In order to satisfy the requirements of terahertz time-domain spectrum system under specific circumstances, an off-axis focus reflective polarization converter in terahertz band is proposed. By combining the principle of phase compensation and phase gradient metasurface, a reflective array containing units is designed. The phase distribution along the metasurface is calculated through the principle of optical path reversibility. Geometric rotation and resonant frequency modulation constitute the phase variation of the unit, which can be superimposed on each other without interference. Compared with the conventional reflective polarization converter in terahertz band, the proposed one could deflect the normally incident terahertz wave while providing larger energy at the focus. The simulation results show that the proposed polarization converter has good performance in both polarization conversion and electromagnetic focus, which has significant practical application in numerous situations.

A simple and novel polarization-dependent phase gradient metasurface (PGMS) is proposed to synthesize a flat-top radiation pattern by dividing the metasurface (MTS) into multiple regions. Each sub-region generates a beam in a particular direction and multiple beams with different directions form a flat-top pattern in the far-field. A flat-top pattern in a single and 3D plane are realized by dividing the MTS into two and four regions, respectively. The proposed MTS consists of a multi-layered elliptical geometry encircled by a square loop. The elliptical shape of the unitcell offers polarization dependent behavior and produces dual-band characteristics for different incident wave polarizations at 10 and 12 GHz. Two microstrip patch antennas operating at 10 GHz and 12 GHz are placed at the focal point of the MTS. The simulated flat-top beamwidths in a single plane with a 1 dB ripple are 36˚ and 34˚ at 10 and 11.8 GHz respectively. Similarly, in 3D space, the beamwidths are 33˚ and 31˚ at 10 and 11.8 GHz, respectively. Both simulated and measured results are presented for 3D flat-top patterns.

The tremendous proliferation of mobile smartphone handsets and their usage worldwide makes human life comfortable, while the radiation hazards associated with them are alarming, especially among children. There is a necessity to minimize the Electro Magnetic Field (EMF) radiation levels. For the evaluation of Radio Frequency (RF) radiation from the mobile phone, one of the dosimetric parameters used is the Specific Absorption Rate (SAR). The RF radiation can be mitigated by incorporating a barrier or a shield of suitable material in the mobile handset design. In the proposed work, the analysis of SAR evaluation absorbed by the human head is determined with the performance of the shielding material called Shielding Effectiveness (SE) using Transmission Line Method (TLM) mathematically. The proposed shielding materials are composed of flexible and transparent thin films. Flexible and transparent thin shielding materials are advantageous over the other shielding materials in reduced size, less weight, non-corrosiveness, and easy processing. These materials include highly conductive Silver film, Silver Nanowire(AgNW) doped with PDDA (poly(diallyldimethyl-ammonium chloride)) polymer single shields, and a laminated shield comprising AgNW/PDDA with PEDOT:PSS (poly(3,4-ethylenedioxythiophene)poly-styrene sulfonate) polymer as lamination. The SARs of planar multi-layered human head models for different ages are estimated at various mobile frequencies with these shields. Under four-layered head models at 6 GHz, adult and child heads absorb 0.0006 W/Kg and 0.000024 W/Kg of RF radiation using pristine Silver film as a single shield. Using a single shield of Ag nanowire and PDDA, the adult and child heads absorb SARs of 0.00058 W/Kg and 0.000023 W/Kg, respectively. With the laminated shield of AgNW/PDDA and PEDOT:PSS as coating material, the same models are exposed to minimal amounts of 0.00054 W/Kg and 0.000012 W/Kg of SAR. At 6 GHz frequency, under seven-layered head models, an adult and a child's head absorb 0.000047 W/Kg and 0.000002 W/Kg of power, respectively, using Ag film. With AgNW/PDDA shield, the adult and child heads absorb a SAR of 0.000046 W/Kg and 0.0000019 W/Kg, respectively. The SARs of 0.000043 W/Kg and a negligible value of 0.0000018 W/Kg are absorbed by adult and child heads individually with the help of AgNW/PDDA/PEDOT:PSS laminated shield. The results exhibit a significant amount of reduction in Specific Absorption Rate with transparent shielding materials compared to SAR absorbed by the head without any shield. This maximum RF exposure rate reduction from mobile phones with the Ag Nanowire/PDDA/PEDOT:PSS laminated shield is achieved for a seven-layered child head model.

