Abstracts:In this letter, a multifunctional low radar cross section (RCS) quantum metasurface antenna (QMA) array with an impedance bandwidth (BW) spanning 1.73 GHz to 2.52 GHz (37.2%) is proposed. The array features two radiation modes: grouped port operating enables omnidirectional direction-finding, while full-port excitation generates a high-gain circularly polarized beam for communication. By using integrated p-i-n diodes, each cell can be tuned independently to control its scattering phase. The QMA array demonstrates monostatic far-field RCS reduction exceeding 10 dB under normal incidence wave across 3.65 GHz to 5.42 GHz (p-i-n on) and 1.95 GHz to 4.34 GHz (p-i-n off). Further code reconfiguration also effectively provides 11 dB RCS reduction at ±24° oblique incidence. It offers broad BW for both impedance matching and RCS reduction, with a high reduction level. For adaptive stealth, each cell employs a quantum superposition state for its scattering properties. Real-time adjustment of the collapse probability based on direction-finding feedback redirects scattered beams matching incident waves, enhancing concealment against multisource radar threats. The cell electrical size is 0.32λL × 0.32λL × 0.08λL, exhibiting compact and low-profile features. This approach provides a novel paradigm and technical solution for the new type of intelligent electromagnetic stealth system.

Abstracts:Dual polarization, reduced focal length, and beam-switching capability are essential for transmitarray antennas in space-limited, polarization-diverse communication systems. Although prior designs addressed these features individually, none have simultaneously realized all three in a single transmitarray. This work proposes a dual-polarized, beam-switching transmitarray employing a folded geometry to reduce the focal length to one-third of conventional structures. The compact profile is achieved by incorporating a phase compensation surface (PCS) composed of polarization-independent reflective unit cells. Each cell provides discrete reflection phases, equalizing the folded and direct ray paths for both polarizations. A 14 × 14 aperture is illuminated by three dual-polarized patch antennas aligned along the y-axis and fed via microstrip lines, enabling ±30° beam steering through port switching and reducing feed blockage. The measured aperture efficiencies are 24.7% in fixed-beam mode and 17.3% in beam-switching mode, with a gain reduction below 1.5 dB compared to full-profile designs, comparable to previously reported single-polarized systems. This design is applicable to high-frequency communication terminals facing constraints in size, polarization diversity, and dynamic beam coverage.

Abstracts:In this letter, a metasurface-based high-aperture-efficiency transmitarray antenna (TA) is proposed. To improve the feeding efficiency, a rectangular waveguide cavity-backed slot array was adapted for the feed. Eight coupling slots (CSs) corresponding to the unit cell (2 × 4) were added to a cavity operating in TE120 mode to improve efficiency and suppress grating lobes. The slot spacing was compactly designed to be 0.48λ0 and 0.4λ0 in the x-axis and y-axis directions, respectively, resulting in a 2 × 4 slot subarray structure. By varying the coupling height between the CSs corresponding to the unit cell and the metasurface (MS), the optimum height was found to be 0.13λ0, within the range of the reactive near-field region. To verify the beamforming performance of the proposed antenna, five types of MSs were designed to operate at beamforming angles θ of −40°, −20°, 0°, 20°, and 40°. The simulated and measured aperture efficiencies for a broadside beam were observed to be 78.2% and 59.3% at 10 GHz and 9.92 GHz, respectively. To our knowledge, the presented 8 × 8 TA achieved the highest simulated aperture efficiency. The measured results confirm that the proposed TA has high aperture efficiency and good beamforming characteristics.

Abstracts:In this letter, a dual-circularly polarized (DCP) antenna with high port isolation and a wide 3 dB axial ratio (AR) coverage is designed for satellite communication applications. Vector projection analysis reveals that the AR degradation of circularly polarized antennas at low elevation angles is primarily attributed to the insufficient vertical polarization components with appropriate phase characteristics. To address this limitation, a metallic cavity structure is introduced to the antenna element to enhance the vertical polarization radiation component. Precise tuning of placed notches further enables phase optimization of these vertical components, achieving high-purity DCP radiation. Finally, the prototype is processed and measured to determine its effectiveness in covering the 1.55 GHz to 1.6 GHz, where port isolation greater than 20 dB and 130° AR beamwidth in the upper hemisphere is achieved.

