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].