Carr-Purcell-Meiboom-Gill (CPMG) is generally used as the measurement sequence of unilateral NMR (UMR) sensors, and the NMR signals collected by the sequence are composed of a series of echo signals. In the traditional CPMG measurement signal, each echo peak value is first taken and then denoised, which would lead to the inaccuracy of the peak point taken, resulting in deviation. To ensure the measurement result more accurate, this paper proposes to employ wavelet technology to denoise the echo signal first, and then take the peak point to analyze the data. Firstly, a simplified model of the spin-echo signal without the influence of gradient magnetic field was established, and white noise was applied to a certain extent. Then, Signal to Noise Ratio (SNR) and Root Mean Square Error (RMSE) were used as evaluation indexes. The denoising effects under different wavelet bases and thresholds were compared. Finally, the Matlab simulation result showed that wavelet analysis had a good effect on the denoising of unilateral NMR spin echo signal.

A problem of scattering of electromagnetic waves by thin impedance biconical vibrators in а free space and in a rectangular waveguide is solved by an asymptotic averaging method and a generalized method of induced electromotive forces (EMF). An influence of the change of vibrator radius upon energy and spatial characteristics is numerically studied. Theoretical results are compared with the experimental data.

We introduce the concept of gap Zak or Chern topological invariants for photonic crystals of various dimensionalities. Specifically, we consider a case where Tamm states are formed at an interface of two semi-infinite Bragg mirrors and derive the formulism for gap Zak phases of two constituent Bragg mirrors. We demonstrate that gap topological numbers are instrumental in studies of interface states both in conventional and photonic crystals.

This paper presents a single element ultra-wideband (UWB) microstrip patch antenna with high directivity. In this work, techniques like partial ground and modification of the patch have been used to achieve the UWB. The designed antenna consists of a modified U-shaped radiating patch with a microstrip feed attached directly to it. The initial U-shaped radiating patch is modified by attaching an inverted trapezium on both sides of the feed line. Two parasitic patches are introduced near the feed structure of the antenna after etching away two rectangular slots with appropriate dimensions. Moreover, the proposed structure consists of a partial ground plane which contributes to the UWB nature. Modifications in the form of square and triangular slot etching are carried out in this part of the proposed structure. The proposed antenna is compact with dimensions of 16 mm × 19 mm × 1.6 mm. Finally, gain enhancement of the proposed structure is done by placing a Frequency Selective Surface (FSS) behind the proposed antenna with an air spacer in between the structures. A novel FSS unit cell is proposed, and its performances are checked experimentally. Later, FSS is combined with the antenna, and measured peak gain of 9.7 dBi is obtained experimentally. The overall size of the structure is 62.5 mm × 52 mm × 24.9 mm.

A broadband phase shifter (PS) with a constant phase based on a negative group delay (NGD) microwave circuit is proposed. The presented broadband PS is composed of distributed microstrip lines and two resistors, which is based on the positive group delay compensation principle. By tuning the electrical length of the phase shift transmission line, the constant phase can be obtained in the range of -360° ~ 0°. For verification, three broadband PSs with the phase shift of -90°, -180°, and -270° (90°) are designed, fabricated, and measured at the center frequency of 1.0 GHz. The measurements show that the -90° PS can achieve a constant phase of -90°±3.0° with a fractional bandwidth (FBW) of 73.1%; the -180° PS can achieve a constant phase of -180°±5.0° with an FBW of 51.1%; and the -270° PS can achieve a constant phase of -270°±4.0° with an FBW of 40.4%. Besides, the return loss is greater than 13.6 dB in the flat-phase bands.

In this paper, an AgileDARN (Agile Dual Auroral Radar Network) radar echo classification method based on support vector machine is proposed. AgileDARN radar echo includes ionospheric backscattering echo, meteor echo, noise interference, etc. With the continuous operation of AgileDARN radar, the amount of data increases rapidly, requiring efficient and reliable classification methods. In order to efficiently classify the echoes of AgileDARN radar, this paper proposes an echo classification method based on support vector machine. By analyzing the characteristics of the autocorrelation function (ACF) of the sampled data and extracting the features, the support vector machine(SVM) classification method is adopted to classify AgileDARN echo into ionospheric backscattering echo, meteor echo and noise interference. The data analysis shows that the classification accuracy of training data set is more than 99%, and that of test data set is more than 95%. Using this classification model to classify 1800 echo data of AgileDARN radar, the classification accuracy is more than 91% compared with the result of manual interpretation.

