This paper introduces an innovative circuit theory of analog voltage compressor (AVC) and decompressor (AVD). This electronic function can also be assumed as an analog voltage converter. Analytically, it acts as power function synthesizer topology designed with an analog nonlinear circuit. The AVC/AVD topologies are based on an operational amplifier associated with resistor and non-linear diode components. Given the positive parameter a>0, the main x-y characteristic of the AVC/AVD is formulated by y=x^a for the input and output x and y, respectively. The synthesis formulas allowing to determine the AVC/AVD parameters in function of a are established. To validate the original AVC/AVD concept, static and dynamic simulations and experimentations with a proof-of-concept circuit using operational amplifier UA741 are carried out. As expected, well correlated x^1/2-AVC and x²-AVD characteristics are realized with the static testing for the voltage range varied from 0 to 9-V and 0 to 3-V for AVC and AVD circuits, respectively. The simulation and experimentation of dynamic test results are in good agreement for the sine wave voltages with frequency varied from DC to 1-kHz. The simulated and experimental results confirm the relevance of the developed compressor/decompressor analog circuit. The AVC/AVD functions for instrumentation system applications can be potentially applied to the amplitude matching especially for digital systems.

Recently we derived generalized sheet transition conditions (GSTCs) for electromagnetic fields at the surface of a metascreen (a metasurface with a ``fishnet'' structure, i.e., a periodic array of arbitrary spaced apertures in a relatively impenetrable surface). The parameters in these GSTCs are interpreted as effective surface susceptibilities and surface porosities, which themselves are related to the geometry of the apertures that constitute the metascreen. In this paper, we use these GSTCs to derive the plane-wave reflection (R) and transmission (T) coefficients of a symmetric metascreen, expressed in terms of these surface parameters. From these equations, we develop a retrieval approach for determining the uniquely defined effective surface susceptibilities and surface porosities that characterize the metascreen from measured or simulated data for the R and T coefficients. We present the retrieved surface parameters for metascreens composed of five different types of apertures (circular holes, square holes, crosses, slots, and a square aperture filled with a high-contrast dielectric). The last example exhibits interesting resonances at frequencies where no resonances exist when the aperture is not filled, which opens up the possibility of designing metasurfaces with unique filtering properties. The retrieved surface parameters are validated by comparing them to other approaches.

The authors propose a new method based on spatial cumulants for estimating the parameters of multiple near-field and far-field sources. The Toeplitz property used in some studies is not applicable to fourth-order statisticsto separate sources components. Therefore, in this paper, a method is proposed to computeoutput cumulants of specified sensors in special arrangements, by which the components of the near-field and the far-field sources are effectively separated using differencing. The angle and range estimations, as well as the classification of the sources, are obtained based on the data from two spatial cumulant matrices. One of them contains the angle information of all sources, and the other only contains the information of the near-field sources. The parameters extraction algorithm is based on the ESPRIT technique; therefore, the proposed method does not require any spectral search. This leads to a significant reduction in computational complexity. Unlike some approaches, the proposed method does not suffer from array aperture loss. Also, the parameters pairing procedure is done automatically. Analysis and simulation results confirm the good performance of the proposed method in terms of computational complexity, estimation accuracy, correct classification of signals, and aperture loss.

Multipacting is electron discharge that occur in components operating in RF high-power electromagnetic fields. In this paper, we will study on a new coaxial structure with ceramic window. A similar structure is utilized in many high power devices for power transfer. Due to the multipactor effect, it will generate huge heat and cause damage to the window, ultimately affect the performance of microwave devices. In order to suppress of the surface multipactor effect and improve the transmitting power, the application of an external DC bias is analyzed and simulated. A Monte Carlo algorithm is used to track the secondary electron trajectories and study the multipactor scenario on the surface of a ceramic window in a coaxial line by using 2-D particles distribution code. Since secondary electron multiplication needs to meet specific resonance conditions, an appropriate DC bias will generate a compensating trajectory and collision, which can suppress the secondary electron avalanche. The optimal value of this external bias voltage that will avoid the multipactor phenomenon in the coaxial line will be calculated by simulation in MATLAB.

In this paper, the reconstruction problem of inaccessible objects buried into a three-part space with locally rough interfaces is solved by Distorted Born Iterative Method (DBIM). DBIM requires the calculation of the background electric field and Green's function in every iteration step via the solution of the direct scattering problem. Here, they are calculated numerically by using the buried object approach (BOA) which is very useful in the solutions of the problems including stratified media with locally rough interfaces. Various numerical applications have been performed to demonstrate the applicability and efficiency of the method. The method was found to be very successful in reconstructing moderate contrast objects when they were buried in the middle space. In this case, the method works effectively even if the buried objects and interface roughnesses have complex geometric structures. Moreover, the multiplicity of buried objects has no negative effect on the reconstruction results. Nevertheless, the results of reconstruction deteriorate when objects are buried in the bottom space. However, the accuracies of them are still on an acceptable level in this situation.

