A polarization-insensitive ultra-wideband absorber based on hybrid structure
Kun Xue, Yifeng Qin, Haoliang Sun, Min Han*, Hongyi Zhu and Shaohua Dong*
As detection technology continually advances, the survivability of targets on the battlefield is significantly challenged. Therefore, microwave absorbers with stealth capabilities have become a focal point of research in modern military science. To address the issues of narrow bandwidth and complex structures in existing absorbers, we propose a model for an ultra-wideband absorber based on a hybrid structure. In this study, we design, manufacture, and characterize a polarization-insensitive ultra-wideband absorber (PIUWA), which demonstrates impressive absorptivity of over 90% across a range of 4-24.53GHz (a fractional bandwidth of 144%). This is achieved by inducing multiple resonance peaks within the hybrid structure. Moreover, the subwavelength periodicity of the PIUWA theoretically contributes to its angular stability under full-wave polarizations. We observed that absorption performance remains stable under incident conditions within 45 degrees. Furthermore, the operational mechanism of the PIUWA is elucidated through an equivalent circuit model, with design validity confirmed via experimental measurements. This study paves the way for the design and fabrication of ultra-wideband microwave absorbers that offer high absorptivity, robust angular stability, and simpler assembly processes, thereby broadening the potential for application in other absorber types.
Introduction: The deployment of advanced detection technologies underscores the urgency of minimizing the detectability of military equipment in contemporary battlefields. Metamaterials, owing to their robust electromagnetic (EM) regulation abilities across various domains, have emerged as promising candidates for crafting stealth materials [1-3]. In recent times, microwave absorbers have garnered considerable attention for their superior performance over conventional frequency selective surfaces (FSS) in reducing the radar cross section (RCS) of multi-station radars. These absorbers achieve this by dissipating incident EM power, thereby enhancing device survivability, as illustrated in Fig.1[4-6]. Early proposals for microwave absorbers included the Salisbury screen and the Jaumann absorber. However, their real-world application has been hindered by constraints such as narrow bandwidths or excessive thickness [7,8]. In light of the evolving requirements of stealth systems, wideband absorbers have gained significant interest. Consequently, circuit analog absorbers (CAAs) were introduced to pave the way for thinner microwave absorbers with broader bandwidths [9].
Broadly, wideband absorbers fall into two structural categories: planar [9-14] and three-dimensional [15-19]. Planar metamaterial absorbers typically achieve broadband absorption through the employment of FSS loaded with lumped resistors [10-12], or by using High Impedance Surfaces (HIS) as lossy layers [13,14]. These lossy layers are typically positioned a quarter-wavelength above the metal plate to optimize absorption. The first case often involves using a Square-Loop-Array (SLA). However, the design of an SLA absorber necessitates numerous lumped resistors; for example, the designs in [10] and [11] each require eight resistors per unit, while [12] requires sixteen lumped resistors per element. The utilization of an SLA is primarily aimed at generating multiple resonances to broaden the absorption bandwidth. However, as shown in [10-12], a single-layer SLA can only produce a maximum of three resonance points. To induce additional resonance points, it becomes necessary to either add more square patterns to the same plane or to incorporate more resonant layers in a unit. Regrettably, these methods not only increase the period or thickness of the absorber but also complicate the manufacturing process. Another approach, which involves the use of high impedance materials such as resistive inks with the appropriate surface resistance as a lossy layer, offers a very limited absorption bandwidth. As seen in [13] and [14], only two or three resonances exist within the absorption band with fractional bandwidths of 112% and 92%, respectively. Moreover, the application of large quantities of resistive ink, which is challenging to spray evenly, can negatively impact the absorption performance in practice. Recently, wideband 3D absorbers, grounded in cavity theory [15-18] or radiation pattern synthesis[19], have drawn the attention of many researchers. However, the assembly process of 3D absorbers is cumbersome due to their complex structure, and some of them can only facilitate fixed-polarization absorption [16,18].
(a) (b)
Fig. 1Comparation of FSS and microwave absorber in the presence of multi-station radars. (a) FSS. (b) microwave absorber and the 4×3 PIUWA units.
Drawing from the above discussion, it’s clear that there are areas for improvement in ultra-wideband (UWB) absorbers. For two-dimensional structures, the goal is to generate more resonances within a limited number of layers. For 3D absorbers, the complex fabrication and assembly process necessitated by their intricate structure warrants simplification. Against this backdrop, we propose an ultra-wideband absorber based on a hybrid 2D and 3D structure in this study. The 3D upper layer of the hybrid structure naturally forms a cascade transmission line between the dipoles, leading to an increased number of resonance peaks and a subsequent widening of the absorbing bandwidth. The proposed PIUWA exhibits a broad absorption band from 4 to 24.53GHz, representing a fractional bandwidth (FBW) of 144%, all while maintaining a compact size of 0.10λL× 0.10λL× 0.11λL (where λLdenotes the wavelength at the lowest cut-off frequency). The proposed PIUWA offers several advantages: 1) In comparison to 2D structures, the PIUWA provides a wider absorption bandwidth without the need for additional materials to support the lossy layer and backplane. 2)When contrasted with 3D structures, the PIUWA has a simpler structure that facilitates easier design and installation. 3) The subwavelength periodicity of the PIUWA is theoretically advantageous for maintaining angular stability and avoiding grating lobes [20].