in measurement accuracy. The simulated and measured results with a variety of nonmagnetic and magnetic materials demonstrate the effectiveness of the proposed method over broadband microwave frequencies.

In multiscattering environment, the channel capacity can be enhanced by employing dual-polarized antennas, and more significantly, using tripolarized antennas. Aiming at providing a wide coverage of both existing wireless communication systems and the new 5G sub-6 GHz band, this article presents a compact ultrawideband tripolarized antenna. The proposed design consists of a monopole-equivalent structure for vertically linearly polarized (LP) omnidirectional radiation and two orthogonal stepped dipoles for ±45° LP broadside radiation. Several important design features are proposed and implemented. The dipoles act as parasitic elements for the monopole structure, which broadens the monopole bandwidth and reduces the mutual coupling. Furthermore, the port isolation is improved by inserting two metallic posts between the crossed dipoles and the ground plane. The measured results indicate that the fabricated prototype achieves an overlapped bandwidth of 84.73% (1.68–4.15 GHz) for

This communication presents a design concept and realization of a patch antenna array with frequency and polarization reconfigurabilities. The frequency tunability is achieved using one short-circuited varactor diode per patch element. These tuning components also facilitate the generation of different polarizations by arranging the patches both horizontally and vertically. The 45° linear polarization (LP) is obtained by feeding all patches with the same phase, i.e., setting all varactor capacitance the same value. The circular polarization (CP) is obtained by slightly detuning the varactor capacitance such that the phase difference between the horizontal and vertical radiating elements is 90°. For validation, a $2\times 2$ array has been fabricated and measured. The array prototype can operate with 45° LP, left-handed, or right-handed CP in a continuous tuning frequency range from 3.6 to 4 GHz (about 10%).

This letter exploits a dual feature of rectangular open-ended ground slot to design an integrated 4G/5G antenna for mobile devices. In the proposed configuration, the antenna footprint of the 4G module is used for the installation of a 5G antenna module. While the entire rectangular slot is structured to operate at 800 MHz and 2 GHz, an array of 4-element Vivaldi antenna operating at 28 GHz is added within the slot space. This array provides end-fire radiation with 11 dBi peak realized gain. To validate the design concept, a prototype is fabricated on a RO-5880 substrate with an overall size of $150\times 75\times 0.51\text{ mm}^3$ . The results show that the design covers two low frequency bands (from 770 to 810 MHz and from 1.9 to 2.6 GHz) with less than $-6$ dB reflection coefficient and a wideband millimeter-wave frequency range (from 26 to 32 GHz) with less than $-7$ dB reflection coefficient. A multiple-input–multiple-output (MIMO) configuration of the antenna is also realized with diversity performances verified.

An approach toward designing and building of a compact, low-profile, wideband, unidirectional, and conformal imaging antenna for electromagnetic (EM) head imaging systems is presented. The approach includes the realization of a custom-made flexible high-permittivity dielectric substrate to achieve a compact sensing antenna. The developed composite substrate is built using silicon-based poly-di-methyl-siloxane (PDMS) matrix and microscale of aluminium oxide (Al2O3) and graphite (G) powders. Al2O3 and G powders are used as fillers with different weight-ratio to manipulate and control the dielectric properties of the substrate for attaining better matched with the human head and reducing antenna’s physical size while keeping the PDMS flexibility feature. Using the custom-made substrate, a compact, wideband, and unidirectional on-body matched antenna for wearable EM head imaging system is realized. The antenna is configured as a multi-slot planar structure with four shorting pins, working as electric and magnetic dipoles at different frequency bands. The measured reflection coefficient (S11) shows an operating frequency band of 1–4.3 GHz. The time-average power density and the amplitude of the received signal inside the MRI-based realistic head phantom demonstrate a unidirectional propagation and high-fidelity factor (FF) of more than 90%. An array of 13 antennas are fabricated and tested on a realistic 3-D head phantom to verify the imaging capability of the proposed antenna. The reconstructed images of different targets inside the head phantom demonstrate the possibility of utilizing the conformal antenna arrays to detect and locate abnormality inside the brain using multistatic delay-multiply-and-sum beamforming algorithm.

