This paper presents a dual-band eight-element multiple-input multiple-output (MIMO) antenna for 5G applications. The 8-element antenna is formed into two 4×4 MIMO systems that operate at 2500-2600 MHz and 1800-2200 MHz bands. The antenna elements are mounted along the perimeter of a rectangular ground plane with a total size of 110 × 80 mm² and are printed on both sides of a low-profile PCB material. Elements radiate through open rectangular slots etched on the antenna's ground conductor. The open slots are excited by T-shaped microstrip lines fed by 50-Ω coaxial connectors. The size of the ground plane's slots, and the T-shaped radiators control the resonance of the antenna's elements. The proposed design employs orthogonal elements to mitigate mutual coupling. The isolation between ports is less than -10 dB. The radiation efficiency ranges from 40% to 65% across operating frequency bands.

Wireless power transmission system (WPTS) based on near-field inductive coupling is an effective way to provide power for gastrointestinal micro-robot. WPTS is normally realized by a Helmholtz coil outside the body and a three-dimensional receiving coil in the micro-robot. Helmholtz coil has two types, circle and square. However, a quantitative comparison for them in the application of WPTS has not been available yet. In this paper, the calculating models of the electromagnetic field intensity (EMFI) for the circular Helmholtz coil (CHC) and square Helmholtz coil (SHC) are built. With the built model, the uniformities of the electromagnetic field (UEMF) of two Helmholtz coils are calculated. The actual coil system is built to verify the correctness of the built models. When the diameter of the CHC and the side length of the SHC are both 40 cm, the available areas (UEMF ≥ 90%) for powering the robot supplied by the CHC and SHC are 39% and 56%, respectively. Also, the consumed powers of the two coils, when identical EMFI is excited, are compared. When the EMFI at the center of the CHC and SHC are both 1 Gs, the consumed powers are 5.09 W and 4.62 W, respectively. The above results show that compared to the CHC, the SHCnot only has better uniformity, but also consumes less power. Thus, it is more suitable for the WPTS.

The aim of this paper is to prove that the power generated by a wearable textile patch antenna experiences reduced absorption in the phantom when the antenna ground-plane is increased. First, the dedicated human torso-equivalent phantom and two antennas were fabricated, which are multi-layered, with orientation normal to the body and made of the same materials. One of the antennas has a double in size ground-plane with regards to the other antenna, while the rest of their dimensions are identical. According to the proposed measurement procedure, once the radiation efficiencies of both antennas are measured in free space and with the phantom, the total absorption coefficient and the phantom losses are evaluated. The comparison of the measurement results proves that the increased ground-plane reduces the absorption on the phantom body of the antenna EM power (by 30.5%). Simulations and measurements were found in good agreement, with maximum deviation between the two up to 6% in terms of radiated efficiency. Hence, the proposed experimental evaluation of the impact of the ground-plane size of a wearable textile patch antenna on the reduction of the power absorbed by the user's body can be considered as a simple, reliable and cost-effective measurement method.

In this paper, a novel compact single element G-shaped Asymmetric Coplanar Strip (ACS) fed antenna and its four-element printed multiple-input multiple-output (MIMO) antenna have been presented with multi-band frequency characteristics. The proposed MIMO antenna has been fabricated on an FR-4 substrate (46 × 46 × 1.6) mm³ with dielectric constant ε_r = 4.4. The desired isolation between the elements (-18 dB) is achieved by placing the antenna elements orthogonal to each other. Simulated and measured results show that return loss (S₁₁) for the proposed MIMO antenna is less than -10 dB in the operating bands, with frequency ranging 2.30-2.45 GHz, 3.36-3.65 GHz, and 4.53-5.88 GHz, respectively, which ensures its operation in multiple frequency bands. Moreover, these bands are obtained for 2.3 GHz WiBro, LTE and 5G NR to cover B40/B42/N30/N40/N97 together with 3.5 GHz/5 GHz WiMAX/WLAN band applications. Meanwhile, the diversity performance characteristics like ECC (Envelope Correlation Coefficient), MEG (Mean Effective Gain), DG (Diversity Gain), Total Active Reflection Coefficient (TARC), and Channel Capacity Loss (CCL) have been calculated and are presented in this paper. The correlation coefficient is found to be less than 0.001 with a diversity gain greater than 9.95, and an acceptable channel capacity loss is less than 0.4 bits/s/Hz.

