This paper proposes a wideband, low-profile, circularly polarized (CP) crossed-dipole antenna with an artificial magnetic conductor (AMC) reflector. The antenna consists mainly of a pair of orthogonal half-wavelength dipoles, two 90$^\circ$ phase-delay rings, four parasitic patches, four pairs of vertically arranged metallic strips, an AMC reflector, and a square ground plane. CP radiation is generated by exciting bow-tie dipole arms via phase-delay rings. Introducing parasitic patches and vertical metallic strips optimizes the surface current distribution, broadening the impedance-matching bandwidth and axial ratio bandwidth (ARBW). Meanwhile, using the AMC structure as a reflector effectively reduces the antenna profile. We fabricate a prototype of the proposed antenna to verify consistency between simulated and measured results. The measured results show that impedance bandwidth (IBW) is 1.51-2.54 GHz (50.9%), and ARBW is 1.53-2.51 GHz (48.5%). The antenna achieves a peak gain of 8.5 dBic and an average gain of 7.05 dBic across the entire operating band. Furthermore, the antenna features an ultra-low profile of 0.12λ_L at the lowest operating frequency.

In this paper, we propose a compact quad-band bandpass filter by using short-ended stepped-impedance resonators (SE-SIRs) and open-ended stepped-impedance resonators (OE-SIRs) in a hybrid structure. The proposed design uses the odd- and even-mode resonances of the two types of resonators to produce four independent passbands in a compact multilayer structure. The first and second passbands are set by SE-SIR resonators at 1.56 and 2.5\,GHz, respectively. The third and fourth passbands are centered at 3.5 and 5.2\,GHz, respectively, due to the OE-SIR resonators. To achieve the quad-band response in a compact size, a multilayer arrangement with electromagnetic broadside coupling between the feeding structure and resonators has been adopted. Finally, the filter is designed, simulated, fabricated, and measured using a Rogers-Ceramic RO4360 substrate with a permittivity of ε_r = 6.15 and thicknesses of (h₁ = 0.3, h₂ = 0.508) mm. Simulated insertion losses were 0.29, 0.26, 0.43, and 0.66 dB, and return losses were 28.65, 15.05, 14.04, and 17.3 dB in each of the four operating bands, respectively. Filter responses from simulation and measurement are contrasted and discussed. The produced filter is very small, covering a circuit area of about 129 mm², without feeding ports.

This paper presents the design, numerical modeling, and experimental validation of a compact, collocated multi-port reconfigurable antenna array optimized for high-density WLAN access points. The proposed structure supports robust dual-band operation covering the 2.4-2.5 GHz and 5.2-5.8 GHz frequency ranges, exhibiting a measured reflection coefficient well below -10 dB across both bands. Polarization diversity is successfully achieved through orthogonally oriented, horizontally and vertically polarized radiation elements, ensuring an isolation higher than 17 dB between adjacent ports. Furthermore, an integrated electronic switching network based on PIN diodes enables flexible radiation pattern control, allowing seamless switching between a 360° omnidirectional coverage mode and multiple directive beam states. The omnidirectional configuration provides a stable baseline for wide-area connectivity, while the directive modes focus electromagnetic energy into specific spatial sectors, boosting the peak total gain from 5.16 dBi to 7.14 dBi. This dynamic beam-steering capability significantly mitigates multi-path fading and optimizes Multiple-Input Multiple-Output (MIMO) system performance in complex indoor environments. The entire array architecture occupies a compact physical footprint of only 200 mm × 200 mm, with excellent agreement between simulated and measured results, confirming its suitability for next-generation WLAN infrastructure.

