As the need for rapid data transmission and dependable wireless networks grows, so does the need for advanced antenna technology. This has become a major focus of modern communication technologies. This paper describes the design of a 4-port multiple-input multiple-output (MIMO) microstrip patch working at 28 GHz in the Ka-band. This antenna is fabricated on a substrate measuring 21 × 21 × 3.97 mm³, composed of FR4, foam, and RT/Duroid 5880. It uses a microstrip feed. Performance enhancements are achieved by positioning the feeds orthogonally, incorporating a U-shaped slot into the MIMO antennas, and implementing a superstrate made of metamaterial (MTM) elements. Additionally, a single-layer MTM superstrate with rectangular slots is created to improve gain while keeping good impedance matching. The design process systematically improves gain and mutual coupling while keeping the overall size compact. The specific challenge addressed by the design is to improve peak gain and radiation efficiency by employing MTM elements operating at 28 GHz. The 4-port MIMO antenna achieves an impedance bandwidth (IB) of 27.11-29.21 GHz, with a peak gain of 14.05 dB, respectively. This antenna is used in next-generation communication systems, vehicular networks, and 5G systems.

This study presents a broadband, switchable, and bi-functional terahertz device based on the phase transition of vanadium dioxide (VO₂). When VO₂ is in the metallic state, the device operates as a linear polarization converter (LPC). When VO₂ transitions to the insulating state, the device functions as a broadband linear-to-circular polarization converter (LTC-PC). Numerical simulations are conducted to verify the device performance. To further optimize metamaterial performance and accelerate the design process, a deep learning framework that integrates convolutional neural networks (CNNs) and the Transformer architecture via an adaptive mechanism is proposed. Numerical simulations indicate that this LPC achieves a polarization conversion ratio (PCR) exceeding 90% across the 1.92-2.93 THz band and maintains angular stability for incidence angles up to 50°. The LTC-PC operates effectively within the 2.40-4.33 THz range. Featuring broadband operation and bi-functional capabilities, the converter holds significant potential for applications in terahertz imaging, sensing, solar energy harvesting, and communications.

A compact four-port circularly polarized multiple-input multiple-output (CP-MIMO) antenna with a dual-layer architecture is proposed for low-altitude communication applications. In compact MIMO arrays of CP-capable monopole elements, strong mutual coupling makes stable CP radiation difficult to achieve. To address this issue, the proposed antenna uses a lower layer for dual-polarized MIMO generation and an upper layer for polarization conversion. The antenna is fabricated on two FR-4 substrates with an overall size of 0.85λ × 0.85λ × 0.084λ. In the lower layer, a dual-polarized feed backplane (DPFB) forms a ±45° dual-polarized MIMO array with port isolation exceeding 17 dB. In the upper layer, a polarization conversion superstrate (PCS) converts the incident dual-polarized waves into CP radiation. The PCS extends the impedance bandwidth by 36%, from 7.55 to 10.08 GHz, and enables LHCP radiation with a 3 dB AR bandwidth of 8.22-8.89 GHz. A gain enhancement of 48% is also achieved. Measured results verify the design and show good MIMO diversity performance.

Accurate simulation of partially coherent imaging is crucial for computational lithography, with Abbe and Hopkins as the two main formulations being used. Although the two methods are equivalent in theory, practical simulators making independent choices between Abbe and Hopkins could hardly produce consistent results that match the desired accuracy owing to the inherently different ways of numerically representing, discretizing, and truncating the illumination source and lens pupil function, etc. Moreover, classical Hopkins models require prior construction and/or eigen decomposition of the high-dimensional transmission cross coefficient (TCC), the prohibitive costs of which hinder timely model verification. To address these challenges, we developed a unified Abbe-Hopkins formulation in conjunction with a TCC-free Hopkins pointwise sampler for efficient cross-model validation. Our formulation supports both Abbe and Hopkins modeling in a single unified framework, with the two simulation modes using exactly the same numerical representations of the illumination source and projection lens. Cross-model verification for randomly sampled points is performed efficiently by evaluating the Hopkins quadratic form through a fast Fourier transform of an image and a few pointwise multiplications between images, without ever explicitly constructing a TCC and eigen-analyzing it. Numerical tests show that the Abbe and Hopkins results agree up to the machine precision level.