This paper proposes a dual-polarized and high gain, four-element based compact multiple-input-multiple-output (MIMO) antenna operating at 5.2 GHz. First, a hammer-shaped antenna has been designed with a gain of 5.3 dBi, impedance bandwidth of 400 MHz, and broadside radiation. A mathematical analysis for radiated electric field and an equivalent circuit model for the hammer-shaped antenna are developed. Using the hammer-shaped antenna as an element, four element MIMO design with shorting walls is proposed. The shorting walls near non-radiating edges improve isolation between the elements by changing the direction of the major lobe. The proposed design has an envelope correlation coefficient (ECC) < 0.15, measured gain of 5.5 dBi, and mean effective gain (MEG) ~ -3 dB. This design has a low profile and single layer planar structure of area 65 mm x 65 mm, which makes it a good contender for portable devices or low-profile hand-held applications in WLAN band.

A high selectivity microstrip dual-mode diplexer with a stepped-impedance opened-end structure is implemented to reduce the size of a dual-mode resonator and suppress the harmonics. The proposed dual-mode resonator structure consists of a microstrip half-wavelength resonator and an open-circuited stepped-impedance stub. The stepped-impedance opened-end structure can control an even mode in the upper and lower desired bands to improve the cutoff responses. The sharp cutoff selectivity of the filter is created to improve the diplexer performance and wide suppress harmonics. The dual-mode diplexer prototype is analyzed, fabricated, and measured. The measured result agrees well with the analyzed result. The simulated and measured dual-mode diplexers are designed at the operational frequency of Tx/Rx at 1.95 GHz and 2.14 GHz, respectively. It is shown that the dual-mode filter has a wide stopband, including the first spurious resonance frequency due to the stepped-impedance stub.

A frequency reconfigurable antenna with dual-band operation is presented. The antenna has a circular radiating patch loaded with an annular slot, and the slotted patch is shorted to the ground plane with four conducting posts. The antenna has three feed ports. Two of the ports are used to excite the slot mode resonant at a lower frequency, and broadside radiation with dual orthogonal linear polarizations can be obtained. The other port is used to excite the monopolar-patch mode resonant at a higher frequency, and conical radiation with vertical polarization can be yielded. To reconfigure the operation frequencies, four varactors are symmetrically placed across the annular slot. The simulated results indicate that the resonant frequency of the slot mode can be tuned from 1.62 to 1.17 GHz when the capacitance of the varactors is varied from 0.6 to 1.8 pF; besides, for each capacitance value, the impedance bandwidth of the antenna operating in the monopolar-patch mode can cover the frequencies from 2.4 to 2.5 GHz. Experiments are also carried out to validate the simulated data.

In this paper, the thinning space is constrained to only outer sub-planar array elements instead of fully filled planar array. Since the amplitude weights of the outer elements have small amplitude excitations, they can be optimized to find the least useful elements and remove them without affecting the desired radiation characteristics. The binary genetic algorithm is used to perform such thinning optimization. Simulation results show that roughly the same performance can be achieved when the number of removed elements in the outer sub-array relative to the total number of the planar array elements does not exceed 19%. In addition, to keep the size of the thinned array equal to that of the original filled array, the perimeter elements were excluded from the thinning process. Also, some constraints on the thinned array pattern are imposed to control the null directions toward interfering signals.

A new reflectionless filter with discharging circuit is presented. Under the premise of keeping the original filter unchanged, a discharging circuit is added. The relationship between the passband and stopband of the discharging circuit and the original filter is similar to the duality of the circuit. Without affecting the performance of the original filter, the reflected energy of the original filter is discharged to the ground through discharging circuit, so as to achieve no reflection of the filter, avoiding the interference to the input. Analytical design equations are provided so that the reflectionless filter can be designed. According to this design method, the reflectionless dual-band bandpass filter is designed and fabricated. Simulation and measurement results are in agreement. It has good reflectionless performance. The feasibility of the design method is verified.

In this article, a hybrid inversion algorithm based on an innovative stochastic algorithm, namely, the bat algorithm (BA) is proposed. Electromagnetic inverse scattering problems are ill-posed and are often transformed into optimization problems by defining a suitable cost function. As typical methods to solve optimization problems, stochastic optimization algorithms are more flexible and have better global searching ability than deterministic algorithms. However, they share a common disadvantage: heavy computing load. This directly restricts the application of the algorithms in high-dimensional problems and real-time imaging environments. To solve this issue, diffraction tomography (DT) is introduced to provide a reference for the initialization of the BA. Furthermore, the hybrid method makes full use of the complementary advantages of linear reconstruction algorithms and stochastic optimization algorithms to improve accuracy and efficiency at the same time. Moreover, in order to avoid the algorithm falling into local extrema, a linear attenuation strategy of the pulse emission rate is proposed to enable more bats to perform global search in the early stage of the algorithm. In the numerical experiments for different types of dielectric objects, the reconstruction results of this hybrid BA-based algorithm are compared with those of the DT and the particle swarm optimization (PSO).

