This paper describes the design methodology of a compact multiband microstrip patch antenna intended for next-generation wireless communication applications. The proposed antenna operates over seven distinct frequency bands: 1.25-1.32 GHz, 2.30-2.44 GHz, 2.50-2.75 GHz, 2.92-3.25 GHz, 3.40-3.65 GHz, 3.70-4.23 GHz, and 4.70-6.0 GHz. These operating bands support a wide range of wireless services, including LTE, 5G communications, Wi-MAX, ISM applications, radar systems, and broadband wireless communications. Multiband performance is achieved through the incorporation of three strategically placed slits in the radiating patch along with a square split-ring resonator (SSRR). By adjusting the dimensions of the slits and the position of the SSRR, the operating frequency bands can be effectively tuned. The proposed antenna occupies a compact footprint of 40 × 40 mm² and consists of a radiating patch, a partial ground plane, and an SSRR structure. Simulation results demonstrate resonant frequencies at 1.3, 2.38, 2.66, 3.0, 3.5, 4.2, 4.9, and 5.7 GHz. Owing to its compact size, multiband capability, and simple structure, the proposed antenna offers advantages in terms of reduced cost, lower system complexity, and miniaturization, making it suitable for modern wireless communication systems.

Due to the high electrical conductivity, relative permittivity, and magnetic permeability of seawater, the propagation behavior of electromagnetic fields differs significantly from that in air. The conductive nature of seawater causes strong eddy current loss and magnetic field attenuation, thereby reducing the effective coupling coefficient and resulting in frequency detuning between the transmitter and receiver coils. Moreover, the marine environment introduces parasitic impedance paths and additional energy dissipation due to the conductive medium, which further decreases transmission efficiency. These unique electromagnetic characteristics make the design and optimization of wireless power transfer (WPT) systems in seawater more complex and challenging than in air, motivating this study to develop and analyze a dual-receiver WPT architecture that improves midrange transmission efficiency under underwater conditions. To address this issue, a single-transmitter dual-receiver (1TX-2RX) WPT system operating in the 300-550 kHz frequency range is designed and implemented. Experimental results demonstrate that, under midrange transmission in seawater, the efficiency of the proposed 2RX architecture improves markedly from 12% in the 1RX system to 25%, while maintaining stable output performance under various receiver coil misalignment conditions. In addition, compared with operation in air, the optimal operating frequency of the 2RX system in seawater shifts leftward from approximately 460 kHz to 410 kHz. To better characterize the impact of seawater on transmission performance, complex impedance and mutual inductance parameters are incorporated into the conventional circuit model, enabling effective representation of the additional losses and coupling attenuation induced by the conductive medium. The predicted load voltage is consistent largely with the experimental measurements, validating the accuracy and applicability of the proposed modeling approach. Overall, this study not only verifies experimentally the feasibility of improving midrange transmission efficiency through a dual-receiver architecture but also establishes theoretically a circuit modeling method suited better for seawater environments, providing useful insights for the design and optimization of marine WPT systems.

The design of large-scale coding metasurfaces poses significant computational challenges, often limited by the prohibitive time required for full-wave simulations necessary for optimization. This paper proposes an efficient design strategy based on a Hybrid Genetic Algorithm, validated through the design, fabrication, and characterization of an X-band metasurface for Radar Cross Section reduction. The proposed design strategy relies on a two-stage optimization process: a fast pre-optimization phase, based on the analytical Huygens-Fresnel principle, generates a preliminary solution which is subsequently refined by a second optimization stage utilizing full-wave simulations. Specifically, the optimization targets a 1-bit coding scheme, where meta-atoms switch between two distinct states with a phase difference of 180 ± 37°. This hybrid approach demonstrates optimal convergence, reducing computational time by 25% compared to traditional full-wave-only techniques. Furthermore, a novel ``spiralling cross'' unit cell topology is introduced. Owing to its delay-line geometry, this structure provides additional degrees of freedom for spectral tuning and supports intermediate phase shifts, thus enabling encoding schemes beyond traditional 1-bit configurations. Experimental results confirm the validity of the proposed approach, demonstrating how the combination of versatile geometry and hybrid optimization effectively overcomes the trade-offs between numerical accuracy and computational efficiency.