This study presents a compact frequency-selective metasurface for transmissive and reflective polarization conversion over the V-band (40-65 GHz). The structure consists of two Rogers RO4003C substrates separated by a 0.2-mm air gap and interconnected by a metallic post to enhance electromagnetic coupling. We performed parametric optimization to obtain multiple polarization-conversion bands. The optimized unit cell has a periodicity of 3.9 mm and employs resonator dimensions of L₁ = 2.6 mm, L₂ = 2.1 mm, W = 1.0 mm, S = 0.3 mm, H = 0.2 mm, and D = 0.3 mm. We conducted full-wave simulations and free-space measurements using WR-19 horn antennas and a vector network analyzer for validation. The proposed structure exhibits frequency-selective polarization-conversion behavior, with the transmission conversion exceeding 90% over approximately 40-49 GHz and near-unity conversion at selected frequencies, while the reflection conversion approaches 100% at resonant frequencies of approximately 58 and 60.5 GHz. The fabricated prototype showed good agreement with simulations, with minor discrepancies attributed to fabrication and measurement uncertainties. The proposed structure is promising for millimeter-wave communications, intelligent electromagnetic surfaces, radar sensing, transmitarray antennas, and future 6G systems.

Virtual synchronous generator (VSG) control is a representative grid-forming strategy for converter-interfaced renewable generation. In a conventional VSG, active-power feedback is calculated directly from terminal voltage and current, allowing voltage distortion and weak-grid-induced oscillatory components to enter the swing loop. This paper proposes a flux-linkage-based active-power calculation that reconstructs electromagnetic flux linkage from the measured converter voltage and evaluates active power through the flux-current relation. The calculation is integrated into a VSG controller with cascaded voltage and current inner loops. A small-signal impedance model and PSCAD/EMTDC simulations are combined to characterize the feedback-interaction mechanism and validate the method under grid-impedance, operating-point, disturbance, and filter-parameter variations. Compared with the conventional voltagebased VSG, the proposed method reduces steady-state active-power peak-to-peak ripple by 56.7-65.8% and frequency ripple by 42.6-66.7% in the grid-impedance sweep, and reduces the non-dc active-power component by 69.9-72.7% across the tested operating points, while maintaining comparable terminal current and voltage THD. Closed-loop active-power-command tests further quantify the associated reactive-power and local terminal-voltage responses under three converter-grid interface conditions. The impedance model identifies a 37.3% lower normalized resonance peak in the 40-250 Hz interaction band, while an electromagnetic transient (EMT) spectral study sampled at 50 kHz characterizes the adjacent 10-40 Hz and 500 Hz-10 kHz bands. These results demonstrate that flux-linkage-based power feedback improves the quality of the signal driving the virtual swing equation within a practical VSG architecture.

To resolve the trade-off between numerical accuracy and computational efficiency when integrating the adaptive cross approximation (ACA) into the characteristic basis function method (CBFM), this paper proposes a hybrid optimization framework incorporated with Krylov subspaces. This framework first employs CBFM to partition scattering targets into multiple subdomains and realize dimension reduction of the impedance matrix. It then adopts the ACA algorithm to perform low-rank compression on far-field coupling submatrices. On this basis, the GMRES algorithm is introduced to implement iterative optimization. The framework uses the approximate solution from CBFM as the initial iterative guess, and it leverages ACA to accelerate constructing Krylov subspaces for reducing iterative computational overhead. Numerical results of two sets of test cases demonstrate that, compared with the traditional coupled CBFM-ACA solver, the proposed method can significantly improve numerical accuracy while maintaining favorable computational efficiency.

This work presents a compact four-port low-profile multi-input multi-output(MIMO) antenna designed for wideband wireless communication over the S(2-4 GHz), C(4-8 GHz), and lower X-band(8-12 GHz) spectrum. The proposed antenna initially exhibits a broad impedance bandwidth extending from 2.5 to 9 GHz under the standard -10 dB matching condition. To improve spectral selectivity and mitigate unwanted interference, U-shaped slots are embedded in the feeding line and radiator, generating two distinct rejection bands at 3.5-3.9 GHz and 5.1-5.9 GHz, corresponding to WiMAX and WLAN frequencies, respectively. Including these slots slightly shifts the upper operating limit to 8.5 GHz. Implemented on an FR4 dielectric substrate (ε_r = 4.4) with a thickness of 1.6 mm, the antenna occupies an overall footprint of 40 × 34 mm². Additionally, circular polarization is achieved across the 6.1-7.9 GHz region, enhancing communication reliability in multipath propagation environments. A common-ground decoupling structure is employed to improve port isolation and suppress mutual coupling, resulting in isolation greater than 20 dB. The antenna demonstrates favorable diversity characteristics, with an envelope correlation coefficient below 0.05 and a diversity gain exceeding 9.95 dB. Moreover, a maximum realized gain of 6.2 dBi is obtained within the operational band. Experimental validation of the fabricated prototype confirms close agreement with simulated results, highlighting its potential for compact wideband multi-antenna wireless systems.

