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In this article, a rasorber with energy-selective passband and ultrawideband absorptive out-of-band is proposed, which combines the advantages of conventional frequency-selective absorber (FSA) and energy-selective surface (ESS). The proposed design consists of an upper lossy layer and a lower active layer. By analyzing the equivalent circuit model (ECM), the lossy layer could generate three transmission zeros, thereby achieving ultrawideband out-of-band absorption independent of the incident power. The active layer embedded with p-i-n diodes exhibits a nonlinear response to the incident intensity of spatial waves, which can be switched between bandpass and bandstop resonances under low and high incident intensities based on ECM. The steady-state and transient-state characteristics of the proposed power-dependent rasorber are analyzed as well. For demonstration, a prototype with $13\times 13$ unit cells is fabricated and measured in free space for a low-power experiment, while two prototypes with $4\times 2$ unit cells and $2\times 1$ unit cells are processed and measured in rectangular waveguides for a high-power experiment. Under low incident power level, a high-selectivity transparent window is successfully realized with simulated (measured) low insertion loss (IL) of 0.9 dB at 3.58 GHz (0.83 dB at 3.44 GHz) and an ultrawide absorption band range of 5.87–17.88 GHz (5.98–18 GHz). As the incident field is higher than 740 V/m, the transmitted power undergoes a rapid reduction in the passband. The maximum shielding effectiveness (SE) reaches 18 dB at 3.58 GHz and the SE is better than 10 dB in the range of 0.5–17.5 GHz. The excellent nonlinear transmission characteristic with ultrawideband out-of-band absorption makes the proposed rasorber have various application prospects in the high-power microwave (HPM) protection and stealth.

Digital predistortion (DPD) has proven to be an efficient method of linearizing power amplifiers (PAs). In recent years, the use of neural networks (NNs) for DPD has gained momentum. This development can be attributed to the general attention NNs have gotten in recent years, but more importantly, to their ability to find efficient solutions to problems that either have no explicit solution, or the existing solution is sophisticated. Previously, DPDs were designed using one of the two main methods that originated from iterative control: direct learning architecture (DLA) and indirect learning architecture (ILA). Today, NN-based DPDs are using a new tool, but the same old architectures. In this article, we used the ability of an NN to break from the classic ways and come up with a new architecture, where a single NN models the entire system. As proof of concept, a 6-W single-transistor GaN PA, amplifying a 5G test signal, is linearized using the proposed method.

A wideband energy capturing adapter based on 3-D meta-structure is presented in this article for wireless power transmission (WPT) system. Compared with conventional planar resonant structure, the proposed 3-D meta-structure for energy collection is designed first. After analyzing the magnetic field magnitude and power loss density distribution of the 3-D meta-structure unit cell, it is found that the induced current lengths are different under ${x}$ -polarization and ${y}$ -polarization, resulting in different equivalent circuits and operating bandwidths. The collecting efficiency of the 3-D meta-structure unit cell under oblique incidence is investigated as well, demonstrating the stable wide-angle reception. Then, a wideband combining network is proposed to collect the alternating current (ac) power from an $8\times8~3$ -D meta-structure array based on multistage T-junction power dividers and impedance transformers. The operating bandwidth of the proposed combining network can cover that of the 3-D meta-structure array. After energy collection, a rectifier circuit based on a multichannel parallel connection is presented for wideband ac-to-direct current (dc) conversion. Due to the nonlinear characteristics of the proposed rectifier circuit, the effects of input power level, load resistance, and operating frequency on rectification efficiency are studied in detail. Finally, a prototype is fabricated and tested for demonstration. Measured results show that the proposed design can achieve over 30% system conversion efficiency $\eta $ at 3.5–8.5 GHz (6.2–6.7 GHz) under ${x}$ -( ${y}$ -) polarization. Simulated and measured results are in good agreement.

This article introduces a novel design for a thin, single-layer, optically transparent microwave metamaterial absorber. The proposed structure is composed of a polycarbonate (PC) substrate with two sides coated with gold films of a few tens of nm thickness via thermal evaporation. A specific periodic pattern is etched onto the upper side of the substrate using the optical lithography technique, while the bottom side serves as the electrical ground plane. The absorber’s performance was analyzed using comprehensive full-wave electromagnetic (EM) simulation and equivalent circuit modeling, which were validated by experimental measurements, showing excellent agreement. The fabricated transparent absorber demonstrated over 90% absorption in a broad bandwidth (BW) from 8.5 to 23 GHz, covering $X$ , $Ku$ , and a portion of $K$ bands, with an average optical transparency of approximately 75%. Our full-wave simulation results showed that the proposed absorber’s performance is highly insensitive to the incident angle and polarization angle due to its symmetrical pattern. The proposed single-layer absorber has a thin, rigid structure with high transparency, making it suitable for various real-world applications.

Temporal late-time reflection and transmission natural mode amplitudes ( $A_{n}$ , $B_{n}$ , $R_{n}$ , and $Q_{n}$ ) of an ${N}$ -layer multilayer structure are examined by separating the whole structure into left and right regions and middle layer. Due to this separation, some relations among these amplitudes are analytically disclosed. It is shown that if the structure has reflection symmetry, then the relation between $A_{n}$ and $B_{n}$ (and $R_{n}$ and $Q_{n}$ ) is in the form $A_{n} = (-1)^{n-1} B_{n}$ (and $R_{n} = (-1)^{n-1} Q_{n}$ ), where $n$ is the mode number. These relationships are independent of material properties. A practical example related to damage or defect detection is considered to show the usefulness of these simple relations. The effect of noise on this detection was examined by introducing a Gaussian white noise (with a signal-to-noise ratio (SNR) ranging from 40, 30, and 20 dB to 10 dB) to the scattering parameters. It is observed that our proposed formalism is resistant to noise when detecting material defects or damages in reflection-symmetric multilayer structures. The effectiveness and feasibility of the derived objective function for a reflection-symmetric multiliayer structure were validated by coaxial line spectral (up to 26.5 GHz) and time-domain (via transformation) measurements of a three-layer structure.