The ideal ultra-wideband (UWB) antenna feed for lens and reflector systems radiates a uniform and customizable beamwidth vs. frequency. Here, a new antenna concept for radiating frequency-independent Gaussian beams with arbitrary bandwidths and beamwidths is reported. It is analytically shown how to resistively load a transmission line network to maintain a Gaussian amplitude taper across an antenna array aperture. In contrast to many other feed antennas, the radiation properties here can be tailored without time-consuming full wave optimizations. The radiated beamwidth, bandwidth, antenna size, radiation efficiency, and gain can all be quickly estimated using the derived closed-form expressions. An example, 16x16 Vivaldi element array is fed with a network of resistively loaded microstrip lines. The simulated array radiates a Gaussian beam with 10 dB full beamwidth of 35°±5° and directivity of 20 dB±1.5 dB over 6.5 GHz-19 GHz (3:1 bandwidth ratio). However, the radiation efficiency is inherently low due to the large loss associated with generating the Gaussian amplitude taper at all frequencies. The example array has a simulated radiation efficiency of 1% at the higher operating frequencies. The array was fabricated and measured. The measured beamwidths agree well with simulation to validate the reported theory. This architecture is a particularly attractive option for feed antennas that require customizable directivities, and can tolerate low radiation efficiencies such as test and measurement.

A band-pass frequency selective surface (FSS) structure using capacitance layers is proposed to improve the performance of angular stability. It consists of band-pass FSSs, supporting dielectrics, and capacitance layers out of band-pass FSS. The supporting dielectrics and capacitance layers work as a transmission line and capacitance impedance matcher. Through the impedance matcher, the bandwidth is stabilized, and insertion loss at passband is reduced from -0.76 dB to -0.39 dB for incident angles up to 60°. The equivalent circuit of the proposed structure is presented, and the Smith chart is given to explain the mechanism of the capacitance layers. Finally, a prototype is manufactured and measured. A relatively good agreement is obtained between simulations and measurements. Therefore, the proposed structure can be an effective solution to improve the angular stability performance of band-pass FSS design.

In this work, a compact planar dual-square ring (DSR) microstrip patch antenna is investigated to acquire dual-band resonance with dual-mode excitation for Wi-Fi/WLAN and 5G-NR based wireless applications. This dual-square ring geometry is employed on single layer dielectric, excited through EM coupling by using a quadrilateral feed patch, which offers massive flexibility in impedance matching for dual-band resonance with minimum coupling effects in common excitation and ground plane. This planar DSR structure shows the resonance at 2.4 GHz and 3.7 GHz frequency bands with bandwidths greater than 100 MHz and 200 MHz, respectively and a maximum gain response of 4.3 dBi with VSWR of <<2. Here the simulation results are verified through experimental results of the fabricated antenna. This proposed antenna design can be configured for Wi-Fi/WLAN application at 2.4 GHz in lower-order resonance mode (TM₀₁) and for 5G-NR application by utilizing the fringing benefits of higher-order mode (TM₁₀) at 3.7 GHz.

The literature lacks detailed information about the electrical properties of the plastic filaments used in 3D printing. This opens the way for research on characterizing the types of materials used in these filaments. In this work, a method for the extraction of the dielectric constant and loss tangent of materials is described. This method, which is suitable for characterizing any dielectric material, is then used to characterize 3D-printed samples based on different filament materials and infill densities over a very wide frequency range [0.02-10 GHz]. The selected materials are Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS) and a semi-flex filament that combines the two important features of flexibility and endurance. These three types are the most commonly used in 3D printing. The two$-$line technique is applied to extract the complex permittivity of the material under test (MUT) from the propagation constant. This method employs the uncalibrated scattering parameters with different types of transmission line for any characteristic impedance. A rectangular coaxial transmission-line fixture has been used to validate the theoretical work through simulations and measurements involving the 3D filament samples.

This paper presents a novel engineered artificial dielectric superstrate for improving the radiation characteristics of a CPW-fed planar antenna. Even though the permittivity of the material used for the superstrate is only 4.4, it attains an effective permittivity of more than 18 because of the periodic pattern printed on it. Due to the high value of effective permittivity an improvement in radiation pattern, impedance matching and gain of the antenna are obtained. From the measured results an impedance bandwidth of 374 MHz from 2.453 GHz to 2.827 GHz is observed for the antenna loaded with superstrate. The periodic pattern is fabricated on a substrate of thickness 1.6 mm, and it occupies an area of 56.45×42.48 mm².