Abstracts:This letter studies electromagnetic scattering by dielectric-loaded conductive sectors using the subdomain method. The subdomain method decomposes two conductive sectors facing each other into two single-sector problems. This enables representing the scattered electromagnetic fields of their individual subdomains using Fourier–Bessel series in local coordinates. The boundary conditions are enforced by mode matching. Surface currents are compared with those obtained from the finite element method and both the convergence rate of modal coefficients and the relative root-mean-square error are examined. The results show that the proposed method produces accurate and computationally efficient solutions for the given problem.

Abstracts:Wireless communications rely on path loss modeling, which is most effective when it includes the physical details of the propagation environment. Acquiring this data has historically been challenging, but geographic information systems data are becoming increasingly available with higher resolution and accuracy. Access to such details enables propagation models to more accurately predict coverage and account for interference in wireless deployments. Machine learning-based modeling can significantly support this effort, with feature-based approaches allowing for accurate, efficient, and scalable propagation modeling. Building on previous work, we introduce an extended set of features that improves prediction accuracy while, most importantly, proving model generalization through rigorous statistical assessment and the use of test set holdouts.

Abstracts:This letter presents an area-weighted conformal finite-difference time-domain (AW-CFDTD) method aimed at improving the accuracy of electromagnetic simulations involving complex material interfaces. Unlike conventional conformal FDTD (CFDTD) techniques, which typically estimate effective material parameters based solely on the fraction of each material along the grid edge, the proposed approach incorporates an additional weighting scheme that accounts for the surface areas associated with the grid edges. Specifically, the effective permittivity and conductivity are computed using the minimum between the material proportion along the grid edge and the corresponding proportion on the adjacent surface area. This dual-weighting strategy leads to a more physically consistent representation of material transitions, thereby enhancing both the numerical accuracy and general applicability of the CFDTD method. To evaluate the proposed AW-CFDTD method, a benchmark simulation of a square dielectric object was conducted and compared with conventional CFDTD, fine-mesh staircase FDTD, and CST. AW-CFDTD showed higher accuracy with lower computational cost than fine-mesh methods, offering an efficient and accurate solution for conformal FDTD modeling, especially in RF and microwave applications with complex material boundaries.

Abstracts:Accurate characterization of the reflection coefficient of reconfigurable intelligent surface (RIS) is crucial for the design and optimization of RIS-assisted communication systems. Conventional measurement methods often rely on specialized instruments, which are costly and bulky. In this letter, we propose a novel harmonic-based reflection coefficient measurement method that enables in-situ and low-cost measurement using standard software-defined radio platform. By applying time-varying square-wave control signals to the RIS, the proposed method can obtain the amplitude ratio and the phase difference between two reflection states through harmonic analysis of the reflected signal. A prototype system is developed and experimentally validated, demonstrating that the proposed method achieves comparable measurement results to that of conventional approaches while significantly reducing hardware complexity.

Abstracts:This letter introduces a low-profile broadband folded transmitarray shared-aperture transceiver designed for frequency-diverse near-field sensing and real-time gesture recognition. The system uses a random encoded folded transmitarray and a dual-polarized feed to generate random transmit–receive fields through polarization control. The folded array operates from 31 GHz to 46 GHz (corresponding to 38% relative bandwidth) and includes a random-coded transmitarray surface and a polarization conversion reflectarray surface. The folded design integrates separate transmitter and receiver components into a shared-aperture transmitter yielding a compact 15.4$\bm{\lambda }$ × 15.4$\bm{\lambda }$ × 2.6$\bm{\lambda }$ system with a height-to-diameter ratio of 0.16. Experimental results demonstrate the effectiveness of the proposed compact system in a real-time classifying ten gestures with an accuracy of 99.6% using a convolutional neural network.

Abstracts:Acoustically actuated antennas exploit mechanical resonance to achieve electrically small and on-chip implementation. However, these unique traits complicate the accurate characterization of their radiation performance. In this work, we systematically identify and quantify two dominant sources of extrinsic electromagnetic radiation in on-chip measurements: 1) substrate coupling that effectively extends the radiating aperture and 2) parasitic radiation from probes and cables. In typical acoustic antenna measurements, both effects can overestimate the measured gain by more than 10 dB, while leaving the resonance response largely unaffected. This behavior is distinct from that of conventional electrically small antennas. Through controlled experiments, we reveal that improper test conditions, including oversized substrates, probe feeding, near-field and nonanechoic environments, can cause radiation gain errors greater than 20 dB. Based on these findings, we develop and validate a standardized test protocol incorporating die-level isolation, ferrite bead loading, wire-bonded printed circuit board (PCB) feeding, and far-field anechoic measurements. These results reveal critical but previously overlooked artifacts in the characterization of acoustic antennas and establish a framework for their intrinsic evaluation.