In this paper, a coupled-line based dual-band branch-line coupler with port-extensions is presented. The configuration of the coupler consists of a single coupled-lines section, two transmission lines, and an easy to analyze L-section impedance matching network at all four ports of the coupler. A detailed theoretical analysis is carried out to obtain the closed-form design equations to determine the design parameters of the coupling structure. It is observed that the proposed dual-band coupler can support wide band-ratio and arbitrary power division. To validate the proposed design concept, a prototype working at 0.9 GHz and 1.8 GHz is fabricated on a 60 mil Rogers 4003C substrate exhibiting excellent match between the simulated and measured results.

For the first time, an extended hemispherical integrated lens antenna on a low-cost substrate, FR408HR, is presented. The antenna is designed for industrial and automotive radar sensor applications operating in the 24 GHz ISM band. The proposed antenna has a gain of 15.2 dBi, low sidelobes, and half-power beamwidth of 16 degrees. To reduce the cost, we used low loss materials; Teflon for the lens and low-cost FR408HR as a patch antenna substrate. The size of the reported 24 GHz antenna is small. The diameter of the base of the lens is 38 mm (3 times of free space wavelength), and its height is 43.5 mm (with an extended height of 24.5 mm). Simulated results match well with measurements.

This paper investigates the original circuit theory on stopband (SB) negative group delay (NGD) passive topology. The basic specifications of SB-NGD function are defined by considering the voltage transfer function (VTF) of the passive circuit. An original design method and experimentation tests of SB-NGD circuit are developed. The innovative theoretical analysis is elaborated from both magnitude and GD analytical expression of the VTF model from the resonant LC-series network passive topology. The mathematical existence condition of SB-NGD aspect is analytically explored in function of R, L, and C component parameters. The formulations of the basic equations enabling the calculation of the lumped components of the SB-NGD passive circuit in function of the desired specifications as NGD cut-off frequencies, NGD value and attenuation are established. To confirm the effectiveness of the original SB-NGD circuit theory, a proof-of-concept (POC) of SB-NGD circuit board is designed, simulated, fabricated and experimented. As expected, despite the equivalent series resistor (ESR) effect of the inductor element, the theoretical modelling, simulation and measurement results are in good agreement. The SB-NGD behavior is confirmed with lower and upper cut-off frequencies, 0.7 kHz and 1.35 kHz, respectively. Furthermore, the corresponding NGD minimal values are -33 µs and -11 µs, respectively.

In this paper, a patch antenna (PA) and its self-complementary structure, slot antenna (SA) are proposed and designed for directly matching the impedance of a rectifierat 2.45 GHz resonance frequency. The structures of these antennas comprise three sections, meandered-line, spiral, and a double-folded geometries, which make their geometrical parameters to be varied in easy manner according to design equations. In order to enhance both the desired level of a complex reflection coefficient of antenna at given resonance frequencies and the specified lower and higher frequencies constituting the impedance frequency bands, a new fitness function is presented. This fitness function is applied in designing broadband or multiband antennas having approximately perfect conjugate impedance matching with the impedance of a rectifier suitably used for RF Energy Harvesting (RF EH) application. An optimization design methodology based on two programs operating in synchronous manner, the particle swarm optimization (PSO) implemented in MATLAB simulation tool anda CST MWS Electromagnetic (EM) solver, is applied to the designed PA as an illustrative example. The simulation results reveal that our design methodology is helpful to obtain an optimized PA (OPA) having good impedance matching at the desired resonance frequency along with appropriate band. Measured result of the fabricated prototype is in good agreement with the simulated ones. Moreover, acceptable features such as small size, omnidirectional radiation, and broadband operation satisfy the (2.4-2.5 GHz) WLAN band, which strongly makesthe OPA a good candidate for RF EH applications.

