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.

A co-planar waveguide-fed symmetrical staircase-shaped ultra-wideband antenna is proposed in this work. This antenna consists of three pairs of rectangular notches, two symmetrical C-shaped slots and two pairs of quarter-circular-ring-slits which are etched on the rectangular radiator and ground plane, respectively. By sequentially inserting three pairs of rectangular notches with proper positions, an excellent impedance bandwidth of 1.55-16.95 GHz (166.51%), i.e., a 10.94:1 ratio bandwidth is obtained. The total volume of the prototype is merely 0.239×0.224×0.004λ_l³, λ_l wavelength of the free space at the lowest operating frequency (i.e., 1.55 GHz). As a result, wider impedance bandwidth, fair gain and better impedance matching of the proposed antenna are obtained. It is observed that the simulation results are in good agreement with the measurement results. The transmission line model (TLM) of the proposed antenna is presented, and it shows the antenna behavior based on the effect of each element. It is observed that the characteristics of the TLM model are close to the simulation result using the CST simulator. The prototype is successfully implemented, fabricated, and compared with the experimental results.

This letter proposes a 2×2 surface-mount dipole antenna array based on via fence for 5G millimeter-wave applications. The dipole antenna element was first proposed, which has a compact size and low cost. Then the via fences are introduced to reduce coupling between adjacent elements and enhance isolation. In this way, compared with a 1×2 antenna array without the via fence, the isolation of a 1×2 antenna array with a via fence is improved by 12 dB at 26 GHz. The elements are extended into 2×2 arrays with and without the via fence, and their performance is evaluated by the evaluation board. The measurement results show that the -10-dB impedance bandwidth of the antenna array is 19% (24.7-29.9 GHz), and the peak gain is 9.5 dBi at 25 GHz. The proposed 2×2 array can be used in the N257 (26.5-29.5 GHz), N258 (24.25-27.5 GHz), and N261 (27.5-28.35 GHz) frequency bands. Low cost, small size, and high isolation characteristics make it one of the candidates for 5G millimeter-wave applications.

This paper presents crosstalk analysis of E-plane multichannel waveguide joints for high frequency. The multi-cavity modeling technique and method of moment are used to analyze the crosstalk. Waveguide has many practical uses in high powered RF systems. When two channel waveguides are joined, the phenomenon of crosstalk will certainly appear, and the reason behind is poor workmanship. The gap appearing at the flange joint causes power coupling to the neighboring ports. In this paper two channel E-plane waveguide joints for frequency range 15 GHz to 18 GHz have been analyzed. Scattering parameters data obtained from cavity model analysis have been verified and compared with CST microwave studio simulated and measured data.

Transcranial magnetic stimulation (TMS) has been widely used in the treatment of varied physical and neuropsychiatric disorders, especially in major depression. The intracranial electromagnetic field is generated by the the time-varying current in the stimulation coil to change the potential of targeted neurons during the treatment. Since different mental disorders correspond to specific stimulation targets and broad stimulation range might raise serious side effects, stimulation focalization is very important in TMS. To achieve focalized stimulation, a novel magnetic stimulation coil with the field shaper and the crescent ferromagnetic core (the FSMC coil) is proposed and optimized in this study. The Finite-Element Method (FEM) is adopted to analyze the relationships between the design parameters of the field shaper and crescent ferromagnetic core and the characteristics of the intracranial electromagnetic field. Compared to traditional single circular coil, the focalization of the intracranial electromagnetic field generated by the optimized FSMC coil can be significantly improved both from 2D and 3D levels. To verify our method, an anatomically realistic human head model with different electrical properties assigned to each tissue of the brain is employed in this paper. We also checked the maximum induced charge density on the targeted plane generated by the optimized coil to make sure that it will not cause any induced neurologic damage.

This article proposes a new type of variable-leakage-flux flux-intensifying permanent magnet (VLF-FIPM) machine and performs optimization and multi-physical field analysis on it. By designing leakage flux bypass and various magnetic barriers, the proposed machine has the variable-leakage-flux characteristic and reverse saliency characteristic of L_d>L_q. Firstly, the evolution process from the conventional interior permanent magnet (IPM) machine to the proposed machine is explained. Secondly, the output torque, torque ripple, core loss and reverse saliency ratio of the proposed machine are optimized by multi-objective comprehensive optimization method. Then the electromagnetic performance of the optimal machine is compared with that of the initial machine and conventional IPM machine. Finally, the temperature field and stress field of the optimal machine in different states are analyzed in detail. Both theoretical results and simulation analysis verify the effectiveness of the proposed design idea and optimization of the VLF-FIPM machine.