In this paper, a novel miniaturized dual-band embedded near-zero index (NZI) metasurface-based patch antenna is presented. A new methodology based on loading a narrowband microstrip patch antenna (resonating at 4.6 GHz) by a metasurface embedded in the middle of the antenna's substrate is introduced. The loaded antenna has a dual-band resonance of bandwidth of 15% and 43% at 2 GHz and 4.6 GHz, respectively. The metasurface layer is an array of square holes such that there is no hole below the patch. The metasurface layer is designed as a near-zero-refractive-index material (NZRIM). By controlling the phase reflection properties of the structure, the antenna gain is increased by 5.5\,dB, original bandwidth increased ten times and the front-to-back ratio improved from 7 to 187. Also, footprint miniaturization of 56.5% with a maximum size of (1.9λ₀)² is obtained. To the best of the authors' knowledge, such enhancement is the largest to date.

A physics-based hybrid implicit-explicit finite-difference time domain (HIE-FDTD) method is developed for electromagnetic modeling of multi-passband frequency selective surfaces (FSSs). Using this self-developed HIE-FDTD simulator, several dual- and tri-passband FSSs are designed and further fabricated. The measurement results are in good agreement with the simulation ones, which prove high accuracy of the self-developed HIE-FDTD algorithm. In addition, the resonant frequencies of the designed FSSs can be effectively adjusted by changing their geometric parameters. This work provides electromagnetic guides of structure and parameter selections for designing multi-passband FSS.

In this article, we present the design and production of a miniaturized adjustable coupler with optimized dimensions of 48 mm in length and 31 mm in width. This coupler offers the possibility of covering all phases [0, 45°, 90°, 120° and 180°]. To be able to achieve this, the proposed coupler can be adjusted through the implementation of six SMV2019-079LF diodes which allow shifting from one phase to another. This new flexibility, in terms of phase shifting, can greatly improve the multifunctional use of this small and efficient coupler, in particular, in comparison with previously improved phase shifting couplers which are limited to one or two phases. The high performance and efficiency have been verified by the results obtained by simulation and measurement.

In this article, a miniaturized, dual-polarized corner-fed microstrip antenna is designed and fabricated at 1.43 GHz for Low Earth Orbit (LEO) Satellite applications. The antenna adopts a Complementary Split-Ring Resonator (CSRR)-inspired structure and slotted patch to achieve miniaturization. This reduces the patch size by 39.4%. Meandered impedance-transforming lines are placed for impedance tuning, and its benefit is demonstrated by both simulated and measured S₁₁ curves reaching lower than -20 dB. Feeding at corner increases its isolation to -25 dB over the whole bandwidth of 40 MHz and reaches lower than -33 dB at the resonant frequency. The antenna is fabricated and tested. Measured results are generally in good agreement with simulations.

In this paper, we present a gain-enhancement technique for reflectarray applications with compact aperture size and a low profile. To increase antenna gain, reflectarrays are constructed as an electrically large aperture, and the feed is required to be of high directivity, which is accompanied by a longer focal length. This increases the dimensions in two aspects, including the physical aperture size and the profile of the overall structure. To obtain high gain with compact dimensions, we develop a reflectarray that uses an active-integrated feeding antenna. This feeding antenna is connected to a microwave power amplifier, which enhances the gain without reducing the half-power beam widths (HPBWs) of the patterns. Accordingly, the feed can be arranged with a shorter focal length, whereas the spillover efficiency is still high. Moreover, the power amplifier contributes additional gain of 20.6 dB, and thus the proposed structure can achieve realized gain as high as 44.5 dB with dimensions of 9.2 × 6.7 square wavelengths. Such a high-gain and compact antenna is particularly suitable for satellite applications.

