This review article explores the advancement of metasurfaces in wearable antenna design for off-body communications. The wearable antenna needs to be compact, flexible, and, most importantly, should have less back radiation. In this context, wearable antennas that are inspired by metasurfaces are a good choice. Metasurface can make the antenna compact and reduce the back-radiated waves, which lowers the specific absorption rate (SAR) and improves the antenna's performance. In addition, the metasurface can also generate circular polarization (CP) by carefully rotating the electromagnetic (EM) waves incident on it and multi-band by simultaneously exciting its multiple modes. Using the aforementioned features provided by the metasurface, the surveys are segregated as single-band with linear polarizations (LP), single-band with CP, dual-band with LP, dual-band with dual polarization, and dual-band with dual CP. Prior to the survey, the challenges and considerations for wearable antenna design as well as the theoretical perspective behind performance improvements are discussed. Also, a conventional unit-cell of the metasurface is theoretically designed using the discussed theories and validated using CST Microwave Studio, which shows good agreement with each other.

In this paper, an antenna sensor based on an electromagnetic bandgap (EBG) structure is proposed to measure the complex permittivity of liquid under test (LUT). The sensor consists of two parts: a detection antenna and an EBG structure. The detection antenna uses a semicircular arc defective ground structure to improve the quality factor (Q-factor). Simultaneously, the EBG structure can be equivalent to a narrow-band bandpass filter, so that the electromagnetic wave can only propagate in a very narrow frequency band. It can further improve the Q-factor of the antenna and realize the precise positioning of the resonance frequency point. The complex permittivity of the LUT can be extracted by measuring the resonant frequency shift and the amount of variation in the Q-factor of the antenna. The test results show that the sensor can detect dielectric values covering the range of 1-25, and the average sensitivity is 2.342%. It combines the advantages of high sensitivity and wide detection range.

The traditional dual-vector model predictive torque control (MPTC) of permanent magnet synchronous motor suffers from the problems of large control computation, large torque ripple, and prediction deviation due to parameter mismatch. To address these issues, an enhanced robustness dual-vector MPTC (ERD-MPTC) control strategy is proposed in this paper. First, in order to reduce the control computation, a fast voltage vector selection table based on a 12-sector voltage vector map is proposed, which reduces the number of prediction iterations from 14 to only 3. Secondly, to reduce the ripple of torque and flux in one cycle, the cost function without weight factor is proposed. This cost function includes the fluctuations at the moment of the switching point. Then, for the bad effects of parameter mismatch, the inductance parameter is estimated by using the amount of error variation between the predicted value and the actual measured value at adjacent moments. So, an ERD-MPTC strategy to enhance the robustness of the prediction model in the presence of parameter mismatch is proposed by integrating the inductance updating mechanism and expanded state observer. Finally, through the experiment, it is shown that the proposed strategy can reduce the torque fluctuation, effectively reduce the adverse effects of parameter changes, and greatly improve the stability of the system.

A CPW-fed hexagon-shaped split ring resonator (H-SRR) antenna consisting of three concentric SRR rings is proposed for triband WLAN/WiMAX and Wi-Fi6 applications. A single port optimized antenna has a size of 43×22×1.6 mm³ with two ports, and a multiple-in-multiple-out (MIMO) antenna based on the same H-SRR design is of size 95×52×1.6 mm³. The use of metallic loadings between the rings led to an impedance bandwidth of 21%/65% for the single-port H-SRR antenna and 33%/66.5% for the dual-port H-SRR antenna in the 2.4 GHz band and 5.2/6 GHz bands. The antennas exhibit a gain in the range of 2-2.7 dB and good radiation characteristics. Also, the proposed antenna design achieves isolation of more than 30 dB without using any de-coupling network making the structure simple and compact. For tri-band applications of the proposed dual port antenna, the MIMO parameters ECC, TARC, DG, and MEG are found about < 0.005, < -10 dB, ≤ 10 dB, and < -6 dB, respectively in the 2.4/5.2/6 GHz bands without any decoupling structure. Measurements with a commercial transmitter at 5.8 GHz confirmed that these antennas offer better Wi-Fi 6 connectivity. Thus, the results confirm that the novel features of the proposed antennas are simple structure, wideband operation, and moderate gain with a compact size in the 2.4/5.2/6 GHz bands, and therefore, these presented antennas are useful in the current WLAN/WiMAX systems as well as upcoming Wi-Fi 6 applications like routers.