Silicon nitride (Si₃N₄) has emerged as a compelling platform for visible-light, quantum-photonic, and nonlinear-optical applications because of its broad transparency window, low propagation loss, and negligible two-photon absorption. However, the moderate refractive index of Si₃N₄ poses a challenge for efficient fiber-to-chip coupling using grating couplers. Although advantageous for wafer-scale testing and flexible layouts, Si₃N₄ grating couplers suffer from low grating strength, which limits their coupling efficiency. Here, we experimentally demonstrate high-efficiency grating couplers on a hybrid amorphous-silicon/silicon-nitride (a-Si/Si₃N₄) platform. Uniform devices are presented for TE and TM polarizations in both the O- and C-bands, while apodized-focalizing designs are demonstrated for both polarizations in the O-band and for TE polarization in the C-band. The fabricated devices achieved record coupling efficiencies of -1.6 dB and -1.2 dB in the O-band for TE and TM polarization, respectively, and -1.3 dB and -2.9 dB in the C-band for TE and TM polarization, respectively. The measured peak wavelengths deviated from their nominal values, primarily because of variations in the refractive index of the fabricated Si₃N₄ and the grating duty cycles relative to their design values. These shifts are well reproduced by simulations incorporating fabrication biases and can be compensated for by adjusting grating parameters or the fiber coupling angle. To the best of our knowledge, these are the highest coupling efficiencies demonstrated to date for silicon nitride grating couplers in the O-band for both polarizations and in the C-band for TM polarization. For TE polarization in the C-band, fabrication deviations shifted the peak coupling wavelength to 1512 nm, resulting in the highest efficiency reported to date for a silicon nitride grating coupler operating in the S-band. These results demonstrate a practical approach for achieving low-loss fiber-to-chip coupling in Si₃N₄ photonic integrated circuits, addressing a longstanding challenge in the platform and supporting the development of scalable systems for datacom, telecom, nonlinear photonics, and quantum technologies.

A miniaturized, multiband, Fibonacci-inspired MIMO antenna is proposed for modern wireless communication technologies, such as sub-6 GHz 5G, WiMAX, WLAN, and Wi-Fi. The proposed antenna incorporates a Fibonacci spiral as the radiating element along with an enhanced defected ground structure (DGS). This configuration provides improved impedance matching, low mutual coupling, and strong diversity characteristics. The antenna employs a slot-loaded decoupling strip and an optimized ground plane to suppress surface currents and enhance isolation between elements. In its final two-element configuration, the antenna exhibits three distinct resonant frequencies at 1.1 GHz, 4.22 GHz, and 5.65 GHz. Both simulated and measured results demonstrate excellent impedance matching, achieving a minimum return loss of -27.22 dB at 4.22 GHz and an isolation level better than -20 dB throughout the operating frequency range. Moreover, the envelope correlation coefficient (ECC) remains below 0.05, while the diversity gain exceeds 9.95 dB. The antenna also achieves a radiation efficiency ranging from 75% to 85%. Surface current analysis further demonstrates that the combination of the Fibonacci-inspired geometry and DGS effectively produces multiband operation while minimizing mutual coupling.