This paper presents a dual-sense circularly polarized (CP) antenna with compact size, high isolation, and wideband characteristics. The antenna consists of a microstrip patch, an array of parasitic metal plates positioned around the patch, and a 90° branch line coupler as feeding structure. Unlike the conventional design method, we demonstrate that the wide isolation bandwidth can be achieved with an imperfect coupler and imperfect matching at the radiating element. The final design with overall dimensions of 0.59λc× 0.59λc × 0.06λc (λc is the free-space wavelength at center operating frequency) was fabricated and tested. Measurements on the fabricated prototype show that the antenna has a bandwidth of 16.5% (5.0-5.9 GHz) with port isolation of better than 15 dB. In addition, a peak broadside gain of about 6.6 dBic is attained. In comparison with other related works in literature, this antenna has advantages of wide operating bandwidth with high isolation while possessing a compact footprint.

A dual-function slot antenna at microwave and millimeter-wave (mm-wave) band is proposed. The design consists of a slot printed on the edge of the structure ground plane. A short-circuited varactor diode (VAR) is used to achieve the frequency tunability from 2.05 to 2.7 GHz (4G, WLAN) with a maximum realized gain of 4.5 dBi. For mm-waveband, the slot works as a connected slot antenna array (CSAA) by using eight periodic feeders with a wide bandwidth of 23–29 GHz (5G) and a maximum realized gain of 12.5 dBi. To enhance the functionality, two slots are orthogonally arranged for multiple-input multiple-output (MIMO) application. The whole structure is implemented using Rogers 5880 substrate with a board size of $70\times 60\times 0.381$ mm3. The envelope correlation coefficient (ECC) and isolation are calculated, showing satisfactory MIMO characteristics. The minimum ECC value is 0.01, while the isolation is more than 20 dB among different feeding ports. Due to the integration of 4G and 5G operations into a single narrow slot, the proposed antenna system is compact, simple, and planar in structure and, thus, attractive for future mobile handheld devices.

A single-layered multiple-input multiple-output (MIMO) antenna system is presented for future wireless devices operating at WLAN (2.45 GHz, 5.2 GHz), 4G LTE (2.6 GHz) and 5G (24 GHz, 28 GHz) bands. The design consists of a dual-band four-element monopole MIMO antenna system that covers the 2.45 GHz and 5.2 GHz bands. A decoupling mechanism is designed in the ground plane based on a tapered slot. This tapered slot has a dual function, working both as a decoupling structure ( $\lambda _{0}/4$ long) at 2.45 GHz and as a tapered slot antenna ( $3\lambda _{0}$ long) at 28 GHz when excited by a proper feeder. The proposed MIMO antenna system is designed on an RO-5880 substrate with overall dimensions of $104$ mm $\times 104$ mm $\times 0.51$ mm. The measured results show that the presented design covers two low-frequency bands (from 2.40 GHz to 5.1 GHz, from 5.1 GHz to 5.6 GHz) with a peak gain of 5 dBi and one high-frequency band (from 23 GHz to 30 GHz) with a peak gain of 11 dBi. The minimum measured isolation between the two antennas is more than 16 dB, while the maximum envelope correlation coefficient value is less than 0.16 showing good MIMO characteristics.

As a combination of three orthogonal dipoles, the tripolarized antenna has been demonstrated to significantly improve signal quality in scattering environment. In this communication, we propose a low-profile tripolarized antenna with wideband characteristics. The tristate includes 0° and 90° linear polarizations (LP) with the main beam at broadside and a vertically LP with omnidirectional pattern in the azimuth plane. The design consists of a monopolar patch and two orthogonal dipoles. The wideband performance for the monopolar patch is obtained from two nearly overlapped radiating modes controlled by an array of shorting wires. For the two orthogonal dipoles, the antenna profile is reduced with the effect of parasitic patches. One of the key design features is to utilize the shorting wire in the monopolar patch to feed the dipoles, allowing a simple design and avoiding any feeding network. The concept is experimentally verified with a prototype exhibiting an overlapped bandwidth of 14% with a low profile of $0.09\lambda $ , where $\lambda $ is the free-space wavelength at the lowest operating frequency. The measured peak gain in two broadside modes and in monopolar mode are 10 and 7.5 dBi, respectively. Measured isolation between any port is better than 31 dB in the whole frequency range.

The design of 5G 4-element antenna array system with the capability of switching its operation between right hand circular polarization (RHCP) and left hand circular polarization (LHCP) is proposed. The CP-reconfigurability is achieved by electronically switching two PIN-diode pairs in the feeding circuit, which consists of a branch line coupler and two feeding arms. The proposed configuration reduces the number of tuning elements and allows the use of imperfect PIN diodes at millimeter-wave frequency. The fabricated prototype exhibits a 3-dB axial ratio bandwidth from 27.2 to 28.35 GHz. A maximum measured gain of about 6 dBic and a radiation efficiency of around 51% over the bandwidth are achieved for both CP states.