This research aimed to design a sea pimp-shaped monopole antenna by using etching and cutting techniques, combined with the addition of reflector, to modify the antenna structure to support the bandwidth standard according to GSM-850 (0.82-0.90 GHz), GSM-900 (0.88-0.96 GHz), DCS (1.72-1.88 GHz), PCS (1.85-1.99 GHz), UMTS (1.92-2.17 GHz), 5G Band40 (2.30-2.40 GHz), LTE41 (2.496-2.690 GHz), and WLAN IEEE 802.11b/g/n (2.4-2.48 GHz). This antenna used a galvanized metal sheet with a conductivity of 3.56 x 10⁷ s/m to fabricate the structure of the radiator, ground plane, and reflector. The reflector modifies radiation patterns and increases the gain of the antenna. The antenna structure used the CST program for simulation to determine the optimal parameters and property values. As a result of replication, the antenna had a dual-band with a reflection coefficient S11 at 915 MHz (736-1040 MHz) of -26.70 dB and a frequency at 2.28 GHz (1.68-2.94 GHz) of -20.15 dB. The antenna gains are 6.70 and 8.47 dBi, an increase of 83.56% and 44.04% over the antenna without a reflector, respectively. The antenna had a unidirectional pattern in all the frequency ranges which can be utilized for the purpose of RF energy-harvesting (RF-EH) systems to provide power to low-power electronic systems.

In recent years, uncertainty analysis is a hot topic in the field of electromagnetic compatibility simulation. The actual electromagnetic environment is simulated by considering the randomness of the model input parameters. However, there are currently two key issues that have not been resolved. One is the curse of dimensionality problem that occurs when there are many random variables. The other is how to establish a random input model with generality and portability. In order to address these issues, this paper proposes a new random input modeling method called random variable black box model. When applying to the Stochastic Collocation Method with dimensionality reduction sparse grid strategy, the applicability of this uncertainty analysis method can be extended to any probability density function, then enabling efficient electromagnetic compatibility simulation uncertainty analysis of high-dimensional random variable models and fundamentally solving the curse of dimensionality problem. Finally, this paper implements a joint simulation technology of the MATLAB software and the COMSOL software to verify the strong portability of the random variable black box model, ensuring that advanced uncertainty analysis methods can be smoothly introduced into commercial electromagnetic simulation software and expanding the application scope of uncertainty analysis.

This paper proposes a compact and broadband coplanar waveguide (CPW)-to-rectangular waveguide (RWG) transition using a resonator with an impedance-matching element. The transition size is as small as 7.66 mm, and the frequency range for which the reflection coefficient is smaller than -15 dB covers from 8.05 GHz to 12.18 GHz (FBW = 40.83%), almost encompassing the full X-band (8.2-12.4 GHz). To reduce the size of the transition, a short-circuited stub phase shifter is used to replace the half-wavelength phase shifter, resulting in a miniaturized CPW-to-RWG transition using the resonator with the impedance-matching element. The transition size is further reduced from 7.7 mm to 5.9 mm. Besides, the frequency range for which the reflection coefficient is smaller than -15 dB covers from 8.05 GHz to 12.38 GHz (FBW = 42.38%), nearly encompassing the full X-band (8.2-12.4 GHz) as usual. To verify the simulation results, both CPW-to-RWG transitions using the resonator with the impedance-matching element are fabricated and measured. The simulation and measurement results are in reasonable agreement.

In this manuscript, a compact four-element antenna array is introduced for global navigation satellite system (GNSS) upper L-band applications. The proposed design is modelled using a higher epsilon substrate to obtain a smaller patch footprint. The array consists of four rectangular right hand circularly polarized (RHCP) patches etched on a circular substrate having a compact diameter of only 125 mm. The patch elements cover the BeiDou B1 (1561.098 MHz), GPS L1 (1575.42 MHz), Galileo E1 (1575.42 MHz) and GLONASS G1 (1602 MHz) bands with an axial ratio below 3 dB. A defected ground structure (DGS) has been integrated in the ground plane of the proposed array along with a novel meta-isolator on the top side between the antennas to achieve a high isolation level of more than 24 dB in the complete band of interest. The proposed antenna array has a high gain of more than 6.9 dBi and a radiation efficiency greater than 93%. A prototype of the proposed array is fabricated, and measured results are presented to validate the design.