This paper presents the design, manufacture, and experimental validation of a novel 3-D pixel antenna with volume-filling characteristics, and the design is based on our Method of Moments (MoM) solver that is efficiently coupled with a global/local optimizer for tailoring the antenna shape and concurrently selecting the location of the feeding port and shorting straps. The design, aimed at operating in the ISM bands of 2.45 GHz and 5.8 GHz, has dimensions under one-tenth of wavelength at the lowest frequency of operation. The optimization results are cross-validated using a commercial full-wave simulator, with a deviation of the reflection coefficient across the operating bands within 3%, showing also a high antenna efficiency of 99.6% and a gain of 1.06 and 4.53 dBi at the matching frequencies, with radiation patterns predominantly oriented towards the top hemisphere. Tolerance and parameter sensitivity studies were also performed. A scaled-up prototype of the antenna was built at a very low cost using standard additive manufacturing techniques, featuring a very good agreement between simulation and measurements, which proves the feasibility of this new kind of complex shape antennas in further applications where compact internal antennas are required.

A novel gain-enhanced microstrip antenna (MSA) with metamaterial planar lens for long-range radio frequency identification (RFID) applications for the 902-928 MHz UHF frequency band is proposed in this paper. The antenna is a combination of a new general-purpose circularly polarized MSA and a novel effective negative refractive index metamaterial (NIM) slab of 25 unit cells, arranged in a 5 x 5 layout, working as a planar lens for gain enhancement. The general-purpose MSA has an impedance frequency band of 828-1015 MHz, a maximum gain of 8.43 dBi at 915 MHz, an axial ratio frequency band of 896-931 MHz and excellent performance for short and medium range RFID applications. The new infinite periodicity NIM slab has a negative refractive band of 886-1326 MHz, a negative electric permittivity band of 888-3406 MHz, and a negative magnetic permeability of band 885-1065 MHz. Together, the general-purpose MSA and the NIM planar lens results in the low-cost gain-enhanced antenna for long-range RFID applications, with an 843-993 MHz impedance frequency band and a maximum broadside gain enhancement of 48.27%, resulting in a 12.5 dBi gain at 902 MHz. Finally, the parametric studies conducted during the design process of the gain-enhanced antenna with metamaterial planar lens are presented.

In order to energize the biomedical implantable electronic devices wirelessly for in vivo health monitoring of patients in an isolated, outdoor and inaccessible environment, an alternate driving energy source is highly desirable. In pertinent to this, a photovoltaic driven wireless energizing system has been explored. The system is designed to convert solar energy to a high frequency energy source so as to facilitate energy transfer through resonant inductive link to the automated bio-medical sensing system allied with the receiver unit. The received power is observed to be 286 mW for the coil separation gap of 3 cm and load value of 40 Ω at the resonant frequency of 772.3 kHz. The automated biomedical smart sensor is competent to acquire the body parameter and transmit the consequent telemetry data from the body to the data recording segment. The real-time body temperature parameter of different living beings has been experimented, and to ensure the accuracy of the developed system, the observed parameter has been matched with a calibrated system. The proposed scheme can be suitable for monitoring wirelessly other in vivo health parameters such as blood pressure, bladder pressure, and physiological signals of the patients.

A wireless power transfer system based on magnetic resonant coupling (MRC) is preferred in many applications as it provides good balance between power transfer efficiency and physical separation distance. However, wireless power transfer to multiple loads through magnetic resonance coupling demands time due to the noteworthy advancement in consumer portable electronic devices. However, operating multiple loads corresponding to their optimum power level is a major concern which mostly depends on the position of the receiving coils with respect to the transmitting coil. This article presents an experimental investigation to find the best suited position of the multiple receiving coils corresponding to a spirally configured transmitting coil for powering multiple loads at their optimal power level. Through this technique multiple electronic devices can be powered up not only in one direction but also in both directions with their optimal power level. The findings will greatly assist the design of a resonant wireless power transfer system for powering multiple loads.

