Bo Li

This communication presents a novel synthesis design method for absorptive frequency-selective reflectors (AFSRs) with arbitrarily specified reflection bandwidths. The proposed method integrates an ultrawideband impedance matching network with a bandstop filtering network to achieve flexible control over absorption and reflection characteristics. Detailed circuit models for both networks are proposed and cascaded to construct AFSRs with either wide or narrow reflection bands. A customized synthesis procedure maps desired real-rational functions to the transfer functions of the respective circuit models, enabling precise determination of circuit parameters based on given design specifications. To validate the theoretical predictions, the single- and dual-polarized AFSR prototypes with the wide and narrow reflection bands are designed, fabricated, and measured. The wide band prototype operates from 1.38 to 8.68 GHz (FBW = 145.1%) with a reflective 3dB bandwidth of 3.31–6.70 GHz (FBW = 67.7%) for TE-polarized incident wave. The narrow band prototype operates from 1.26 to 8.29 GHz (FBW = 147.2%) with reflective 3dB bandwidths of 4.90 to 5.19 GHz (FBW = 6.1%) for both TE- and TM-polarized ones. The simulated and experimental results consistently confirm the effectiveness of the proposed synthesis design method for designing AFSRs with arbitrarily specified reflection bandwidths.

Huygens metasurfaces (HMS) formed by Huygens sources can flexibly control electromagnetic waves to generate any desired radiation pattern. This capability is precisely what holographic multiple-input multiple-output (HMIMO) communications seek to achieve. Therefore, deploying HMS at the base station provides an attractive solution for next-generation wireless communication systems. Against this background, this study establishes an electromagnetic channel model to analyze the effective degree of freedom (EDoF) of HMS systems, in which EDoF directly characterizes the spatial multiplexing capability of HMS systems. This aspect has not been addressed in previous research. Specifically, we first model the channel of HMS using the electric and magnetic dyadic Green’s functions in electromagnetic theory. Since the channel is unique to HMS, it is specially called the Huygens channel. Then, we analyze the EDoF of the Huygens channel. Finally, by comparing with existing EDoFe studies based on the electric dyadic Green’s Function (EDGF) channel, the results demonstrate that the Huygens channel exhibits a larger EDoF in the near-field region. This provides important insights for the design and optimization of future communication systems, such as the HMIMO communications system.

This paper presents a novel synthesis method for designing arbitrary-order and ultra-wideband three-dimensional (3-D) 90° polarization-rotating structures (PRSs) using Chebyshev functions. The proposed PRSs consist of orthogonally arranged single-layer printed circuit boards (PCBs) with slotline configurations, enabling efficient conversion of incident linearly polarized waves from the x-direction to the orthogonal y-direction over a wide frequency range. By leveraging Π-type and T-type shunt short-circuited stub topologies, the design achieves even- and odd-order wideband filtering responses, respectively. A systematic synthesis procedure is developed, mapping the circuit model’s transfer function to a Chebyshev response with specified return loss (RL) and fractional bandwidth (FBW). Theoretical, simulated, and experimental results demonstrate that the fourth- and fifth-order PRS prototypes exhibit wide operational bandwidths (75.6% and 118%, respectively), low insertion loss (IL) (84%) under oblique incidences up to 40°. The proposed 3-D PRSs offer significant advantages in terms of angular stability, design flexibility, and performance, making them suitable for advanced applications in polarization manipulation and wideband frequency-selective systems.