Assis predicted that based on Weber's electrodynamics, an alternative direct-action model formulated before Maxwell, a charge accelerating inside a sphere at constant electric potential, should have a measureable effective mass. Although initially some experiments appeared in the literature that indeed claimed such an effect, all recent studies found no evidence. All experiments so far used either discharges or electrons with non-constant accelerations that could mask the existence of Assis's prediction. We performed an experiment using a Perrin tube, which produces a beam of electrons with a constant velocity that can be deflected by Helmholtz coils to hit a Faraday cup. The tube assembly was put inside a spherical shell, which could be charged up to 20 kV. Any effective mass of the electrons would have changed their position on the Faraday cup. We found no variation of the electron position within our experimental accuracy, which rules out Assis's effect by two orders of magnitude. This confirms Maxwell's theory and the fact that electrostatic potential energy cannot be localized to individual charges.

A Negative Group Delay (NGD) filter prototype design based on cascaded identical 2nd-order baseband stages is presented. The prototype design achieves an NGD-bandwidth product that in the upper asymptotic limit for a distributed design is a function of out-of-band gain in decibels raised to the power 3/4. This is an improvement of previous cascaded first-order designs that have an NGD-bandwidth functional dependency of out-of-band gain in decibels to the power of 1/2. The bandwidth is taken as the 3 dB amplitude response bandwidth. The corresponding NGD design upshifted to a non-zero center frequency, i.e. a Band-Stop Filter (BSF) design, is shown to be possible to implement with Sallen-Key topology, and an example is presented for a 500 MHz center frequency and a 100 MHz (20%) 3 dB bandwidth. The filter shows a 4.05 ns negative group delay with a 1.28 ns in-band variation and a 3-dB amplitude response over the bandwidth of 100 MHz, achieving an NGD-bandwidth product of 0.405. An in-band distortion metric is presented, which can be evaluated for any specified time-domain input waveform. It is shown that the bandwidth, order of filter and desired distortion for a particular input waveform are interrelated. Therefore, the proposed in-band distortion metric constitutes another trade-off quantity to be checked for any type of NGD design.

Single patch designs of a microstrip antenna with a U-slot or a pair of rectangular slots (E-shape) provide a single band circularly polarized response, and hence they are not useful in frequency and polarization agile applications. In this paper, a modified design of a Ψ-shape microstrip antenna is proposed for dual band and dual sense circularly polarized response. Use of unequal length rectangular slots in the modified patch, optimizes the inter-spacing between the modified TM21 and TM22 resonant modes, surface current distributions and impedance levels at them to yield dual band circularly polarized response. An impedance bandwidth of 1992 MHz (37.05%) is obtained which completely covers the axial ratio bandwidth of 11.84 and 5.67%, in the two bands with frequency ratio of 1.3 in between them, thereby satisfying the requirements of frequency agile systems. Over the impedance and axial ratio bandwidth, the antenna exhibits nearly broadside radiation pattern with a gain of around 7 dBi. A design methodology based on the simple parametric formulation is presented, which helps in realizing a similar antenna in the specific frequency band. The proposed antenna can find applications in frequency and polarization agile systems where the signal loss due to the interference and jamming can be reduced.

This paper introduces a model and design of an innovative bandpass (BP) negative group delay (NGD) distributed circuit. The passive circuit topology under study is constituted by fully distributed elements without lumped components. The NGD passive structure is implemented as a ladder shape topology composed of distributed transmission line (TL) elements. The S-matrix model is established from TL-based equivalent Z-matrix operations of TLs with respect to the ladder geometry. As a proof of concept, a two-cell ladder prototype is designed in microstrip technology, which is simulated, fabricated, and tested. The calculated and simulated measurements are in very good agreement with the validation of BP NGD behaviour. NGD value is better than -3 ns with centre frequency between 3.56 and 3.68 GHz over more than 30 MHz NGD bandwidth being observed. The circuit operates under insertion loss better than 5 dB and reflection loss better than 8 dB. This innovative BP NGD passive circuit can be useful in the RF and microwave engineering area, for example, to reduce the signal propagation delay in the upcoming and 5G telecommunication systems.

The electrostatic sensor is a rapidly developing particle monitoring sensor. This paper applies sensor array to inverse the information carried by detected multiple charged particles precisely. It breaks through the constraint that the detailed information of particles cannot be obtained in previous studies. The proposed method can be widely applied to oil line and gas path debris monitoring. The sensing mathematical model and the finite-element model are established. A compressive sensing-based method is proposed to invert the information of charged particles. Through simulation and experimental verification, the method can accurately estimate the centroid of multiple particles, the total charge quantity of the particle cluster, the spatial position of each particle and the charge quantity carried by each particle in the multiple particles with a low error rate when the multiple particles are distributed near the pipe wall of flow channel.