In the ever-evolving landscape of engineering and technology, the optimization of complex systems is a perennial challenge. Cavity filters, pivotal in Radio Frequency (RF) systems, demand precise tuning for optimal performance. This article introduces an innovative approach to automate cavity filter tuning using Q-learning, enhanced with epsilon decay. While reinforcement learning algorithms like Q-learning have shown effectiveness in complex decision-making, the exploration-exploitation trade-off remains a crucial challenge. The study conducts a thorough investigation into the application of epsilon decay in conjunction with Q-learning, employing the well-established epsilon-greedy strategy. This research focuses on systematically decaying the exploration rateε over time, aiming to strike a balance between exploring new actions and exploiting acquired knowledge. This strategic shift serves to not only refine the convergence of the Q-learning model but also remarkably elevate the overall tuning performances. Impressively, this optimization is achieved with a notable reduction in the number of tuning steps, demonstrating an efficiency boost of up to 45 steps.

The present study examines the negative group delay (NGD) behavior of a circular curved (CC) coupled-line (CL) microstrip circuit with a bandpass (BP) characteristic. The novel CC CL-based circuit is derived from the curved li-topology which demonstrates BP-NGD functionality. The basic theoretical approach enabling the BP-NGD analysis is introduced. The BP-NGD function main properties related to NGD center frequency, NGD value, and NGD bandwidth are defined. Despite the progressive NGD research work, it was wondered how the RF printed circuit board trace geometrical parameters such as curvature radius and angle change the microwave communication parameters. To verify the BP-NGD concept feasibility, different microstrip prototypes are designed, simulated, fabricated, and tested as the proof of concept (POC). Thus, a developed empirical study of CC microstrip structures corroborating well-correlated simulations and experimental results is examined. Moreover, deep sensitivity analyses for geometrical design parameters were performed using commercial tool full-wave simulations. The obtained results provide insights into the effects of CC-structure inter-space and curvature angles on the inherent BP-NGD parameters. The proposed NGD circuit is potentially useful in the future in RF and microwave engineering for signal delay correction. Additionally, it helps in understanding the characteristics of microstrip PCB traces that are important for optimizing signal integrity (SI), power integrity (PI), and electromagnetic compatibility (EMC).

By calculating the compensation phase distribution from the radiation fields of the feed, two approaches are proposed for reflectarrays (RAs) generating circularly polarized orbital angular momentum (CP-OAM) beams with higher mode purity. Particularly, if the radiation fields are extracted in spherical coordinates rather than Cartesian coordinates, the required phase distribution for generating a CP-OAM beam of +1/-1 mode can be directly obtained according to our mathematical derivation which shows that the spherical components of left-/right-hand circularly polarized (LHCP/RHCP) fields naturally involve the OAM phase term of +1/-1 mode. To better demonstrate our work, the CP-OAM RAs with both smooth and corrugated horns of RHCP as feeds are designed by three approaches: the conventional approach (CA) based on approximation of phase center, and the two approaches based on simulated radiation fields in Cartesian coordinates (CCA) and spherical coordinates (SCA), respectively. Full-wave simulation results show that the OAM mode purity can be enhanced by either CCA or SCA, and the SCA can produce even higher mode purities than CCA when an offset feed is employed.

This paper proposes a new low loss high dielectric substrate made of CaTiO₃ incorporated butyl rubber (CBR). The synthesized material has a dielectric loss of 0.0048 and dielectric constant of 11.7. A novel circular anisotropic structure is used in the design of metamaterial unit cell. This metamaterial structure exhibits near-zero refractive index at 2.4 GHz. The extraction of refractive index is done from the scattering parameters using generalised sheet transition condition. An array of 3 × 3 unit cell is the metasurface superstrate, which is proposed for the gain enhancement. It is kept above the microstrip patch antenna working at 2.4 GHz. The proposed superstrate provides a gain enhancement of 4.6 dB. The height between the antenna and the superstrate is optimized to 0.088λ. The enhancement of gain on metasurface substrate with different loss tangents is analyzed. The simulation and measurement results of antenna with superstrate show good agreement with a peak gain of 7.6 dBi. The radiation efficiency of the antenna is increased by 42%.