Metamaterials offer great promise for engineering electromagnetic properties beyond the limits of natural materials. A typical example is the so-called spoof surface plasmons (SPs), which mimic features of optical SPs without penetrating metal at lower frequencies. Spoof SPs inherit most of the properties of natural SPs, including dispersion characteristics, field confinement, localized resonance, and subwavelength resolution, and therefore are highly expected to offer a new solution for low-frequency applications. With the development of spoof SPs, three different theories have been introduced. The first one is the description of subwavelength corrugated metal surfaces by a metamaterial that hosts an effective plasma frequency. The second one is developed with high-index contrast grating, which can realize propagation with ultra low loss and localization with ultrahigh Q-factor resonance. The last one is structural dispersion induced SPs, a perfect low-frequency analogue of optical SPs, realized by exploiting the well-known structural dispersion waveguide modes only with positive-ɛ materials. Here, the developments of these three theories including propagation and localized SPs are reviewed, focusing primarily on the fundamental and representative applications.

The study of outer-rotor coreless bearingless permanent magnet synchronous generator (ORC-BPMSG) is intended to pave the way for the future of high-speed flywheel energy storage systems. A multi-objective parameter optimization method is proposed for the outer-rotor coreless bearingless permanent magnet synchronous generator with the aim of improving the fundamental wave content of the generator's output voltage, reducing harmonics and optimizing the suspension force at the same time. Firstly, the basic parameters and operating principle of the generator are described. Then, the response surface (RS) method is used to obtain the objective functions for the total harmonics distortion (THD), the mean value of the suspension force and the suspension force pulsation. The optimal optimizations of the ORC-BPMSG are selected by establishing the pareto solution set through the improved multi-objective particle swarm optimization (MOPSO) algorithm. Finally, the optimal ORC-BPMSG prototype is fabricated, and the performance of the prototype is verified. The experiments show that the optimized generator output voltage has fewer harmonics and operates reliably.

We study TE-wave propagation in a hollow waveguide with a graded transition from a lossy right-handed material (RHM) filling the left-hand half of the waveguide to the impedance-matched lossy left-handed material (LHM) filling the right-hand half of the waveguide. The transition between the two media is graded along the direction perpendicular to the boundary between the two materials (chosen to be the z-direction), and the permittivity ε(ω, z) and permeability μ(ω, z) are chosen to vary according to hyperbolic tangent functions along the z-direction. We obtain exact analytical solutions to Maxwell's equations for lossy media, and the solutions for the field components confirm the expected properties of RHM-LHM structures. Thereafter, a numerical study of the wave propagation over an impedance-matched graded RHM-LHM interface is performed, using COMSOL software. The numerical study shows an excellent agreement between the numerical simulations and analytical results. Compared to other solution methods, the present approach has the advantage of being able to model more realistic smooth transitions between different materials. However, in the limiting case, it includes correct results for abrupt transitions as well. In the present approach we also introduce the interface width as an additional degree of freedom that can be used in the design of practical RHM-LHM interfaces.

The uplink and downlink of modern satellite communication systems operate on different frequency bands. A novel dual-band dual circular polarization filter antenna with a single port is proposed in this paper. The dual-band characteristic of the antenna is obtained by exciting stacked patches. The antenna supports right-handed circular polarization (RHCP) at lower frequency band and left-handed circular polarization (LHCP) at higher frequency band, respectively. The feeding network is realized with a strip line, and the antenna can be equivalent to a parallel circuit. If the lower frequency patch works, the higher frequency patch presents high impedance, and vice versa. Therefore, the antenna has excellent filtering performance. The measurements of antenna prototype are in good agreement with the simulation results. The impedance bandwidth is 5.80-6.10 GHz and 9.20-10.64 GHz, and the axial ratio (AR) bandwidth is 40 MHz for lower band and 180 MHz for higher band, respectively. Meanwhile, the radiation pattern is stable in the operation frequency bands.

In order to solve high torque ripple of permanent magnet synchronous motor (PMSM) for marine electric propulsion under the current control methods, the improved model predictive current control (MPCC) of PMSM for marine electric propulsion based on the mathematical model of three-phase PMSM is proposed. First, the stator current prediction model is derived based on the forward Euler method. Then the first optimal voltage vector is obtained by the value function, and the second optimal voltage vector, and the second optimal voltage vector and the first and second optimal voltage vectors' respective action times are obtained by the q-axis deadbeat control, which are directly fed back to the inverter. The proposed control method is verified by simulation and hardware in the loop simulation experiment. The experiment results show that, in comparison with the direct torque control based on space vector modulation (SVM-DTC) in the case of motor speed and torque mutation, the torque ripple of motor is reduced by 9.40% and 4.80% respectively based on improved MPCC. The feasibility and effectiveness of the proposed method are verified by the simulation and experiment results.