This article describes a four-element ultra-wideband (UWB) Multiple-Input Multiple-Output (MIMO) antenna for the Sub-7 GHz. The antenna element is an elliptical patch slot antenna with a coplanar waveguide (CPW) feed. To make the bandwidth wider, we add step-by-step parasitic stubs based on the results of the Characteristic Mode Analysis (CMA) analysis, which can control multiple modes better. The four-element antenna is connected to a common ground structure with a central cross-shaped isolation stub and also inhibit the propagation of surface waves. It can be used in the range from 1.08 GHz to 8.70 GHz (155.8%), which is fabricated on a low-cost FR4 substrate. In this range, the isolation between antenna elements exceeds 20 dB. The performance of the design is very good. The envelope correlation coefficient (ECC) is less than 0.02, and the diversity gain (DG) is greater than 9.96.

Accurate prediction of magnetic interaction forces is important for electromagnetic actuators, inductive coupling systems, and calibration devices. This paper presents a unified analytical and numerical framework for evaluating the axial magnetic force between two finite-dimensional, perfectly coaxial, air-core cylindrical coils under steady currents. The model assumes uniform purely azimuthal current density and neglects radial and axial current components, winding-pitch effects, magnetic materials, misalignment, and transient phenomena. Starting from the Biot-Savart law and Lorentz force formulation, the coil-coil interaction integral is derived and reduced using cylindrical symmetry, leaving only the axial resultant force. Three complementary methods are developed in MATLAB: a semi-analytical elliptic-integral formulation, a direct trapezoidal numerical-integration method, and a filament-based mutual-inductance method. The methods are computationally benchmarked for representative thin-wall, moderate finite-radius, mixed-radius, and large finite-radius coil geometries. The results show consistent force predictions, with relative half-spread values below approximately 4% for the cases considered. Discretization sensitivity and error-source analysis are included to clarify numerical accuracy and convergence. The proposed framework provides a transparent benchmark for axial force evaluation in idealized coaxial air-core coil systems.

The proposed symmetric quad circular radiator antenna with a semicircular etched-slot DGS was designed and fabricated on an FR4 substrate of size 50 × 50 × 1.6 mm³. The antenna provides impedance bandwidths of 2.24-2.94 GHz, 3.84-3.98 GHz, and 4.92-5.06 GHz, with resonant frequencies observed at 2.44 GHz (-40.3 dB), 2.86 GHz (-17.9 dB), 3.92 GHz (-14.9 dB), and 4.98 GHz (-10.6 dB). These operating bands make the antenna suitable for WLAN, ISM, and other sub-6 GHz wireless communication systems. The antenna also achieves a realized gain above 5 dB at the operating frequencies, indicating stable radiation characteristics. The diversity performance was evaluated using standard metrics. The ECC remains below 0.5, which lies within the acceptable range for diversity operation. In addition, the diversity gain varies between 9.88 and 10.02 dB, while the channel capacity loss stays below 0.6 bits/s/Hz across the frequency range. A compact symmetric quad circular radiator antenna incorporating a three-segment semi-circular bridge along with a semicircular etched-slot mdefected ground structure (DGS) is presented for multi-band wireless communication applications. These results confirm that the proposed antenna provides reliable radiation and diversity performance for practical wireless communication applications.