This paper develops a wideband spin-decoupled unit cell to form high efficiency wideband spin-decoupled metasurface (MTS), which can achieve more versatile Bessel beams with independent control of the beam direction, polarization and Orbital angular momentum (OAM) mode for near field applications. The MTS is designed for wideband dual Bessel beams: Beam-I (RHCP, θ₁=30˚, φ₁=180˚, l=1), Beam-II (LHCP, θ₂=30˚, φ₂=0˚, l=0), where φ and θ are the azimuth and elevation angles, respectively; l is the OAM mode; and RHCP(LHCP) represents the right (left) hand circular polarization. Compared with conventional phase gradient MTSs, the proposed MTS achieves more versatile functionalities and better performance: wideband (35.3%), dual Bessel beams, circular polarization (CP), high aperture efficiency (AE) 40%, carrying OAM modes, high ratio of non-diffraction distance/aperture size (6.41), high conversion efficiency for Bessel beams (33%), and high OAM purity (78%-99%). Simulated and measured results agree well, and validate the design method. The proposed unit cell can be used to design other high performance multifunctional Bessel beams. The designed Bessel beams have potential applications in dense channel high capacity communication, efficient wireless power transfer, high-resolution imaging, medical treatment.

The polarization of the received signals can be effectively matched regardless of the orientation of the receiving antenna using circularly polarized antennas. A compact printed monopole antenna with a circular ring patch is presented in this paper. The proposed antenna will be able to provide two polarization states: linear and circular. The employment of a modified Minkowski fractal with 1st iteration construction on an open circular ring supports a circularly polarized band having a center frequency of 1.812 GHz and fractional bandwidth (FBW) of 20.033%. The upper band is achieved with a linearly polarized wave having a center frequency of 3.386 GHz and a fractional bandwidth of 11.399%. The proposed antenna is fed by an ungrounded co-planar waveguide (UCPW), enhanced with an impedance transformer to match at 50 ohms with the characteristic impedance. The ground planes around the feed line are defected by ground structures to improve the antenna gain. The fabrication and measurement of the proposed antenna prototype are presented to validate the theoretical results. Measured results support the theoretical findings well.

In order to improve the stability of sensorless high-speed operation of permanent magnet synchronous motors and effectively expand the speed range, a voltage closed-loop flux weakening sensorless algorithm based on the active flux model is proposed. Firstly, the mathematical model of sensorless control of the PMSM is designed based on the active flux model, and the state of the PMSM flux weakening operation is analyzed. Then, based on the voltage closed-loop flux weakening control method, the corresponding flux weakening control algorithm is analyzed and designed. Meanwhile, based on the active flux model, the speed and rotor position of the motor are observed by sliding mode observer and phase-locked loop method. After that, the flux weakening control method and the sensorless control method are combined to realize the sensorless flux weakening control method to improve the stability of the control system. Finally, the proposed algorithm is validated on the experimental platform. The experimental results show that the proposed method can prevent the system from losing control during flux weakening processes, effectively improve system stability, and have smaller angle errors. The speed convergence time is shortened by 80% compared to the non flux weakening control.

With combining the advantages of the hybrid excited machine and field modulation machine, hybrid excitation field modulation machine (HEFMM) exhibits obvious merits of controllable flux operation and independent flux distributon paths. A total copper loss minimization model is established to determine the optimal ratios of field current, d-axis and q-axis currents in low speed region. In high speed operating region, one flux weakening fuzzy control strategy combining with particle swarm optimization (PSO) algorithm for HEFMM was proposed, which improves the dynamic characteristic and decrease the harmful influence of parameters variation during the operation region of HEFMM. The correctness and effectiveness of the proposed flux weakening fuzzy control strategy were verified by the simulation data and experimental results, which demonstrated that the this current optimization method based on PSO algorithm can effectively reduce the total copper loss of the machine by 22%, the range of speed regulation with higher efficiency are ontained.

In this paper, a hexa-slot wheel shaped fractal antenna is proposed for UWB applications. Fractal structure (wheel shaped hexa slots) is developed by inscribing hexagons and circles inside a circular patch monopole antenna. Then orthogonal MIMO antenna with polarization diversity is proposed that operates from 1.39-15.18 GHz with an impedance bandwidth of 13.79 GHz. Its performance is analyzed at three resonant frequencies 5.8 GHz, 8.3 GHz and 10 GHz. It exhibits good diversity performance with ECC < 0.035, DG very close to 10, isolation > 35 dB, CCL < 0.35 bits/s/Hz values. The proposed MIMO fractal antenna achieved an average radiation efficiency of 83.11% with 3.26 dB maximum peak gain. TARC characteristics and time domain behavior using group delay are also examined. Simulated and measured results are in good agreement and hence the proposed MIMO antenna with polarization diversity is well suitable for UWB applications.

