Yong-Ling Ban

The multi-input multi-output (MIMO) technology employing a larger number of antenna elements will be utilized in sixth-generation (6G) communication systems to enhance spectral efficiency. This letter presents a transparent MIMO antenna capable of embedding up to 16 antenna elements in display through the use of a metal mesh film, demonstrating its potential for application in 6G MIMO terminal device. Through corner truncation design, two orthogonal half-wavelength resonant modes are excited to achieve U6G band coverage from 6.425 to 7.125 GHz. To enhance the isolation between two adjacent input ports, weak-field regions and edge-cut techniques without through-hole structures are utilized, achieving 15 dB isolation at a compact distance of 1.5/1.7 mm (0.033/0.038 λ at the center frequency). Additionally, the antenna features a low profile of 0.9 mm (0.02 λ).

When the Half Maxwell fish-eye (HMFE) lens is utilized for achieving Radar Cross Section (RCS) enhancement, total reflection occurs due to the mismatch between the refractive index of the semi-circular surface of the lens and that of air, resulting in a narrowed −3 dB beamwidth of RCS. By reducing the maximum refractive index n_max of the HMFE lens, an improved HMFE lens reflector is proposed, which can effectively suppress total reflection, thereby broadening the −3 dB beamwidth of RCS. Moreover, an innovative theory is proposed to calculate the ratio of the total reflection power to the incident power within the HMFE lens, which verifies the rationality and correctness of the method of reducing n_max. Compared with the conventional impedance matching method, this method has the advantages of a wider beamwidth and a larger peak RCS. In addition, this paper also proposes a formula for calculating the RCS of the HMFE lens reflector, and the optimal RCS curve is obtained based on the Genetic Algorithm (GA). Simulations and measurements show that the proposed reflector achieves a −3 dB RCS beamwidth of 80° to 100° and a peak RCS of 9.2 dBsm across 2–18 GHz, with dimensions of 162 mm × 162 mm × 118 mm. Compared to a same-radius Luneburg lens reflector, the proposed improved HMFE lens reflector maintains comparable peak RCS while reducing longitudinal profile by one-third.

This letter presents a novel radar cross section (RCS) enhancement structure: a dual R-KR lens retrodirective array (LRDA). A single R-KR LRDA is first designed by utilizing the broadband and wide-angle beam scanning capabilities of the R-KR lens. However, this configuration exhibits significant monostatic RCS fluctuations across the angular range, as observed in the simulation results. An analysis of the field distribution identifies the primary causes of these fluctuations. To address this issue, a dual R-KR LRDA is proposed, in which two single R-KR LRDAs are arranged in a parallel, laterally staggered configuration. Simulation and measurement results indicate that the dual R-KR LRDA achieves a maximum monostatic RCS fluctuation of approximately 3 dB over an angular range of ±55° within the frequency range of 8.1-12 GHz under linearly polarized (LP) incidence, demonstrating enhanced retrodirective performance.

A novel technique for the design of a three-dimensional multi-angle retrodirective metasurface (3D-MRDM) is put forward. Firstly, based on Floquet theory and the surface impedance theory, the surface current required for accurate retroreflection of the multi-angle retrodirective metasurface (MRDM) is deduced. Secondly, subcells of the MRDM are refined according to the surface current so that the MRDM can achieve high-efficiency retroreflection of multiple angles in the two-dimensional plane. The MRDM is finally extended to a 3D-MRDM by a specially designed honeycomb-meta (HM). A 3D-MRDM that can simultaneously achieve the efficient retro-reflection of a transverse electric (TE) polarized wave incident at seven angles on all three primary channels at every 120° azimuthal interval is designed as a conceptual example. Due to the unique configuration of the HM, the seven incident angles still have high retrodirectivity when the azimuth of the incident wave deviates from the three primary channels by ±5°. The measured results verify that the 3D-MRDM enables the retroreflection of seven incident angles simultaneously over an azimuthal range up to 30°. The design approach offers new perspectives for expanding the MRDM's angular domain, opening up opportunities for its application in radar electronic warfare as calibration targets or mounted on target drones.

Concentric ring arrays (CRAs) are highly regarded for their ability to produce nearly invariant radiation patterns and constant beam widths in the azimuth plane. However, the scanning beam synthesis of sparse CRAs presents higher computational demands than their broadside beam synthesis, leading to the failure of approximation methods and challenges in large-scale array design. To address this, we propose a layout optimization method for sparse CRAs that minimizes sidelobe levels while satisfying physical constraints. Initially, the ring configuration is re-fined through scaling and rotational adjustments to maximize the degrees of freedom in scanning beam synthesis. To achieve this, we develop a second-order cone programming model based on Taylor expansion by considering variables including the radii and the azimuthal rotation angles of rings. Additionally, an adaptive sampling strategy for radiation patterns is proposed to reduce optimization resources by focusing on at least three one-dimensional planes rather than the entire two-dimensional space. To overcome the lack of foresight in iterative optimization, we design the initial taper density solution using linear programming. Multiple numerical experiments validate the effectiveness of the proposed method, demonstrating successful array synthesis across various scales and limited scan angles.

While phased arrays (PAs) offer flexible beam forming and wide-angle scanning capabilities, the high cost of transmit/ receive (T/R) modules significantly limits the development of large-scale arrays. This study addresses the optimization design problem of large-scale sparse concentric ring arrays (SCRAs) for wide-angle scanning with suppressed sidelobes under practical constraints. To address the challenge of co-optimizing array sparsity and beam scanning performance, this study first introduces the concept of the sparsity lower bound (SLB) to thoroughly account for the effect of neglecting higher-order harmonics on approximation calculations. Next, by decoupling the dual optimization variables (ring radius and weights) in the sidelobe minimization problem, an innovative array configuration design framework based on alternating iterative optimization is proposed. To increase algorithm robustness, a Gompertz curve model is introduced to establish an efficient initial solution selection mechanism. Multiple examples show that the array optimization achieved by the proposed method is highly competitive. The most notable achievement is the achievement of a wide-scan sparse CRA with 176,697 elements (35.08% of the fully populated array), an ultralow sidelobe level of -48.48 dB, and an optimization time of only 732 seconds.

A novel wideband ultrathin planar retrodirective metasurface (RMS) is proposed. The structure is composed of anisotropic wideband retrodirective unit cells (WRUs), achieving retroreflection of the TE polarized wave from 6.8 GHz to 8 GHz at 30∘ of incidence. Each WRU is composed of a stack of patches separated from one another by thin dielectric substrates, and the entire structure supported by a ground plane. The bandwidth of retroreflection can be optimized by adjusting the patches in the WRUs to form a low-profile RMS. The equivalent circuit (EQC) model of the proposed WRU was established, providing a fast and efficient method for analyzing the wideband performance of the WRU. Additionally, at incident angles in the 25∘-35∘ range, the proposed RMS still achieves a relatively wide bandwidth of retroreflection, with the 3 dB bandwidth depending on the incident angle, providing greater opportunities for practical applications of the RMS. As a proof of concept, a prototype composed of six supercells was designed, fabricated, and tested, with a supercell consisting of 22 WRUs. Both full-wave simulation and experimental results demonstrate that the proposed RMS achieves a relatively wide bandwidth of retroreflection at incident angles in the 25∘-35∘ range. At the nominal incident angle of 30∘, the measured results show that the RMS achieves retroreflection within 16.2% fractional bandwidth, with an overall thickness of only 0.049λ.