This paper presents a miniaturized implantable antenna with multi-resonances operating at the Medical Implant Communication Service (MICS) band (402-405) and Industrial, and Medical (ISM) band (433.1-434.8 MHz, 868-868.6 MHz, and 902-928 MHz) for advanced biomedical applications. The proposed antenna is notably compact, occupying a volume of 10 × 10 × 1.27 mm³ (equivalent to 127 mm³). Multi-resonance frequencies are generated by incorporating a shorting pin and a meandered resonator structure. The proposed antenna exhibited wideband characteristics, with bandwidth ratios of 39.5% and 28.8% at 402 and 915 MHz, respectively. Moreover, the performance of the implantable antenna was further validated in different organs within a realistic human body model, such as the heart, stomach, small intestine and colon. The practical performance of the fabricated antenna prototype was validated using a tissue-equivalent liquid phantom. Additionally, to evaluate the transmission performance under real-world scenarios, an on-body antenna matched with the implanted antenna was designed for an in-body to on-body transmission setup. Under the maximum safe input power of 25 μW, link budget analysis demonstrates that data can be transmitted at a rate of 10 Mbps over distances of 9.5 and 12 cm in the MICS and ISM bands, respectively. The simulated and experimental results verified the feasibility of substituting a realistic human model with a homogeneous muscle model in the design of an implantable antenna system and demonstrated a strong potential for diverse implantation scenarios and future biotelemetry applications.

This paper presents a novel key-shaped miniaturized four-port MIMO antenna designed for ultra-wideband (UWB) applications. The proposed antenna consists of four orthogonally and symmetrically arranged key-shaped radiating elements. By integrating semi-circular patches at the edges of rectangular radiators, multiple resonance modes are excited to significantly broaden the impedance bandwidth and optimize matching. To suppress mutual coupling within a compact footprint, an enhanced cross-shaped defected ground structure (DGS) is implemented, which effectively blocks surface wave propagation by utilizing narrow rectangular stubs. Experimental results demonstrate that the antenna achieves a continuous impedance bandwidth from 4.4 to 26 GHz, corresponding to a fractional bandwidth of 141.5%. Throughout the operating band, the port isolation remains better than 28 dB, while the envelope correlation coefficient (ECC) is lower than 0.00013, ensuring excellent diversity performance. Furthermore, the antenna, fabricated on an FR4 substrate with a compact size of 47 × 47 × 1.6 mm³, maintains a radiation efficiency between 67.4% and 92.6%. With its superior bandwidth, high isolation, and exceptionally low correlation, the proposed design offers a robust solution for high-performance UWB communication systems.

A novel flexible hybrid rectenna for simultaneous RF energy harvesting (RF-EH) and dedicated wireless power transfer (WPT) is proposed. The rectenna comprises a notched broadband omnidirectional microstrip antenna (2.8-6.3 GHz with a 4.7 GHz notch) and a 4.7 GHz directional dielectric resonator antenna (DRA) located on the microstrip antenna. At the 4.7 GHz notch band, electromagnetic energy is confined around an L-shaped strip and a split-ring slot of the microstrip antenna, exciting a TM₂₀₁^z resonant mode in the DRA that produces a narrow radiation beam at 4.7 GHz. This enables a frequency-domain complementary operation: ambient energy is harvested across the broadband region for low-power applications, while dedicated power is efficiently received at 4.7 GHz for high-power operation. A broadband rectifier employing a dual-channel impedance matching architecture is further proposed, in which two parallel branches cooperatively extend the rectification bandwidth. Measurements demonstrate that the rectenna achieves efficiencies above 45% across 2.8-6.3 GHz, with a peak efficiency of 57.9%. In addition, the use of Polydimethylsiloxane (PDMS) as the substrate provides high flexibility and excellent conformability. These features make the proposed rectenna well-suited for powering electronics on curved surfaces, compact devices, and curved-surface Internet-of-Things (IoT) nodes, such as robots, drones, in-vehicle applications and industrial robotic arm units, enabling reliable conformal deployment on non-planar equipment and distributed IoT systems.