In this paper, we develop two new algorithms for snow water equivalent (SWE) retrieval based on the volume scattering snow at X (9.6 GHz) and Ku (17.2 GHz). Significantly, neither algorithm requires a prior on grain size or on scattering albedo. The two algorithms are based on modifications of the previous algorithm published in our previous two papers (Zhu et al. 2018, 2021). The physical model is the bi-continuous DMRT model, and a parametrization is carried out over a look-up table of DMRT results. The parameterized model gives the X and Ku band co-polarization backscatter as a pair of equations in terms of two parameters SWE and scattering albedo at X band (ω_X). By directly inverting the pair of equations for, σ^X (SWE, ω_X) and σ^Ku (SWE, ω_X), we show that there are at most a pair of solutions which have SWE values that are far apart in most cases, facilitating identification of the correct solution. The first algorithm described in this paper, labelled an algebraic algorithm, uses inversion alone and does not employ a cost function. The proposed algebraic algorithm is validated with multiple airborne data sets and three years of tower-based snow observations. The robustness of the no-prior approach was validated with the airborne observations, by using a prior SWE value that is intentionally far (75% different) from the true SWE. For the validation using tower-based data, three years of observations from the NoSREx experiment in Sodankyla, Finland were used in which the previous SWE result helps to correctly choose between the two solutions. The second cost function-based algorithm finds the SWE and ω_X pair which minimizes the difference between the observed volume scattering σ^X,obs and σ^Ku,obs and the model-predicted volume scattering σ^X,mod and σ^Ku,mod. The cost function uses prior information on SWE, also based on a time series starting with zero/low SWE. NoSREx data is used to show results from this approach. The new algorithm combined with time series eliminates needs of ancillary information of SWE and grain sizes, making the algorithm useful for level-2 products of a satellite mission.

In this paper, we propose a new sparse linear array (SLA) that enjoys a hole-free difference co-array (DCA) and closed-form expressions for its sensor positions. The proposed configuration is valid for arrays containing seven or more sensors (N≥7). Exact expressions for the array aperture and achievable degrees of freedom (DOFs) have been derived. Numerical simulations were performed using MATLAB to reinforce the theoretical understanding. The main aim of this study is not to claim superiority over any existing SLA design, but to report that we have found a new number sequence that can act as an SLA. Except for being highly susceptible to mutual coupling, the proposed array has all the desirable features of a good SLA. We observed that the proposed array is on par with other SLAs for N<20. However, for 20 or more sensors, the array aperture does not scale rapidly in proportion to the added sensors and fails to match the resolution and/or DOFs offered by other sparse arrays. Nevertheless, the proposed sparse array is based on a unique and previously unknown number sequence.

Wave propagation in forests at L-band has essential applications in satellite communication system design, foliage penetration (FOPEN), and remote sensing of forest canopy and soil using passive, active, and reflectometry techniques. In this work, we propose applying the fast hybrid method (FHM) for full wave simulations of forests. The FHM significantly improves CPU time and memory efficiency for full-wave electromagnetic solutions. In this paper, we present simulations of forests of up to 72 trees with heights up to 13 m with FHM. Spatial distributions of electric fields at the bottom plane of the trees are illustrated, showing constructive and destructive interferences. The electric field distributions show that the amplitudes of the electric fields can be as large as twice that of incident waves. The transmissivities are computed and averaged over realizations based on the electric fields underneath the forest. The simulations were performed on a desktop and required a CPU time of only 1346 seconds and the memory of 16.5 GB for the case of 72 13-m tall trees, demonstrating that the FHM method is substantially more efficient than the available commercial software. The results show that the L-band signals can penetrate forests to sense the soil moisture and detect targets hidden within forests, as evidenced by significant electric field intensities under forest canopies. Also, we illustrate that GPS signals can penetrate forests and be successfully received by GPS receivers. In the study on clustering effects, we present two distinct solutions for transmissivities, each corresponding to different spatial distributions of trees while maintaining the same average tree density.