A compact, flexible, ultra-high-frequency (UHF) radio-frequency identification (RFID) tag antenna is proposed for the long-range identification of non-metallic logistics objects with robust horizontal readability. The antenna consists of an open-frame meandered radiator, an upper transverse coupling path, a central interdigital coupling section, and a lower comb-shaped multi-branch section. Together, these coupled sections extend the effective current path, improve antenna-chip power transfer, and establish a distributed current response over the antenna. The fabricated tag measures 40 mm × 29.5 mm × 0.05 mm. Under an effective isotropic radiated power of 3.28 W, the maximum measured read range is 16.6 m at 919 MHz in free space, with a 0°-360° xoy-plane read range of 15.2-16.6 m. Attachment tolerance is evaluated on PP plastic, folded denim, cardboard, and wood, yielding maximum read ranges of 16.7, 13.2, 11.4, and 11.2 m, respectively. Across all five cases, the horizontal stability coefficient C_s = R_min/R_max is at least 0.895. The proposed coupled multi-section topology provides a distributed-current strategy for simultaneously improving angular readability and attachment robustness in flexible UHF RFID tags.

This paper provides a detailed discussion of anomalous dispersion studies of polyaniline carbon nanotube composites (PANI-CNT) in the X-band frequency range. Dielectric properties are used to demonstrate the material's anomalous behavior. Measurement analysis is performed using a network analyzer; the anomalous medium shows a phase advance, while the other shows a gradual phase decrease. By correlating these characteristics, we can observe their significant impact on the material's microwave properties. We present PANI-CNT powder throughout the study. PANI-CNT exhibits enormous microwave enhancement characteristics compared to standard materials. This paper examines various microwave parameters, including permittivity and shielding efficiency. The novelty of the anomalous effect lies in the significant enhancement of shielding efficiency it produces.

This manuscript presents a simple, novel design of a dual broadband absorber with merged double bands and double-split concentric ring resonators in each quadrant-based metamaterial absorber (DBBMDBMMA). The proposed structure exhibits two experimentally validated absorptance broadbands; one extending from 7.73 GHz-9.54 GHz and the other extending from 10.65 GHz-13.27 GHz. The two broadbands cover X-band and Ku-band, respectively. The DBBMDBMMA is fabricated on a double-sided FR4 copper plate with electrical dimensions of 0.5λ_o × 0.5λ_o × 0.05λ_o (λ_o at 8.5 GHz). The unit cell of the metamaterial absorber consists of concentric double split-ring resonators in each quadrant, with splits oriented vertically opposite and separated by a 90° inclination difference. The novelty lies in the response and design of the proposed structure that exhibits dual broadbands extending in two microwave bands without using parasitic elements, a multilayered structure, or an extended substrate width. The response of the DBBMDBMMA structure is stable with variation in incidence and polarization angle. The metamaterial behaviour is studied in terms of effective values of normalized matched impedance, permeability and permittivity. The proposed DBBMDBMMA exhibits reflectance of -21.62 dB at 8.59 GHz, -26.18 dB at 8.88 GHz, -15.11 dB at 11.24 GHz, and -18.63 dB at 12.86 GHz. The first absorptance peak is contributed by outer split-ring resonators in quadrants II and IV; the second absorptance peak is contributed by outer split-ring resonators in quadrants I and III; the third absorptance peak is contributed by inner split-ring resonators in quadrant II and IV; and the fourth absorptance peak is contributed by inner split-ring resonators in quadrants I and III. The proposed DBBMDBMMA structure exhibits polarization and wide incident angle insensitive response that finds applications in stealth technology and RCS reduction.

This paper proposes a compact harmonic communication rectifier for a simultaneous wireless information and power transfer (SWIPT) receiver front-end employing quadrature frequency-shift keying (QFSK) modulation. First, we theoretically analyzed the feasibility of communication using the second harmonic of QFSK signals. Second, a compact harmonic communication rectifier is proposed by combining a frequency-selective duplexer and an impedance-matching network. The duplexer with two output branches partially extracts the low-power second harmonic for communication and fully transmits the fundamental wave for wireless power transfer. The matching network with π-type can transmit the fundamental wave to the diode for rectification and adjust the energy-information power ratio for practical requirements. Finally, the proposed rectifier is fabricated and measured with good agreement between simulation and measurement results. A rectifying efficiency of 76.7% is achieved for QFSK-modulated waves with a carrier frequency of 2.5 GHz and a frequency offset of 20 MHz at an input power of 14 dBm. The extracted second-harmonic power is -5.3 dBm with an energy-to-information power ratio of 64:1. The signal-to-noise ratio (SNR) is 30 dB and the bit error rate (BER) is less than 10^-12.