The second-order small-slope approximation (SSA2) is an analytical approximation method applied to electromagnetic scattering simulations on rough surfaces. However, the conventional serial SSA2 method involves a quadruple integral, and each integration requires a fast Fourier transform (FFT). As a result, the method demands very high memory and suffers from very low computational efficiency. In this paper, a parallel SSA2 method is proposed by combining region decomposition with Message Passing Interface (MPI) programming. The OpenMP technique also adopted to further improve the algorithm's performance. To avoid the frequent communication overhead caused by spectral shifting in traditional parallel FFT methods, the matrix data is carefully allocated, which effectively removes the related communication bottleneck. In addition, by using the separability of the two-dimensional (2-D) FFT, each 2-D FFT is implemented using two sequential one-dimensional (1-D) FFTs. This design limits inter-node communication to a constant upper bound. Numerical results show that when the number of computer nodes reaches 10, the parallel efficiency of the proposed method exceeds 80%, which confirms the effectiveness of the proposed method.

This article introduces a microwave system operating in the low-frequency band of 1.6-2.8 GHz, specifically designed for biomedical applications. Tumor detection in the human brain was achieved by monitoring variations in antenna S-parameter response. A high-gain antenna was positioned on the skull's surface, whose gain and directivity are enhanced by backing it with a Frequency Selective Surface (FSS) array. This arrangement effectively channels energy toward human tissues, which facilitates tumor detection. The combined Antenna-FSS structure improved the gain and directivity by 5.3 and 5.1 dB, respectively. Simulations were conducted using a multilayered skull model consisting of the skin, skull, and brain. The experimental validation was done by performing measurements using a near-realistic human brain phantom and by inserting a tumor in the brain area of the fabricated phantom. The study revealed that the measured S-parameters vary by approximately 11.38 dB when a 6 mm × 6 mm tumor is introduced into the brain region. Additionally, S-parameters are analysed by varying shapes, sizes, and locations of tumors within the brain. The study's findings indicate that variations in the characteristics of the S-parameter can be potentially utilized for detecting tumors in the human brain.

Lightning electromagnetic fields are essential inputs for protection system design, electromagnetic compatibility analysis, and lightning location system calibration. Although most computational studies rely on a single ``typical'' current waveform, real lightning exhibits significant variation in peak current, rise time, and temporal characteristics. This paper presents a systematic investigation of how return stroke current parameters influence radiated electromagnetic fields using the finite-difference time-domain (FDTD) method. Five representative waveforms are examined: typical subsequent stroke (12 kA, 40 kA/µs), first return stroke (28 kA, 12 kA/µs), severe subsequent stroke (25 kA, 80 kA/µs), fast-rising subsequent stroke (20 kA, 120 kA/µs), and rocket-triggered lightning (16 kA, 20 kA/µs). Simulations are performed over homogeneous ground (σ = 0.001 S/m, ε_r = 10). Electric and magnetic field components are computed at near-field (r = 50 m) and far-field (r = 5 km) distances, both underground (d = 2 m) and above ground (h = 10 m). Results show strongly distance-dependent behavior: at 50 m, current rise rate (di/dt) dominates, with fast-rising 16 kA strokes producing fields comparable to slower 22 kA events. At 5 km, peak current governs all components, with 28 kA first strokes producing fields 1.5-2× higher than subsequent strokes. Field attenuation from 50 m to 5 km varies from 130× to 3000× depending on waveform frequency content. The azimuthal magnetic field exhibits uniform attenuation (225-350×) and ground independence, supporting its use in lightning location systems. Triggered lightning underestimates severe natural strokes by 40-60% at 50 m, narrowing to 20-30% at 5 km. These findings indicate that worst-case protection scenarios must be distance-specific: fast-rising strokes govern buried infrastructure within 100 m, while first strokes dominate overhead systems beyond 1 km.