A novel algorithm is developed to realize the optimal synthesis of a sparse nonuniform-amplitude nonuniform-distribution planar array (SNANDPA) in microwave power transmission (MPT) systems. The dual compression factor particle swarm optimization (DCFPSO) algorithm and the subarray partition technique are adopted to realize the optimal synthesis of SNANDPA. The DCFPSO algorithm first optimizes beam collection efficiency (BCE) and side-lobe level outside the receiving region (CSL) of SNANDPA which ensure efficient energy transmission of an MPT system and suppress the influence of electromagnetic wave radiated by antenna array on the environment. The subarray partition technique then simplifies the feed network to minimize the system cost. SNANDPA parameters including transmitting aperture, receiving aperture, BCE, CSL, power pattern, element weight, and element distribution, can be obtained efficiently via the proposed method. Representative numerical cases under the different numbers of subarray and elements conditions are analyzed and compared with those of other two traditional MPT array models. Experimental results show that, when the transmitting aperture is 4.5λ×4.5λ and the square receiving region u₀=v₀=0.2, BCE and CSL are 94.96% and -17.09 dB, respectively, and only 64 elements and 8 amplifiers are required. We conclude that the proposed model can be used to create an efficient and low-cost MPT system.

A compact asymmetrically fed U shape slot antenna for 5G smartphone application is presented. The antenna consists of two major components that are responsible for generating broadband circular polarization (CP) (i) the microstrip patch feed structure employed at the top plane and (ii) U shape slot mechanism employed at the ground plane. The U shape ground radiator is approximately half wavelength slot mode which is responsible for lower band CP operation. The planar monopole structure is formed by the microstrip line feed. This planar monopole is a quarter wavelength mode used for achieving higher band CP operation. By combining these two CP modes generated by U shape slot and port-line structure, a broadband CP antenna is designed. The circular polarization is achieved with lesser complexity in the design structure. To get the practical validation of the simulated design, it is then fabricated and tested for measured results without using power divider. The measured axial ratio bandwidth covers from 2.00 GHz to 6.50 GHz (4.50 GHz) and the -10 dB Impedance Bandwidth (IBW) covers from 1.66 GHz to 8.10 GHz (6.44 GHz) respectively. The isolation between two asymmetric port-lines is greater than 14.5 dB within 3-dB axial ratio bandwidth.

In this paper, a numerical electromagnetic model of a low-cost detection modality for red palm weevil pests in palm trees using resonant-based microwave sensors is presented. The developed sensor is based on the complementary split-ring resonator concept. The complementary resonator is easily modeled using printed circuit board technology, where the transmission response from two ports at ends of 50 Ω-matched transmission line is recorded. The microwave sensor has been designed to work at the 2.45 GHz ISM-band and is placed underneath a finite size 3D model of a palm tree trunk infested with the red palm weevil pest. For comparison purposes, the numerical simulation results are compared against a reference case of a healthy palm trunk. The results show the capability of the proposed numerical electromagnetic model in detecting presence of the red palm weevil in palm trees.

An equivalent source model based on neural network is proposed to rapidly estimate the magnetic radiation characteristics of linear synchronous motor (LSM) in electromagnetic suspension (EMS) maglev system. The equivalent source is composed of electric dipoles and a closed three-dimensional (3-D) surface, and is developed in terms of source reconstruction method. A few sampling data of magnetic field simulation results serve as the input information to determine the unknown current distribution on equivalent source model. To solve the inverse radiation problem and characterize the whole radiation pattern with high efficiency, the current distribution signature of equivalent model is fitted into artificial neural network models. Separate neural network models are fitted under different phases of winding excitation, which enables the low-frequency magnetic field estimation under both 3-phase balanced operation and unbalanced operation. The equivalent source model is extended to estimate LSM radiation in multi-source environment, and the comparison with numerical simulation verifies its accuracy and efficiency.