In this paper, an eight-element MIMO smart antenna system consisting of two different array modules for handheld device is proposed. The system is composed of two antenna array modules. The first module is a six-element array operating in N78 (3.3-3.8 GHz) band for 5G, which achieves MIMO functions for receiving and beam scanning for transmitting. The second module is a two-element antenna array, which operates in LTE/WWAN/N78 (0.7-0.91 GHz, 1.63-2.61 GHz, 3.3-3.8 GHz) bands. To take full advantage of the existing antenna resources in the mobile device, the six elements in the first module are combined with the two elements in the second module to form an 8-element array in the overlapping N78 band. Good isolations and envelope correlation coefficients are achieved in the receiving mode by loading L-shaped slots for the combined module. The distribution of excitations for the combined array in the transmitting mode is optimized by the method of maximum power transmission efficiency to direct the beam to the desired direction with maximum possible gain, and is realized by an in-house designed beamforming controller. The impacts of the environments on the antenna array performance are investigated.

In this paper, we present a general solution for time-harmonic electromagnetic fields with its electric and magnetic fields parallel to each other (E || B fields) in source-free vacuum and demonstrate that every time-harmonic E || B field is composed of the superposition of two counter-propagating Beltrami fields. We show that every E || B field can be categorized into one of two cases depending on the time dependence of the function that describes the proportionality between the electric and magnetic fields. After presenting the mathematical definition of a Beltrami field in electromagnetism and its handedness, we perform a detailed analysis of time-harmonic E || B fields for each case. For the first case, we find the general solution for the E || B fields using the angular-spectrum method and prove that every first-case E || B field can be generated by superposing two oppositely traveling Beltrami fields with the same handedness. For the second case, we deduce the general solution for the E || B fields by employing complex analysis and demonstrate that every time-harmonic E || B field is composed of two counter-propagating planar Beltrami fields with opposite handedness.

The research goal of low-resolution radar aircraft target classification is to analyze the category of the given low-resolution radar aircraft target echo. In existing solutions, the feature extraction methods based on rotating modulation spectrum have good performance, such as the complex cepstrum method, autocorrelation method, cycle diagram method, autoregressive model power spectrum method, and singular value decomposition method. Most of these methods are more complicated in calculations, and practical applications often require higher pulse frequencies and longer observation times, which are greatly restricted. In this paper, a classification method based on ensemble empirical mode decomposition and multifractal correlation (CMEEMDMFC) is proposed. The basic design idea is to obtain the intrinsic mode functions (IMFs) by using the signal decomposition ability of ensemble empirical mode decomposition (EEMD) and select some components which are beneficial for improving the signal-to-noise ratio (SNR) for recombination. Then extract the corresponding multifractal correlation (MFC) features from the new signals for recognition. For verifying the validity of the model, a comparison model was selected to test on the same data set. Experimental results show that the proposed model performs well in classification accuracy.

We studied electromagnetic wave propagation in a system that is periodic in both space and time, namely a discrete 2D transmission line (TL) with capacitors modulated in tandem externally. Kirchhoff's laws lead to an eigenvalue equation whose solutions yield a band structure (BS) for the circular frequency ω as function of the phase advances k_xa and k_ya in the plane of the TL. The surfaces ω(k_xa, k_ya) display exotic behavior like forbidden ω bands, forbidden k bands, both, or neither. Certain critical combinations of the modulation strength m_c and the modulation frequency Ω mark transitions from ω stopbands to forbidden k bands, corresponding to phase transitions from no propagation to propagation of waves. Such behavior is found invariably at the high symmetry X and M points of the spatial Brillouin zone (BZ) and at the boundary ω = (1/2)Ω of the temporal BZ. At such boundaries the ω(k_xa, k_ya) surfaces in neighboring BZs assume conical forms that just touch, resembling a South American toy ``diábolo''; the point of contact is thus called a ``diabolic point''. Our investigation reveals interesting interplay among geometry, critical points, and phase transitions.

Substrate integrated E-plane waveguide (SIEW) was invented recently to design E-plane waveguide devices on printed circuit board, which cannot be achieved by using the conventional substrate integrated waveguide (SIW). This paper is the first time to present an E-plane displaced SIEW junctions bandpass filter. The proposed design is shorter than the recently published SIEW septa filter and has a smaller footprint than several other SIW filters. It is designed by mapping an equivalent E-plane waveguide filter to its SIEW implementation. A filter prototype is built and measured for validation.