This paper presents the design, fabrication and experimental validation of a novel compact high-gain decagonal microstrip patch antenna designed for noninvasive soil moisture sensing. The antenna utilizes the strong correlation between its dielectric properties and soil moisture content, which directly influences the antenna's reflection coefficient and resonant frequency. The decagonal geometry using a Taconic TLY-5 substrate achieves a high gain of 7 dBi and an excellent radiation efficiency of 96.5% compared to the FR-4 substrate. The prototype antenna fabricated on Taconic TLY- substrate resonates at 2.445 GHz frequency with a measured reflection coefficient of -34.3 dB, validating the high-performance characteristics predicted by full-wave simulations using CST Studio Suite 2018. The sensing capability of the fabricated antenna is carefully tested across sandy, loamy, and clay soils, demonstrating a predictable downward shift in resonant frequency as moisture content increases. A linear regression analysis is performed on the experimental data from sandy, loamy, and clay soils, yielded a high coefficient of determination (R² ≈ 0.9) for all three soil types, establishing a calibration model to accurately convert the resonant frequency shifts into soil moisture values. The obtained sensitivity values for soil moisture detection are 4.76 MHz/% for sandy soil, 5.15 MHz/% for loamy soil and 4.89 MHz/% for clay soil respectively. The compact size, high gain and consistent performance across different soil samples make this proposed antenna a better solution for precision agriculture and environmental monitoring through integration into wireless sensor networks.

This paper presents the configurations of U-slot cut hexagonal microstrip antennas loaded with shorting posts, which realize a reduction in the center frequency of axial ratio bandwidth as well as the total substrate thickness. Three configurations employing four, eight, and twelve shorting posts positioned around a hexagonal patch are presented. Shorting posts loading adds to the inductive component in the antenna's input impedance, which yields a reduction in total substrate thickness and frequency. Among three variations, the U-slot cut hexagonal patch employing eight shorting posts yields the optimum result. It achieves axial ratio bandwidth of 2.23%, for 0.033λ_cAR reduction in the substrate thickness and 55 MHz (4.5%) reduction in the center frequency of axial ratio bandwidth. All these results are for marginal reduction in peak broadside gain. Against the U-slot cut hexagonal patch on a reduced substrate thickness, shorting posts loaded antenna achieves higher axial ratio bandwidth with 56 MHz (4.81%) reduction in the center frequency of axial ratio bandwidth. Considering all these results, the proposed design offers a compact circular polarized solution, while employing a resonant U-slot. For the obtained antenna characteristics, the proposed designs can find applications in GPS L5 and L2 bands. Experimental validation for the proposed configurations has been carried out where the measured results show close agreement with simulated ones.

This manuscript describes the design of a scannable Cosecant² Shaped Pattern Generating Phased Array Antenna (PAA) intended for airborne synthetic aperture radar (SAR). The Cosecant² Shape avoids the requirement of sensitivity time control correction at receiver side. A C-band linear array antenna consisting of 16 elements is designed, analyzed, developed, and characterized. The array antenna is embedded with a U-slot and Defected Ground Structure (DGS) integrated microstrip elements. The introduction of a DGS provides an improvement of 6.5 dB in cross-polarization levels when scanning. Additionally, the null perturbation technique is employed to synthesize the excitation coefficients for the Cosecant²-Shaped pattern, aiming to achieve the minimum current taper ratio. These synthesized coefficients are combined with a progressive phase for beams that can scan within a ±30° angular range. The proposed linear array, along with its excitation and phasing network, has also been developed and characterized in an anechoic chamber.

Magnetic Coupled Resonant (MCR) Wireless Power Transfer (WPT) is typically used for electrical charging, offering high tolerance to misalignment and wider transmission range. However, MCR-WPT is assumed to be a two-port circuit, including transmitter (Tx) and receiver (Rx), and exhibits lower efficiency than conventional inductive power transfer. Various studies have been proposed to increase the 4-coil MCR-WPT efficiency, but further challenges remain due to the turn technology complexity. A relevant and simple design solution is developed in the present paper that enables the optimization of Power Transfer Efficiency (PTE) by minimizing implementation cost. To achieve that goal, the transfer- and resonator-distances, TD and RD, respectively were optimized through theory, both circuit and 3-D electromagnetic (EM) simulations via 3-D modeling and experimentation. The validation PTE results obtained from analytic calculation, simulation and experimentation affirm that the maximum efficiencies of 94.10, 90.15% and 69.35% were obtained at optimal positions around RD = 7.5 mm and TD = 100 mm, respectively. The slight difference of the obtained PTE among theory and simulation with experiment is due to the setup instrument imperfection. The performed study is useful for the WPT charging systems, such as electronic sensors, wearable devices, and communication systems.