In this paper, a novel behavioral model for the receiver front-end is presented. This model allows the accurate prediction of the nonlinear effects of the receiver front-end including the in-band distortion, intermodulation and harmonic generation. The behavioral model is a block-oriented model that consists of three blocks, the frequency conversion block, nonlinear block， and memory linear block. The nonlinear block and memory linear block are represented by the polynomials in time domain respectively, which can characterize the high-order nonlinearities and the strong memory effects by the appropriate adjustment of the polynomial order. An original model parameter identification procedure that can efficiently estimate the model parameters by using the specific input-output data is also proposed. Moreover, the presented behavioral model and identification procedure are assessed by the experiment with the excitation of single-tone signals, multitone signals and WCDMA signals, respectively. The comparison between the measurement and model simulation suggests that the behavioral model has good accuracy of the prediction of the nonlinear effects of the receiver front-end.

This paper proposes an absorption-transmission-absorption (A-T-A) type frequency-selective rasorber (FSR) with high selectivity that is loaded above a polarization conversion structure (PCS) and applied to a circular polarization (CP) slot antenna array for ultra-wideband radar cross section (RCS) reduction. Outside the operational frequency band (out-of-band) of the antenna, the energy of the incident electromagnetic (EM) wave is directly absorbed by the FSR, whereas from within the operational frequency band (in-band) of the antenna, the incident EM wave penetrates the FSR and irradiates it on the PCS placed on the lower layer of the FSR structure, which meets the phase cancellation condition and is diffused at the same time, thereby realizing the in-band RCS reduction. Due to the lower insertion loss in the passband, higher quality factor (Q value) in the transmission band, and wider absorption band, the proposed FSR can minimize the gain loss (only 0.2 dB) of the CP slot antenna array and widen the RCS reduction bandwidth to 135.5% (5-26 GHz). In addition, due to the central symmetry of the FSR and PCS structures, the CP slot antenna array has monostatic RCS reduction performance for both horizontally polarized (HP) and vertically polarized (VP) incoming waves.

Magnetoacoustic concentration tomograghy of magnetic nanoparticles (MNPs) with magnetic induction (MACT-MI) is a multi-physics field imaging method based on the coupling effect of magnetic field and acoustic field. In order to generate a gradient magnetic field with higher uniformity by using lower current excitation, this paper takes the magnetic field gradient of 0.3 T/m as the design objective and utilizes the Beetle Antennae Search Algorithm to optimize the parameters of the excitation current of multiple coils. The uniformity of gradient magnetic field generated by four typical six-coils structures with different radii and distances was compared with that generated by the Maxwell coil, and then the optimal structure of six-coils was determined. By using the finite element method, the physical process of MACT-MI was numerically solved according to the optimization results, and we obtained the one-dimensional and two-dimensional distribution images of magnetic force and sound pressure. The results show that compared with the Maxwell coil, the multi-coil structure can effectively reduce the current excitation and provide a higher uniform gradient magnetic field, which makes the magnetic force of MNPs more uniform and promotes the complete reconstruction of the sound source. These research results can provide research ideas for the optimization of MACT-MI system and lay a foundation for subsequent experiments and even clinical practice.

Shale oil and gas are unconventional oil and gas resources that can be used as alternative energy sources in the future. Shale reservoirs are the new growth point for the exploitation of oil and gas and development of China's oil and gas industry. The heterogeneity of the shale stratum determines the complexity of its mining. Accurate identification and detection of its oil-bearing characteristics are principal tasks in the oil shale deposit evaluation and economically exploitable interval division. Dielectric logging cannot rely on traditional resistivity logging curves, and it is not affected by the formation water salinity, which can provide the formation water porosity. Combined with other types of logging, it can effectively evaluate the formation oil saturation. In this study, we applied a new type of high-frequency dielectric logging tool in the production of shale oil, developed by the 22nd Institute of China Electronics Technology Group Corporation, based on different dielectric constants of oil, rock matrix, and water. We first introduced the principle of dielectric logging and the major advantages of the dielectric logging tool, and further proposed a new complex refractive index model with clay correction and explained the processing methods, which improved the accuracy of calculating the formation water saturation. Furthermore, the developed technology was applied and evaluated in the Songliao Basin.