Conventional reconfigurable intelligent surfaces (RISs) mainly focus on in-band performance. This article introduces a novel methodology for absorptive filtering reconfigurable intelligent structures (AFRISs) that can effectively steer incident waves to desired directions while simultaneously absorbing out-of-band interferences. Initially, the reflection response of an AFRIS element is modeled through an equivalent circuit model (ECM). For analytical clarity, the ECM is divided into three distinct components: the bandpass section, the phase-shifting section, and the bandstop section. The analysis reveals that the absorption bandwidth of the AFRIS element is determined by the bandpass section, the reflection bandwidth is controlled by the bandstop section, and the reflecting phase is governed by the phase-shifting section. This framework enables the customizable reflection response of the AFRIS element. Subsequently, a 2-bit AFRIS element with 10% reflecting bandwidth and 80% absorption bandwidth is designed and simulated using a single-pole four-throw (SP4T) switch. Both theoretical analysis and simulated results validate the proposed design concept. Ultimately, a prototype of AFRIS with $19\times 9$ elements is fabricated and measured for verification. The measured 1-dB reflection bandwidth compared the maximum reflection ranges from 3.78 to 4.22GHz (11%), while the 10-dB absorption bandwidth spans from 2.37 to 3.66GHz and 4.34 to 5.74GHz (67.3%). The beam scanning angle range exceeds 60°, and the efficiency is greater than 50% within a $\pm ~40^{\circ }$ range. The measured results closely align with the theoretical and simulated ones, indicating that the proposed AFRIS holds significant potential for future wireless communication applications due to its customizable in-band and out-of-band performance.

In this Letter, a novel synthesis method for the dual-polarized ultrawideband microwave absorber (MA) under specific oblique incidence operation is proposed, utilizing a polarization separation design. Each MA element is composed of slotline resonators and thin-film resistors. Accurate equivalent circuit models (ECMs) are then established to explain the absorption mechanism. The transfer function of the ECM is mapped to the desired real rational function to determine circuit parameters for specific design specifications. To verify the theoretical predictions, prototypes with the same and different specifications for dual polarizations are designed under various large incidence angles. Additionally, a more generalized prototype with different specifications is fabricated and measured. All simulated and experimental results validate the efficacy of the synthesis design method, which presents an accurate and efficient solution for designing dual-polarized ultrawideband MAs under specific oblique incidence operation.

This work introduces a dual-band reflectarray antenna with low out-of-band radar cross section (RCS), which is constructed with three-dimensional frequency selective structures (3-D FSS) elements. The element comprises two parts in serial: a single short-ended slot to provide required phase shifts and a cascaded short-ended slot loaded with resistors to absorb out-of-band incident waves. Two 3-D FSS element arrays arranged in two orthogonal directions operate independently in the C-band and X-band, respectively, achieving dual-band responses. A prototype of the proposed reflectarray antenna was designed, simulated, fabricated and measured. Both simulations and measurements demonstrate that the antenna can achieve a pencil-beam radiation pattern at both C band and X band. At C band, the antenna achieves an aperture efficiency of 55.1% and a directivity of 24.6 dBi, while at X band, it achieves an aperture efficiency up to 56% and a directivity of 29.8 dBi. Furthermore, the proposed antenna demonstrates RCS reduction over 10 dB compared to a PEC of equivalent size, reaching up to 146.5% and 148.4% bandwidth for TE- and TM-polarized incidences, respectively.

This communication presents a unified method for extracting and characterizing the equivalent circuit parameters of frequency-selective surface/structure (uniformly abbreviated as FSS), by introducing the concept of numerical short-open and short-open-load calibration techniques using a “field-circuit” combination approach. Each periodic element of an FSS is modeled as a two-port network, which is then divided into three cascaded sub-networks: two spatially excited feedlines and a core element loading. By establishing the perfect standards of ideal short-circuit, open-circuit, and matching load at reference planes along all the feeding parts, the entire network parameters can be solved across a wide frequency band under periodic boundary conditions. A suitable equivalent circuit model (ECM) is then formulated, allowing for systematic and quantitative characterization of all circuit parameters via extractions. The effectiveness of the proposed method is demonstrated by extracting simple and regular 2-D element models, as well as 3-D stacked slotline element models. The method yields accurate extraction results with specified ECM for arbitrary incident angles or polarizations within fundamental mode operation.