A numeric-analytical solution of a problem concerning an impedance vibrator with local asymmetric excitation is derived in the thin-wire approximation. Solution correctness is confirmed by satisfactory agreement of numerical and experimental results from well-known literary sources. Based on the optimization modeling, the design of the impedance antenna characterized by three resonant frequencies intended for mobile communications operating in GSM 900, GSM 1800, and WiMAX ranges is developed. The analysis of basic electrodynamic characteristics of the vibrator antenna has proved the possibility of practical applications of this antenna for phones, portable radio stations, electronic gadgets, and base stations.

Radar data collected on two sides of a horizontally dissipative layered medium are required to invert for the medium parameters. The two-sided reflection and transmission responses are reduced to two single-sided reflection responses. One is the measured dissipative medium response, and the other is the reflection response of the corresponding effectual medium, which has negative dissipation. Marchenko-type equations are solved using these two reflection responses. The obtained focusing functions in the dissipative and effectual media are used to invert for the permittivity and the permeability under the assumption of weak dissipation in reflection. Once these parameters are known, the travel times are used to estimate the layer thicknesses. Finally, the focusing functions are used to estimate the conductivity in each layer. The method does not require any model information and runs as a fully automated process. A numerical example shows that the method works well for a horizontally dissipative layered medium. Statistical analysis for several noise models shows that the method is robust at least up to 40 dB additive and multiplicative white noise.

Simultaneous measurement of temperature and strain using multi-core fiber (MCF) with an in-line cascaded symmetrical ellipsoidal fiber balls structure of Mach-Zehnder interferometer (MZI) is presented. The sensor is fabricated by using an ordinary fusion apparatus. The thermo-coupling effect is realized through Germanium (Ge)-doped central and hexagonal distributed outer cores of MCF. A high-quality transmission spectrum is obtained with a fringe visibility of 12-15 dB and higher extinction ratio. The sensor exhibits superior mechanical strength compared with the fragile structures, such as tapered, etched, misaligned and offset fibers. The temperature sensitivity of 137.6 pm/°C and 68.1 pm/°C in the range of 20-90°C, and the strain sensitivity of -0.42 pm/με and -1.19 pm/με in the range of 0-801 με are obtained, when probe ``L'' is 40 mm and 20 mm, respectively. Simultaneous measurement of temperature and strain can be achieved by solving the coefficient matrix and tracing the wavelength shifts in the interference spectrum. Besides, the sensor has many advantages, such as high sensitivity, easy fabrication, simple structure, being stable and inexpensive, which may find potential applications in the field of optical sensing.

Analog predistortion is an efficient method for improving the linearity of power amplifiers. This paper presents a simple and tunable analog predistortion linearizer with low insertion loss, capable of reducing the non-linearity effects of microwave power amplifiers. The linearizer employs Schottky diodes as a distortion generator and does not require any additional matching circuit. By controlling the DC bias of the diodes, various combinations of characteristics can be obtained; therefore, this structure can be used to match different device behaviors. Experimental validation using a ε_r = 3.38, 20-mil thick Rogers substrate at the center frequency of 2 GHz shows that the fabricated linearizer can provide up to 7.5 dB gain expansion. The fractional bandwidth and insertion loss of the linearizer are 10% and 1.7 dB, respectively. The simulated and measured results are in good agreement with each other. To illustrate an approach for compensating the limited phase characteristics of the presented structure, the design and simulation of a dual-branch linearizer utilizing the reflective Schottky diode predistortion linearizer as a nonlinear unit are also presented.

This paper describes a U-net based Deep Learning (DL) approach in combination with Subspace-Based Variational Born Iterative Method (SVBIM) to provide a solution for quantitative reconstruction of scatterer from the measured scattered field. The proposed technique can be used as an alternative to conventional time consuming and computationally complex iterative methods. This technique comprises of a numerical solver (SVBIM) for generating the initial contrast function and a DL network to reconstruct the scatterer profile from the initial contrast function. Further, the proposed technique is validated against theoretical and experimental results available from the literature. Root Mean Square Error (RMSE) value is used as the metric to measure the accuracy of the reconstructed image. The RMSE values of the proposed method show a significant reduction in the reconstruction error when compared with the recent Back Propagation-Direct Sampling Method (BP-DSM). The proposed method produces an RMSE value of 0.0813 against 0.1070 in the case of simulation (Austria Profile). The error value obtained by validating against the FoamDielExt experimental database in the case of the proposed method is 0.1037 against 0.1631 reported for BP-DSM method.