A simple design configuration of a broadband polarization-insensitive double-layered microwave absorber is presented here. The proposed absorber is designed using indium tin oxide (ITO) based on thin resistive film. The novelty of structure is to achieve large absorption bandwidth with more than 99% absorption. The proposed structure is modeled for 20 dB absorption bandwidth at normal incidence from 6.3 GHz to 14.2 GHz spanning over C-band, X-band, and Ku-band. Under oblique incidence the proposed structure is stable up to 60˚ for TE polarization and 45˚ for TM polarization. To understand the operating principle of absorption of proposed structure, an equivalent circuit is derived, and surface current distribution is also studied. A fabricated sample is measured, which validates our simulation.

As the explicit finite-difference time-domain (FDTD) method is restricted by the well-known Courant-Friedruchs-Lewy (CFL) stability condition and is inefficient for solving numerical tasks with fine structures, various implicit methods have been proposed to tackle the problem, while many of them adopt time-splitting schemes that generally need at least two sub-steps to finish update at a full time step, and the strategies used seem to be an unnatural habit of computation compared with the most widely-used one-step methods. The procedure of splitting time step also reduces computational efficiency and makes implementation of these algorithms complex. In the present paper, two novel one-step absolutely stable FDTD methods including one-step alternating-direction-implicit (ADI) and one-step locally-one-dimensional (LOD) methods are proposed. The two proposed methods are derived from the original ADI-FDTD method and LOD-FDTD method through some linear operations applied to the original methods and are algebraically equivalent to the original methods respectively, but they both avoid the appearance of intermediate fields and are one-step method just like the conventional FDTD method. Numerical experiments are carried out for validation of the two proposed methods, and from the numerical results it can be concluded that the proposed methods can solve equation correctly and are simpler than the original methods, and their computation efficiency is close to that of the existing one-step leapfrog ADI-FDTD method.

This paper discusses the design of a 6-m Cassegrain optics based multiband reflector antenna integrated with beam waveguide (BWG) optics, which consists of an ellipsoidal mirror and three plane mirrors. The presented antenna has been simulated, and 75.8% and 76.8% aperture efficiencies have been achieved at 0.225 THz and 0.338 THz, respectively. The initial design parameters of elements of BWG network are computed using fundamental Gaussian beam parameters. The simulated results of the antenna including aperture efficiency have been presented and discussed in detail. The antenna has been designed for the ground based THz telescope for radio astronomy.

Free-electron radiation results from the interaction between swift electrons and the local electromagnetic environment. Recent advances inmaterial technologies provide powerful tools to control light emission from free electrons and may facilitate many intriguing applications of free-electron radiation in particle detections, lasers, quantum information processing, etc. Here, we provide a brief overview on the recent theoretical developments and experimental observations of spontaneous free-electron radiation in various structured environments, including two-dimensional materials, metasurfaces, metamaterials, and photonic crystals. We also report the research progresses on the stimulated free-electron radiation that results from the interaction between free electrons and photonic quasi-particles induced by the external field. Moreover, we provide an outlook of potential research directions for this vigorous realm of free-electron radiation.

This paper proposes an efficient and accurate scattered field prediction method based on Kirchhoff Approximation called `Mirror Kirchhoff Approximation' (MKA) which is suitable for evaluating the shadowing effect by a metal cuboid. The disadvantages of conventional methods, such as low accuracy of Kirchhoff Approximation (KA) for metal cuboid and high computational complexity of Method of Moment (MoM) for a shadowing object at millimeter wave (mmWave), have motivated the establishment of an efficient and accurate prediction method for a metal cuboid at mmWave. The proposed method solves the previous issues by introducing the ray-based reflection into conventional KA. The idea and detail formulations of the proposed method are presented. The proposed method is validated by comparing with MoM and KA in terms of complexity and accuracy. The results imply that the proposed method presents good accuracy with low calculation time. The MKA has a maximum 8.3 dB improvement compared with conventional KA. Calculating time is improved by 392-915 times compared with MoM.