A cross-shaped composite with frequency selective surfaces is applied in the design of a broadband polarizer. The structure is multilayer, and the unit cell of each layer consists of a composite cross and a metal strip. The design takes advantage of both the similar amplitude and the stable phase difference of nearly 90° of the two orthogonal components of transmitted waves. The bandwidth of the polarizer reaches 74%, from 5.15 to 11.20 GHz, which is able to generate circular-polarization waves from incident linear waves. A sample polarizer is fabricated and tested. According to the measurement results, the polarizer operates from 5.75 to 11.20 GHz at normal incidence. In addition, its working frequency band can still cover the range of 5.80 to 11.00 GHz when the incident angle increases to 20°.

In this letter, a 16 × 16 Ku-band circularly polarized phased array is proposed. The antenna element comprises a cross dipole, a metal cavity, and a threaded SMP connector. The metal cavity makes the gain of the antenna increased and the mutual coupling between the array elements is reduced, thus ensuring good impedance matching and axis ratio (AR) during the two-dimensional beam scanning. The mutual coupling of the center element at center frequency is -26.6 dB when the cavity is added. The proposed phased array achieves the impedance bandwidth (| S ₁₁ | ≤ - 10 dB) of 10.1% (13.1-14.5 GHz) when scanning ±40° in the xz plane and yz plane. AR of the main lobe is less than 2 dB in the total scanning range. The gain fluctuation is less than 3 dB during scanning. This letter presents the measurement and simulation results, verifying that the phased array has good performance.

A broadband linear-to-circular transmission polarizer using a right-angled frequency selective surface is presented in this paper. The new proposed polarizer has both advantages of wide operational bandwidth and high transmission coefficients. To verify the design, a new polarizer working in X-band is optimized and fabricated. Experimental results show that the measured axial ratios lower than 3 dB range from 6.42 to 13.70 GHz, with low insertion loss. Meanwhile, the polarizer can operate over a broad range of frequencies from 6.75 to 10.75 GHz, when the incident angle increases to 25°.

In this article, a frequency selective surface sensor that can be used to detect the refractive index is designed. First, the absorber with single resonant frequency and high absorption is optimized and analyzed. The resonant frequency can be changed by loading a dielectric with different refractive indexes on the absorber. When the thickness of the unknown dielectric is constant, the resonant frequency of the sensor is uniquely related to the refractive index of the unknown dielectric. The linear relation between the two is obtained by simulation fitting, and the performance of the sensor is verified. $S_{{\text{fre}}}$ of the proposed sensor is ${1}{\rm{.311}} \times {1}{{0}^{9}}\,{\rm{Hz}}/{\rm{RIU}}$ (refractive index unit), and the figure of merit (FOM) can reach 7.2778 RIU ¹ .

This paper presents a broadband circular polarizer based on multilayer gradual frequency selective surfaces. The unit cell includes two identical circles and a bar, whose dimensions decrease layer by layer with a reduction factor. This design can provide a good phase difference that is close to 90°. The bandwidth of the polarizer reaches 66.7%, from 7.9 to 15.8 GHz, which can convert the incident linear polarization waves into circular polarization waves. A sample polarizer is fabricated and measured. The results show that the operation frequency ranges from 7.75 to 15.2 GHz, coinciding with the simulation. Meanwhile, the working frequency band covers the range of 8.2–14.5 GHz when the incident angle increases to 25°.

Millimeter-wave systems require absorbers with high absorption efficiency, broad bandwidth, and multi-functional tunability. However, existing devices based on composite structures and materials face design challenges: traditional methods hardly achieve all three indicators simultaneously, often sacrificing one performance for another, making “high absorption, broad bandwidth, multi-function” integration highly difficult. To overcome this fundamental trade-off, this study designs a W-band (75–110 GHz) composite absorber, integrating VO₂ and graphene, and combining periodic resonant structures with a backplane-free phase-change wire-grid architecture. When integrated into the periodic resonant structure, the VO₂ wire-grid provides thermal ‘on-off’ switching of absorption, whereas graphene offers electrical tunability—both achieved by modulating the effective surface conductivity and thus the impedance matching condition. Guided by an XGBoost-based closed-loop inverse design framework, the XGBoost model optimizes parameters, with convergence achieved after four closed-loop simulation iterations. Results show the VO₂ wire-grid absorber performs excellently at 90° incidence: it achieves near-perfect absorption (>99%) over a 21 GHz sub-band (82–103 GHz), showing an 80% improvement in average absorption compared to copper(Cu) backplanes, under identical unit-cell geometry and simulation conditions, with each configuration independently optimized for peak performance. Graphene provides a maximum modulation amplitude of depth of 68%. Innovations of this study include constructing a periodic resonant wire-grid structure that abandons traditional backplanes and uses wire-grids to expand loss areas, enabling dual-mode multi-physics control through co-integration of VO₂ and graphene, and establishing a machine learning closed-loop framework for parameter optimization. It explores a new paradigm for multi-functional tunable absorbers, providing high-performance solutions for MMW systems such as radar, communications, and biomedical imaging.