An improved scattering (S-)parameters extraction method, based on the forward and backward propagating waves under oblique incidence on metamaterials (MMs), is proposed to accurately extract electromagnetic parameters for asymmetric uniaxial MMs in a broad frequency range. The proposed approach equivalently models asymmetric MMs as two isotropic media (distinct from the 3 × 3 matrix-form anisotropic medium). To validate the effectiveness of the proposed method, a low-thickness asymmetric absorptive frequency-selective surface (AFSS) and a high-thickness 7-layer absorber were designed, simulated, and analyzed.

This paper presents a wideband four-port microstrip antenna operating from 2.75 GHz to 6.75 GHz with frequency reconfigurability and controllable notch characteristics. The antenna employs an asymmetric radiating structure to realize circular polarization around 5.5 GHz, while multilayer graphene(MLG) pads are introduced to enable bias-controlled frequency tuning and adjustable band rejection. The four-port configuration, implemented on an RT/Duroid 5880 substrate (ε_r = 2.2, thickness = 1.6 mm), achieves inter-element isolation better than 20 dB without additional decoupling structures. The proposed design also exhibits strong diversity performance with an envelope correlation coefficient below 0.02 and diversity gain above 9.97 dB. The results demonstrate that the proposed antenna provides a compact and low-complexity solution for wideband and reconfigurable sub-6 GHz wireless communication applications.

We have developed a carbon nanotube organic silicone rubber (CNT-OSR) composite medium, composed of methyl trifluoropropyl silicone rubber as the matrix, with different mass fractions of carbon nanotubes added and formed through vulcanization using a bis (cyclopentadiene) vulcanizing agent. The CNT-OSR composite media with carbon nanotube contents of 2wt%, 5wt%, and 8wt% were tested, and the maximum absorption and shielding efficiencies of the media for terahertz waves in the 0.5-1.0 THz frequency range were found to be 69.77 dB, 76.28 dB, and 63.69 dB, respectively. Through impedance matching theory analysis, the absorption and shielding effectiveness of the medium for terahertz waves were confirmed. Additionally, the composite medium exhibits excellent hydrophobic properties. It provides a simple and feasible approach for developing lightweight, efficient, and multifunctional terahertz wave absorbing and shielding materials for the next generation of terahertz wireless communication.

The dielectric constant, which is the real part of the complex permittivity, of composite materials at microwave frequencies was investigated in this study. Ceramics of titanium dioxide, calcium titanate, and strontium titanate with high dielectric constants of 100, 170, and 300, respectively, were selected. Ceramic powders were spread in the polyethylene matrix to form composite samples. The dielectric constants of the composite samples were measured to determine their matching conditions with the mathematical curves of five well-known mixture equations. These five mixture rules were then applied to estimate the dielectric constants of the three selected ceramics from the measured dielectric properties of the composite samples with various volume percentages of ceramic fillers. The mathematical equations of the potential theory errors of the five mixture rules for the dielectric constant estimation were derived and discussed. One of the five rules was selected and modified to obtain a new empirical mixture equation. This proposed empirical equation can significantly improve the accuracy of dielectric constant measurements for the selected ceramic materials. An empirical mathematical relation of the new mixing rule with the dielectric constant of the ceramic is then concluded.

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.

A high-performance Two-Dimensional Photonic Crystal (2DPC) demultiplexer is proposed for application in Dense Wavelength Division Multiplexing (DWDM). Simultaneous high-field confinement and higher modal coupling are achieved using a new hybrid cavity geometry design, which consists of a square cavity with an inner rod radius (r = 110 nm) and a circular cavity with an outer rod radius (r = 100 nm). It is an operating silicon platform featuring a square lattice, bus waveguide, and four drop ports. Plane Wave Expansion (PWE) and Finite Difference Time Domain (FDTD) simulation methods reveal a large photonic bandgap (0.27-0.37a/λ) and excellent spectral performance, including a 98.75% average transmission efficiency, a high Q-factor of 7281, and precise 0.8 nm channel separation. System-level verification, Lumerical INTERCONNECT, and eye diagram and BER analyses were used to test signal integrity. The hybrid geometry also has a smaller footprint and improved integration, making it a suitable design for next-generation optical communication systems.

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.

