In this paper, a 3-D frequency-selective surface (FSS) with high selectivity and large angular stability of 70° is proposed. The equivalent circuit model of the single-layer FSS is first presented and analyzed, offering physical support to the operating principle using the odd- and even-mode analysis method. By cascading two single-layer FSSs, the resulting 3-D double-layer FSS operates as a fifth-order bandpass filter. The filling design of dielectric block, metal block and waveguide wall makes the 3-D FSS produce different magnetic coupling effects in parallel and perpendicular to the electromagnetic wave propagation direction to obtain an electromagnetic filtering response with broadband and good roll-off characteristics. By introducing the outer dielectric bonding to the 3-D FSS, the angular stability is greatly improved from an angle of 0°-70°. Finally, the veracity of the design is verified by physical processing and testing.

A machine learning-assisted quasi-bisection method (MLAQBM) is proposed for the broadband optimization of the pixelated patch antennas. Distinct from traditional MLA optimization methods, MLAQBM employs a coarse-to-fine pixel partition and machine learning optimization to iteratively refine the shape of the antenna. Two implementation strategies are introduced. In the first strategy, larger pixels are initially partitioned and samples are collected. A surrogate model is then established using convolutional neural networks (CNN), and an initial shape is derived through particle swarm optimization (PSO). Again, further refinement is achieved by finer pixel partitioning and subsequent CNN-based PSO. The second strategy leverages a pre-existing broadband E-shaped patch design, which is then subdivided into smaller pixels. By collecting samples, training a CNN surrogate model, and performing PSO optimization, a broader bandwidth is realized. Compared with the reported MLA optimization methods, MLAQBM establishes a stable surrogate model that is robust, easy to converge, and adaptable to different optimization objectives. To validate the proposed strategies, the optimized pixelated patch antennas are fabricated and measured. The measured results demonstrate that the bandwidth of the proposed patch antennas achieves 24.5% and 32.5%.

A fast method is presented for designing ultra-wideband absorbers composed of resistive surfaces (RSs), substrates and metallic plate. A general effective permittivity for RSs printed on stratified dielectrics under considering metallic plate is derived to obtain the closed-form relationships between resistance, capacitance and absorber parameters. An equivalent circuit (EC) is developed for absorbers to combine with genetic algorithm to quickly achieve an optimal value of lumped elements, thereby calculating absorber parameters. Utilizing the proposed approach, an absorber within 0.64-41.7 GHz with reflection loss lower than -10 dB is obtained efficiently. The close match between the EC model and full-wave simulation results shows accuracy of the proposed approach. To further verify the proposed method, a 1.16-19.2 GHz absorber was designed and manufactured, and the measurements agreed well with the EC and full-wave simulations.

This letter presents an ultra-broadband metamaterial absorber with angle-insensitive characteristics covering the L, S, C, X, and Ku bands. The proposed metamaterial absorber achieves wave-absorbing properties through the applications of a cross-shaped substrate printed with high-resistance carbon paste and a frequency-dispersive magnetic material (FDMM). Under the combined effect of the two substructure, the absorber realizes an ultra-wide absorption band from 0.84 GHz to 20 GHz with at least -10 dB reflectivity. In addition, the proposed composite metamaterial absorber shows a wide-angle absorbing performance for both TE and TM polarized waves, and the angular stability is up to 60° for TM polarization. With the application of the FDMM, the thickness of the metamaterial absorber is limited to 26.5 mm (0.074). A prototype is fabricated and measured, and the measurement results agreed well with the simulation.

A multilayer frequency selective rasorber (FSR) with both broad passband and ultra-wide absorption band is proposed in this letter, utilizing graphene-based films and via-based winding inductors. It is constructed by two lossy layers with a broad passband and a third-order frequency selective surface (FSS) placed at the bottom. The unit cell of the top lossy layer is composed of a square graphene film at the center and four arms of via-based winding inductors terminated with a gap capacitor, which extends the absorption band at low frequency and broadens the transmission band at medium and high frequencies. The second lossy layer contains a square graphene film at the center and four strip lines connected to spiral inductors with parasitic capacitance, which enables absorption at high frequency while maintains transmission at low and medium frequencies. The bottom FSS consists of two square patches coupled through a square ring slot for third-order wideband transmission and acts as the ground of the lossy layers. The proposed FSR achieves ultra-wideband impedance matching from 1.56 GHz to 18 GHz (168%), with a passband range of 7.15 GHz to 12.4 GHz (54%) and a minimum insertion loss of 0.56 dB.

A smart stealth radome should absorb incident electromagnetic (EM) waves across an ultra-wide frequency band while maintaining transparency in specific windows for radar and communications signals. This can be achieved using absorption-transmission-absorption (ATA) frequency selective rasorbers (FSRs), though realizing ATA FSRs with both broad bandwidths and sharp transitions remains challenging. Here, we combine multiple dispersion anomalies to construct an ATA FSR with nearly square window-like spectral response. The proposed structure adopts a cascaded configuration comprising two lossy layers and a bandpass frequency selective surface (FSS). Through collaborative dispersion engineering and equivalent circuit model, theoretical guidance is provided for the design and optimization of the spectral response. Simulated results show that the FSR achieves a wide −1 dB transmission band from 7.9 to 12.1 GHz (42%), along with two absorption bands exceeding 90% in 1.81~6.7 GHz (115%) and 13~18.55 GHz (35%). The average roll-off rate of transmission (−20 to −1 dB) reaches about 19.2 dB/GHz, while that of absorption (20% to 80%) is approximately 280%/GHz, outperforming previously reported designs. The measured results of the fabricated samples are generally consistent with the simulated ones. Our design holds significant potential for various radomes, which are quite effective in facilitating high performance EM compatibility and stealth.