A miniaturized four-port multiple-input multiple-output (MIMO) antenna for ultra-wideband (UWB) applications is presented. The proposed UWB MIMO antenna has a compact size of 34 × 34 mm². Four antenna elements are placed orthogonally, and the element is connected to the feed line through metal vias in the substrate. These metal vias increase the bandwidth of the high frequency part of the antenna. A T-shaped slit, a rectangular slit, and a triangular chamfer are etched on the ground between two adjacent antenna elements. The working bandwidth of the antenna is 2.5-11.6 GHz, covering the entire UWB application band. The isolation between antenna elements is more than 18 dB within the operating bandwidth. Details of the design methodology and results are presented and discussed. Envelope correlation coefficient is computed, and it is within the acceptable limit, which validates the design concept for building a compact MIMO antenna system with good performance.

A dual-band subharmonic mixer that employs both the second and fourth harmonics of a local oscillator signal in the mixing process is demonstrated for WIFI application. The design results in a simple and cost-effective mixer as it requires only one local oscillator (LO). A quarter-wave stepped impedance stub has been used to suppress both bands of radio frequency (RF) signal. The proposed dual-band subharmonic mixer is designed for two RF bands with the center frequencies at 2.45 GHz and 5 GHz using a single LO frequency at 1.3 GHz. For mixing purpose, the second and fourth harmonics of LO are utilized. Experimental measurements show high port-to-port isolation and achieve minimum conversion losses of 6.87 dB and 10.0 dB at 2.59 GHz and 5 GHz, respectively. The 3-dB RF bandwidth is 2.3 to 2.95 GHz for the second harmonic and 4.8 to 5.5 GHz for the fourth harmonic of LO signal. The input P1-dB compression points for two modes of the mixer are -9 dBm and -5 dBm, respectively. The RF-to-IF isolations are more than 18 dB (maximum 36 dB) and 20 dB (maximum 33 dB), over both the RF bands.

The design of a compact Pi-shape microstrip antenna for dual-polarized wideband response is proposed. The Pi-shape geometry is realized by modifying a compact C-shape patch. The two stubs placed on the Pi-shape patch edge, optimize the spacing in between the higher order TM₂₀ and TM₃₀ modes with respect to the fundamental TM₁₀ mode which yield a bandwidth of more than 430 MHz (>35%). On an air suspended substrate, antenna exhibits broadside gain of more than 5 dBi over the impedance bandwidth. The orthogonal surface current variations across the TM₁₀, TM₂₀ and TM₃₀ modes realize polarization agility satisfying the requirements of GSM900/navigation satellite applications/ISM900. With respect to the band start frequency, the proposed configuration offers 11% reduction as compared with the equivalent rectangular microstrip antenna. Further, by defining the resonant length at each of the Pi-shape patch modes, the formulation for their resonant frequency is proposed. The Pi-shape antennas redesigned using them at the given fundamental mode frequency yield similar dual polarized wideband responses offering bandwidth of > 35%.

This paper investigates the effect of an external plane wave on a Multi-conductor transmission line (MTL) located above a multilayer soil directly in the time domain. An improved finite-difference time-domain (FDTD) method is used, in conjunction with the Vector Fitting (VF), to obtain the recursion relations of voltages and currents along the line by discretizing the equations in time and one-dimensional space. The source terms of the coupling equations are efficiently obtained in the time domain based on the Gaver-Stehfest algorithm. An equivalent model is also established in this work, where the geometry with three conductors is reduced to two conductors. Finally, some examples are presented to illustrate the effect of the soil and the plane wave on the transient.

In this paper, a novel mechanical-variable-flux flux-intensifying interior permanent magnet (MVF-FI-IPM) motor is proposed, which employs a mechanical flux-adjusting device and owns the characteristic of L_d>L_q. The magnetic poles can be rotated by the mechanical device to vary the leakage flux and adjust the angle of magnetization direction relative to the d-axis. The characteristic of L_d>L_q is achieved through the adoption of surface flux barriers. The topology structure and operation principle of the machine are introduced. Then, the operation of the mechanical flux-adjusting device is analyzed by virtual prototype technology. Based on the two-dimensional finite element method (FEM), the electromagnetic characteristics of the proposed motor and FI-IPM motor are compared. Finally, the results show the proposed motor with a better flux-weakening capability and a lower risk of irreversible demagnetization than that of the FI-IPM motor.