To meet the stringent space constraints and diverse connectivity requirements of modern intelligent connected vehicles, a compact MIMO antenna system designed for microwave and millimeter-wave (mm-wave) vehicle-to-everything (V2X) communications is presented. The proposed antenna features a compact footprint adaptable for integration into space-limited automotive modules, such as shark fin antenna housings. By employing a structure reuse technique, the system integrates a four-element microwave MIMO array and two orthogonal mm-wave phased arrays within a size of 30 mm × 30 mm × 2 mm. In the microwave band, a parasitic patch is introduced to achieve dual-mode resonance, ensuring a wide bandwidth for reliable control signaling. Two orthogonal rows of metallized cavities serve a dual purpose: acting as decoupling structures for the microwave MIMO system and functioning as mm-wave arrays to enable two-dimensional beam scanning. This capability is crucial for overcoming blockage effects in dynamic vehicular environments. Experimental results demonstrate that the proposed antenna achieves wide coverage in the microwave band (4.62-5.11 GHz) and high-gain beam scanning (±40°) in the mm-wave band (25.8-30.4 GHz). The measured isolation exceeds 17 dB with an envelope correlation coefficient below 0.11, validating its suitability for next-generation vehicle terminals.

Free-space electromagnetic waves can be coupled into on-chip propagating surface waves (SWs), a process that holds great promise for receiver front-ends in wireless communication systems. However, it has traditionally faced challenges in coupling efficiency and in controlling the on-chip wavefront of SWs. To address these challenges, we design and experimentally demonstrate SW couplers operating in the terahertz regime based on metal-insulator-metal resonators. Our devices achieve not only broadband and highly efficient coupling, with an efficiency exceeding 60% over a 20 GHz bandwidth, but also enable directional steering of the excited SWs to designated on-chip ports. In this way, mode conversion and onchip routing functionalities are seamlessly integrated into a single compact component. Based on this design, we fabricated devices and implemented corresponding terahertz wireless communication links, successfully demonstrating 16-QAM data transmission in both single-link and dual-link configurations.

This paper reviews recent advances in acoustic computation and modeling, specifically bridging effective medium theory (EMT) and biomedical ultrasound imaging. To achieve this, we examine how EMT provides the physical foundation for wave-based imaging through homogenized parameters, focusing on image reconstruction across diverse systems ranging from single pulse-receivers to multi-input and multi-output (MIMO) tomography. Furthermore, we highlight cross-disciplinary insights from computational optics, such as the transport of intensity equation and ptychography, while addressing acoustic-specific challenges like aberration correction and wave interference. In light of these challenges, emerging solutions are discussed, including ultrasound matrix imaging (UMI) via transfer matrix methods, inverse-designed matching layers, and hardware-accelerated approaches like the Krimholtz-Leedom-Matthaei (KLM) electro-acoustic model for ultrafast imaging. Ultimately, by integrating physical understanding of effective media with advanced computational algorithms, these developments provide a robust framework for the future of high-resolution 3D ultrasonography and acoustic holography.

To achieve synchronous and uniform amplification of dense multi-carrier signals, this paper proposes a multi-frequency nondegenerate parametric amplifier (PA) based on a nonlinear spoof surface plasmon polariton (SSPP) waveguide. By engineering the dispersion characteristics of a varactor-diode-loaded waveguide, we realize an SSPP platform that exhibits minimized phase mismatch for three distinct signal-idler pairs under a constant pump frequency (13.348 GHz) and a fixed bias voltage. Experimental results show that the amplifier delivers highly uniform gains exceeding 20 dB for three closely spaced carriers at 6.363, 6.489, and 6.549 GHz, effectively emulating a three-frequency-shift keying (3FSK) signal. This work demonstrates a fixed-condition amplification scheme that requires no dynamic tuning, offering a promising solution for amplifying densely spaced carriers in integrated communication systems.

In this study, a single-layer substrate and via-free transmit-reflect-array antenna based on a metasurface is proposed. The array antenna comprises transmissive and reflective unit cells arranged alternately in sequence. To achieve a 360° phase coverage, two sets of antisymmetric U-shaped lines are etched on the top and bottom layers of the substrate to form the transmissive unit cells, and multi-layer stacking and vias are avoided. Moreover, by adjusting the lengths of a split-ring structure with phase delay lines for reflective unit cells, a 360° phase coverage is achieved. The measurement results demonstrate that the antenna simultaneously generates a reflective focused beam with a peak gain of 20.8 dBi and a transmissive +1-mode OAM vortex beam with a peak gain of 20 dBi and a mode purity of 90% at 17 GHz.