Fast, accurate modeling of permanent magnet synchronous machines (PMSMs) is essential for efficient design optimization, particularly in high-power wind turbine applications where severe magnetic saturation frequently occurs. While pure analytical models offer rapid computation, they often fail to accurately account for nonlinear saturation, which significantly affects the machine's performance, particularly the back-electromotive force (back-EMF). This study proposes a computationally efficient 2D hybrid analytical framework to investigate the impact of magnetic saturation on the back-EMF in PMSMs. The proposed approach couples an exact analytical subdomain (SD) technique, which is used to resolve the magnetic field in linear regions, with a nonlinear magnetic equivalent circuit (MEC) model for the active ferromagnetic parts. By iteratively updating the relative permeability in the MEC, the method accurately captured local saturation effects and reflected them in the global air-gap flux density. The developed hybrid analytical model (HAM) was used to analyze the total harmonic distortion (THD) and peak values of the back-EMF under saturated conditions. To validate the proposed method, results were compared with those obtained from a 2D finite element analysis (FEA). The comparison shows that the hybrid approach achieves an excellent compromise, delivering a high accuracy comparable to that of the FEA while drastically reducing computational time, making it highly suitable for iterative design and optimization process.

Compared with widely used half-duplex relaying, the development of full-duplex (FD) relaying devices enables simultaneous transmission and reception, which can be desirable for spectrally efficient 6G Internet of Things (IoT) networks. To improve the reliability of FD relaying, maximum ratio transmission (MRT) can be incorporated into an FD decode-and-forward (DF) relaying architecture, making the resulting scheme practical for diverse applications. In this study, critical performance metrics, such as the outage probability, symbol error rate and channel capacity were derived for the Nakagami-m fading channel model. Numerical examples obtained via Monte Carlo simulations corroborate the theoretical expressions and demonstrate that MRT can efficiently mitigate residual self-interference while preserving low receiver complexity. These findings provide useful insights for system designers to consider FD-DF relaying with MRT as a practical and effective solution for next-generation wireless ecosystems.

This paper proposes a dual-port Ultra-Wideband (UWB) Multiple-Input Multiple-Output (MIMO) antenna with triple notch characteristics based on Coplanar Waveguide (CPW) feeding. The antenna achieves triple notch performance by introducing two L-shaped and one inverted U-shaped electromagnetic band gap (EBG) structures on the back of the substrate and etching inverted U-shaped grooves on the feed line, thereby suppressing interference from WiMAX, WLAN, and the C-band. The simulation and experimental results show good agreement, verifying the feasibility of the proposed antenna. The results show that this antenna covers the entire UWB band from 3.08 to 10.87 GHz, with a reflection coefficient of less than -10 dB, mutual coupling coefficient of less than -20 dB, envelope correlation coefficient (ECC) of less than 0.0072, and diversity gain (DG) exceeding 9.9997 dB within the operating band. It meets the requirements of high-performance MIMO systems for channel independence and transmission efficiency, and has potential applications in modern UWB communication systems.

Large metropolitan areas have planned different types of rail transit integrating into transportation hubs due to the needs of integrated development. Owing to scenarios such as parallel or cross-line operation of a junction station, a complex electromagnetic environment may affect the normal operation of BTM equipment. Therefore, it is necessary to study the influence of the trackside electromagnetic environment on the BTM antenna. Taking the scenario in which the parallel section of a mainline railway and an urban rail transit enters the station, as well as the situation in which two trains are adjacent at the junction station, as an example, the model of the BTM antenna is established to analyze the electromagnetic interference of urban rail transit trains in operation. The induced voltage of the BTM antenna in urban rail transit trains is calculated based on electromagnetic environment data from the electromagnetic disturbance source collected on site to analyze the influence on its normal operation. The amplitude of the induced voltage at the BTM antenna port is lower than its normal operation scenario when the parallel section of a mainline railway and an urban rail transit enters the catenary hard across section. The amplitude satisfies the requirement of its starting electromagnetic environment when the urban rail train stops at the platform while the mainline train passes at speed. Under a multi-network integration background, research results show that the trackside electromagnetic environment along the mainline railway will not cause an electromagnetic interference problem for the BTM antenna of the urban rail train.