A compact exponentially tapered balanced antipodal Vivaldi antenna for Phased array systems is proposed in this paper. The proposed design implements slots at the edges to improve impedance bandwidth typically at lower frequencies. The antenna is coupled to a 50 Ω microstrip line between the signal conductors of the middle layer and ground plane. A detailed parametric analysis has been carried out to determine the optimized dimensions and to achieve desired antenna performance. A prototype of the antenna (56×28×1.6 mm³) was fabricated and measured to validate the simulation results. It is revealed that the antenna has a wide impedance bandwidth of 120% over 5-20 GHz and measured gain of the antenna increases from 2.6 dB to 8.0 dB in the whole operational frequency band. The small aperture width which is typically 28 mm is the attractive feature of the proposed design. Therefore, compact size, high gain, ultrawide bandwidth, and directional radiation characteristics of the proposed design may be suitable for advance radar systems.

In this work, a substrate integrated waveguide slot array filtering antenna for dual band applications is presented. This novel design performs the functions of both a filter and an antenna simultaneously. The main intention of this work is to design a circuit that separates the frequencies in a dual band operation. The antenna is designed as an integration of two parts; the upper part operates at 10.2 GHz while the lower part operates at 16.4 GHz. In each part, an array of five longitudinal slots is incorporated, as well as a SIW antenna with complementary split ring resonators that operate as a band pass filter at the front end. Each slot array antenna is designed for a specific frequency band, and its function depends upon its preceding band pass filter. The two band pass filters allow only signals from the frequency bands for which they are designed, to their corresponding slot array antennas. This technique, along with properly spaced metal vias of the SIW antenna, prevents any leakage and hence reduces interference in dual band operation. Both the band pass filter and the antenna can be built on the same planar board. The antenna is fed through a microstrip to SIW taper transition. CST Microwave Studio software is used for optimization and simulation of the structure. The antenna was built on an RT Duroid 5880 and tested to investigate practical validation. The antenna has a bandwidth of 1.9 GHz, from 9.2 GHz to 11.1 GHz in the X-band, and 2.2 GHz, from 15.6 GHz to 16.9 GHz in the Ku band. The gain pattern is unidirectional in nature and has low side lobe levels of -24 dB and -21 dB at resonant frequencies. A noticeable difference that is greater than 20 dB between co-polarization and cross-polarization is observed. The dimensions of the antenna are 56 mm x 32 mm x 0.508 mm. There is an excellent similarity between the simulated and measured results.

Volatile organic compounds (VOCs) have received increasing attentions recently. They are important for air quality monitoring, and biomarkers for diseases diagnosis. For the gas sensor community, various detection technologies were explored not only to detect total VOCs, but also aim for sensor selectivity. Commercially available VOC sensors, such as metal oxide based or photoionization detectors, are suitable for total VOCs but lack of selectivity. With the advancement of optical spectroscopy, it provides a good solution for specific VOC detections. In this review, various spectroscopy techniques are summarised focusing on increasing sensor sensitivity and selectivity. The techniques included in the paper are, non-dispersive infrared, multi-pass cell spectroscopy, cavity enhanced absorption photoacoustic spectroscopy and Fourier transform infrared spectroscopy. Each technique has its pros and cons, which are also discussed.

A bat-shaped microstrip patch antenna is proposed with tri-band characteristics (11.0-11.5 GHz), (11.8-12.3 GHz), (13.0-14.3 GHz) and impedance bandwidth 4.4%, 4.1%, and 9.5% at resonant frequencies 11.37 GHz, 12.2 GHz, and 13.5 GHz, respectively. The proposed antenna exhibits peak gain of 2.6 dBi. The proposed microstrip patch antenna shows the dual band circularly polarized characteristics with two bands (11.0-13.0 GHz) and (13.2-14.0 GHz) and 3 dB axial-ratio (AR) impedance bandwidths 15% and 5.2%, respectively. Investigation of proposed antenna is done using evolution technique. The results are verified experimentally in terms of reflection coefficient, gain, axial ratio, and radiation pattern.