The curvature effects of curved metasurface (MTS) lead to oblique incidence and different unit radiation normal vectors (DURNVs). Oblique incidence causes a reduction in scattering amplitude and degrades focusing efficiency (FE), and DURNV distorts the radiation pattern of curved MTSs. To the knowledge of the authors, for the first time, this paper proposes a phase amplitude modulation and phase modulation (PAM-PM) combined modulation technique for cylindrical MTS to generate a high signal-to-noise ratio (SNR) and high FE three-dimensional (3D) shaped near field with a spiral cross-sectional shape. In addition, a near field with controllable spatial positions is a practical application requirement, and this paper provides a method to establish a 3D-shaped near field with controlled spatial positions. The proposed cylindrical MTS with PAM-PM modulation technique outperforms the PM technique significantly, achieving an SNR above 13 dB and an FE of 38.1%. For cylindrical MTS with only PM, there exists some noise, and the FE is 33.2%. This proposed modulation technique can be applied to 3D near-field systems based on conformal MTS, including wireless power transfer, radiometric temperature sensors for hyperthermia, and medical imaging systems.

A compact, double split-ring H-shaped resonator-based polarization-independent dual-band metamaterial microwave absorber (MMA) with an outstanding absorption efficiency was designed and analyzed for satellite communication applications. The H-shaped resonator-based unit cell was printed on an FR4 material using copper as the conducting material. Copper material was chosen for the radiating patch and the ground plane. A detailed parametric analysis was performed by tuning the geometrical parameters of the H-shaped MMA to achieve dual absorption bands with broad bandwidth and high absorptivity. The recommended H-shaped absorber exhibits dual absorption bandwidths of 710 MHz and 1630 MHz with FBW of 22.95% and 19.35%. Furthermore, it maintains an absorptivity of greater than 90% across the entire spectrum of dual operating bands with almost perfect absorption of 99.99% at the resonant frequencies (3.17 GHz, 7.78 GHz) of each band. The MMA maintains an area of 0.09λ × 0.09λ. A simulated absorber model was fabricated, and the results have been tested using an Anritsu Combinational Analyzer (MS2037C) for experimental validation. The simulated and tested outcomes of the developed prototype are in strong alignment, rendering the absorber suitable for S-band and LEO and geostationary satellite uplinks/downlinks, with specific bands often at 7.145-7.235 GHz (uplink) and 8.4-8.5 GHz (downlink).

Megahertz wireless power transfer (MHz-WPT) enables compact resonant components; yet the matching, compensation, and filtering stages used in conventional systems can dominate loss and standby dissipation at MHz operation. To address this issue, this work proposes a compact 13.56 MHz WPT architecture in which impedance transformation is integrated into the resonant hardware. A self-resonant transmitting coil is co-designed with a Class-E power amplifier to shape the reflected load toward the optimum operating condition, thereby removing the external compensation network, the additional matching stage, and the lumped-element LC output filter. The analysis shows that, when the receiver is removed, the effective load becomes dominated by the transmitter resistance, inherently suppressing delivered power without sensing or closed-loop control. A prototype delivers 9 W over 30 mm with 81.5% end-to-end DC–DC efficiency, while under receiver absence, the DC input power decreases from 11 W to 1.15 W. These results demonstrate a simplified and robust MHz-WPT architecture with reduced component count and inherently low standby dissipation.

A low-profile, dual-band, shared-aperture antenna with a large frequency ratio is presented, based on a meshed patch and an AMC-backed Fabry-Perot (F-P) cavity. By taking advantage of the weak frequency sensitivity of grid slotting in meshed patches, the upper meshed patch is utilized as both the parasitic patch for the low-frequency antenna and the partially reflective surface (PRS) for the high-frequency F-P cavity, thereby simplifying the overall structure. Meanwhile, the AMC ground is employed to control the reflection phase and reduce the cavity height to λ/4, which enables both antennas to share the same aperture within an 8-mm profile. A prototype is fabricated and tested at 1.6 GHz and 15.14-15.46 GHz. Measured results demonstrate a frequency ratio of 1:9.6, a peak gain of 6.2 dBi at 1.6 GHz, a peak gain of 11.8 dBi in the high-frequency band, and a port isolation better than 17 dB. The proposed antenna features compact size, low profile, and efficient structural reuse, making it attractive for integrated multi-band communication systems.