To address the issue of significant fluctuations in radial electromagnetic forces in variable-leakage-flux reverse-salient-pole permanent magnet synchronous motors (VLF-RSPMs), which make stable operation at high speeds difficult, this paper combines an analysis of existing VLF-RSPMs with a novel optimization method to propose a multi-objective hierarchical optimization design method for VLF-RSPMs, which incorporates vibration and noise suppression. First, the paper analyzes the radial electromagnetic force model, natural frequencies, and electromagnetic vibration model of the VLF-RSPM. Second, based on the optimization objective of vibration and noise suppression, parameters with high sensitivity are optimized first through sensitivity analysis. Subsequently, the modal characteristics, radial electromagnetic forces, noise, and stress of the optimized VLF-RSPM are analyzed in detail. Finally, an experimental VLF-RSPM prototype and a test platform for measuring radial electromagnetic force fluctuations are designed. The results demonstrate that the multi-objective hierarchical optimization design method ensures the operational reliability of the optimized VLF-RSPM, enabling it to meet the requirements for high-speed operation.

This study addresses an issue with high-energy laser directed energy weapon performance assessment when applied to the problem of countering swarms of uncrewed aerial systems (UAS). Queueing theory provides a suitable modelling framework for the performance assessment of such systems, as a single server queue can process only one threat at a time, based on the order in which threats arrive at the theatre of operation. Consequently, this introduces delays into the processing of sequences of threats. Delays in such queues typically have time-dependent service times, due to the target's movement. This results in considerable complexity in terms of producing performance predictions through stochastic models. In recent applications of queueing theory to directed energy systems an ad hoc approximation has been used to estimate the delays that threats experience while waiting for service. This approach involves approximating the processing delay of a given threat by a constant value. In particular, it has been estimated by measuring the delay as a product of the expected service time and the number of threats present less one. Such an approximation can result in severely reduced and inaccurate performance predictions. In the current study, the mean delay will be used instead, and improvement on the aforementioned approximation will be demonstrated through explicit examples of swarm UAS defeat.

Wireless power transfer systems suffer from frequency splitting and drift due to horizontal coil offset, which degrade both efficiency and output power. This paper derives a sixth-order equation to calculate resonant frequencies at the maximum efficiency point (Freq-MEP). The frequencies at the maximum power point (Freq-MPP) are determined by differentiating the output power with respect to the operating frequency. Both Freq-MEP and Freq-MPP are shown to vary with horizontal offset. Based on this analysis, a segmented frequency-tracking method is proposed to achieve simultaneous high efficiency and high output power under varying offset conditions. The effectiveness of the developed magnetic resonant WPT system is validated through numerical simulations and experiments.

For small, low-power wireless power transfer (WPT) devices, maintaining constant output power and high transmission efficiency over a wide range of coupling variations remains a significant challenge. The double semicircular coplanar (DSC) coils, featuring unique magnetic field distribution, provide a potential solution for enhancing misalignment tolerance. This paper analyzes the transmission characteristics of a WPT system employing DSC coils. A circuit model for the WPT system with DSC coils is first developed, and its fundamental transmission characteristics are studied. The tolerance of output power and transmission efficiency to positional offsets (X, Y, Z) over various frequency bands is then evaluated. Finally, an experimental DSC coil system that features a stable frequency bandwidth and is largely insensitive to frequency shifts is built. Its misalignment tolerance forms a cuboid spatial zone. Within this zone, namely when the offset of the receiver in X and Y is under half its diameter, the output power and transmission efficiency remain constant, supported by a uniform magnetic field. When the receiver offset is greater than half its diameter, the magnetic field uniformity and mutual inductance change abruptly, leading to a sudden variation in output power and transmission efficiency.