Time-modulated metasurfaces (TMMs) enable low-observable and reconfigurable radar deception, but existing approaches remain limited in synthesizing complex range-velocity (R-V) false-target clusters because of forward-designed modulation schemes and single-platform operation. This paper presents a task-oriented framework for multidimensional false-target synthesis against frequency-modulated continuous-wave (FMCW) radars. An analytical model explicitly connects TMM temporal modulation with the FMCW R-V estimation chain. Intra-frame modulation reshapes the echo spectrum to generate programmable range offsets, while inter-frame control modifies the slow-time response to generate programmable velocity offsets. The modulation-sequence design is then formulated as a combinatorial inverse-design problem and solved using Physics-Informed Initialization-Assisted Particle Swarm Optimization (PIIA-PSO), which exploits analytical R-V relations to construct a physically informative initial population, thereby reducing ineffective early-stage exploration and improving convergence efficiency toward high-quality realizable solutions. The framework is further extended to distributed multi-platform cooperation to increase spatial degrees of freedom and enable coherent energy aggregation. Results show range and velocity errors below 10% for single-platform synthesis and improve the overall SSIM by 7-10 percentage points under distributed configurations, demonstrating enhanced synthesis fidelity, flexibility, and scalability.

This paper presents a compact, low-cost, frequency-domain bistatic radar system using two identical square microstrip patch antennas fabricated on an FR-4 substrate (εr = 4.4, thickness = 1.6 mm). The antenna incorporates a suspended air-gap configuration to enhance bandwidth and radiation efficiency, and is designed to resonate near 3.3 GHz. The antenna was designed and optimised using CST Microwave Studio and experimentally validated using a Vector Network Analyser (VNA). Simulation results demonstrate a peak gain of 6.6 dBi and radiation efficiency of 83.2%. The fabricated antenna exhibits a measured peak gain of 7.0 dBi at 3.23 GHz and a |S₁₁| ≤ −10 dB bandwidth from approximately 3.0 to 3.5 GHz, with a measured resonance minimum of −14.3 dB at 3.27 GHz. The bistatic radar system operates over a swept frequency range of 2.8 to 3.8 GHz, acquiring complex S₂₁ parameters at each discrete frequency step. The received S₂₁ data is processed using background subtraction to suppress antenna coupling and background clutter, followed by an Inverse Fast Fourier Transform (IFFT) for range-domain target localisation. The proposed system is experimentally validated in single-target and multiple-target scenarios. Closely spaced targets are further characterised using a CLEAN iterative deconvolution algorithm as a post-processing step. Consistent target localisation across all scenarios is confirmed by two-dimensional radar images obtained from cross-range scanning. The results demonstrate the feasibility of a low-cost bistatic radar platform for short-range sensing applications including security scanning, remote target localisation, and multi-target detection

In this study, a coplanar waveguide (CPW)-fed half-moon monopole ultra-wideband (UWB) antenna is proposed, simulated, fabricated, and experimentally validated for ground-penetrating radar (GPR) applications. The antenna is specifically designed to provide high forward gain and stable wideband performance via integrating a full-copper metallic reflector. The proposed antenna exhibits good impedance matching and satisfactory reflection coefficient characteristics across the UWB frequency range of 3.1-10.6 GHz. By incorporating the metallic reflector, a significant radiation enhancement is achieved. The antenna attains a peak gain of 8.7 dBi at 6 GHz and maintains a gain above 6.5 dBi throughout the operating bandwidth. This gain improvement and suppression of back radiation are particularly important for GPR systems, where deep subsurface penetration and high-resolution imaging are essential. The integration of a metallic reflector provides a simple yet highly effective approach for gain enhancement without complex structures or advanced materials, as confirmed by both simulated and experimental results. These findings demonstrate the suitability of the proposed antenna for practical high-gain UWB GPR and broadband sensing applications.