Gauss integral theorems for electric and magnetic fields, Faraday's law of electromagnetic induction, magnetic field circulation theorem, theorems on the flux and circulation of vector potential, which are valid in curved spacetime, are presented in a covariant form. Covariant formulas for magnetic and electric fluxes, for electromotive force and circulation of the vector potential are provided. In particular, the electromotive force is expressed by a line integral over a closed curve, while in the integral, in addition to the vortex electric field strength, a determinant of the metric tensor also appears. Similarly, the magnetic flux is expressed by a surface integral from the product of magnetic field induction by the determinant of the metric tensor. A new physical quantity is introduced - the integral scalar potential, the rate of change of which over time determines the flux of vector potential through a closed surface. It is shown that the commonly used four-dimensional Kelvin-Stokes theorem does not allow one to deduce fully the integral laws of the electromagnetic field and in the covariant notation requires the addition of determinant of the metric tensor, besides the fact that the validity of the Kelvin-Stokes theorem is limited to the cases when determinant of metric tensor and the contour area are independent from time. This disadvantage is not present in the approach that uses the divergence theorem and equation for the dual electromagnetic field tensor. The problem of interpreting the law of electromagnetic induction and magnetic field circulation theorem cannot be solved on the basis of the Lorentz force in the absence of charges, and therefore requires a more general approach, when transformation of the field components from the reference frame at rest into the moving reference frame is taken into account. A new effect is predicted, according to which the circulation of magnetic field can appear even in the absence of electric current and with a constant electric field through the contour, if the area of this contour would change. By analogy with electromagnetic induction, for the magnetic field circulation to appear it is important that electric field flux passing through the area of the contour would change over time.

An improved Taguchi's method (ITM) is proposed in this paper. The dynamic reduced rate function linked with the contributions of each parameter is used to increase the convergence speed. An extra procedure is added in the ITM to determine whether the experiment results in orthogonal array meet the termination criterion which is neglected in the traditional Taguchi's method (TM). Three experiments, including the syntheses of linear arrays and the designs of an E-shaped antenna and an ultra-wide band monopole, are conducted to investigate the performance of the proposed method, and the results are compared with those of traditional TM and other meta-heuristic methods. The results show that the same or even better results are obtained by the improved TM with fast convergence speed.

In this paper, a design method that employs simulated annealing (SA) algorithm to create stub structure of a dual-layer microstrip patch antenna for circular polarization is presented. Firstly, based on established controls of SA algorithm, a series of stub structures have been created automatically on the stacked parasitic element - (Split Ring Resonator) SRR of antenna. The desired stub structure is chosen according to the generation of orthogonal modes that produce circular polarization through the electromagnetic coupling to the driven patch with an SRR-shaped slot. Then, a dual-layer microstrip patch antenna with a Z-shaped stub and left-hand circularly polarized (LHCP) characteristic is obtained by employing the assisted design. The designed antenna is simulated, optimized, fabricated, and measured. The results show that the microstrip patch antenna with Z-shaped stub has a simulated minimum axial ratio of 1.64 dB at 2.4 GHz, and the measured peak gain can be up to 5.87 dBi.

Due to the exponential growth of the number and scale of wind farms, wind turbine clutter has become the main factor that limits the detection performance of weather radar systems. As a consequence of the rapid rotation of wind turbine blades, conventional ground clutter filters are ineffective at removing wind turbine clutter (WTC). An improved range-Doppler joint interpolation for WTC suppression is proposed in this paper. The proposed algorithm firstly exploits the frequency-domain transformation technique to improve the signal-to-noise ratio (SNR), so that the interpolation algorithm can recover the weather signal in the case of low SNR. Then, the weather signals recovered by one-dimensional interpolation in range domain and Doppler domain are calculated, respectively, and the two-dimensional joint interpolation is performed based on two-dimensional weighted coefficients calculated via a least mean squares criterion. Theoretical analysis and simulation results show that the proposed algorithm effectively suppresses the wind turbine clutter and significantly reduces the bias in radial velocity estimation caused by WTC contamination in low SNR environments.

A circularly polarized (CP) half-mode substrate integrated waveguide (HMSIW) cavity-backed antenna is developed by a novel design method in this paper. The single-fed antenna contains two coupled HMSIW cavities with orthogonal polarizations. Its design method evolves from filter synthesis procedure, which takes the advantage of equivalent circuit model to accelerate the optimization. The antenna radiates high-purity right handed CP waves with measured axial ratio (AR) of 0.33 dB at 3.55 GHz, while its gain and AR bandwidth are comparable to other CP SIW antennas. With the robust design method, the proposed antenna is competitive in practical applications.

We propose a camouflage device that can greatly reduce scattering in the microwave frequency using only uniform copper plates with no internal structuring (no metamaterials). The camouflage device is designed by optical surface transformation (OST), which is derived from transformation optics but much simpler than transformation optics. The key of our design is to choose suitable arrangement and lengths of these copper plates that satisfy Fabry-Perot condition. The proposed camouflage device can work when the detecting wave comes from a wide-angle range (not only works for some discrete angles). The proposed method will give a new and simple way to design and realize camouflage device.