Int. J. Electron. Commun. (AEÜ) 154 (2022) 154329 Available online 19 July 2022 1434-8411/© 2022 Elsevier GmbH. All rights reserved. Regular paper A highly effcient low-profle tetra-band metasurface absorber for X, Ku, and K band applications Adnan Yousaf a , Mudassir Murtaza b , Abdul Wakeel a , Saiqa Anjum a a Department of Electrical Engineering, Military College of Signals (MCS), National University of Sciences and Technology (NUST), Islamabad 45710, Pakistan b Department of Electronics Engineering, University of Engineering and Technology, Taxila 47080, Pakistan A R T I C L E INFO Keywords: Metamaterial absorber (MA) Split ring resonators (SRRs) Cyclic-4 (C4) symmetry Scattering parameters (S-parameters) Tetra-band absorption ABSTRACT This manuscript proposes a highly effcient multiband, low-profle, and single-layered metamaterial absorber (MA). The unit cell of the proposed MA structure is based on split-ring resonators (SRRs) consisting of an outer square-shaped ring, four circular rings, and an inner vectorial-symmetric star shape design, implemented on the cost-effective FR4 substrate. Our proposed MA is highly effcient as it achieves tetra-absorption peaks at f 1 = 8.0 GHz, f 2 = 13.1 GHz, f 3 = 16.08 GHz, and f 4 = 19.2 GHz with 99.69%, 99.99%, 97.59%, and 99.99% absorptivity, respectively. The proposed structure is 0.65 mm thick, with 1/58, 1/36, 1/30, and 1/25 of respective free-space wavelengths. The proposed design has a polarization insensitivity (ϕ) up to 90 ◦ and an angular stability (θ) up to 70 ◦ , owing to the sub-wavelength size and improved cyclic-4 (C4) rotational symmetric structure of the unit cell. To understand the electromagnetic behavior and absorption methodology of the proposed structure, surface current distribution and electric feld results have been examined. A prototype MA design was fabricated and tested to validate the performance of the design. Under normal incidence of an electromagnetic (EM) wave, the MA prototype achieves near-perfect absorption at the desired resonant peaks. Variations of θ and ϕ were also examined on the fabricated model and the results found were consistent with the simulated ones. 1. Introduction Metamaterials are artifcial-structured designs formulated to realize diverse EM properties not found in the naturally existing materials. The exotic properties, achieved by the combination of metals and the dielectric materials in the metamaterials, enhance their applications from the microwave to visible spectrum [1]. Primitive techniques, like Faraday’s effect and birefringence, though, can be used to manipulate the polarization of the incident EM wave. However, they result in rigid, expensive, and bulky designs [2]. Metamaterials, on the other hand, are nanoscale artifcially designed structures that provide more fexibility in the design, customization, and miniaturization with diversity in terms of applications, simultaneously [3]. The concept of metamaterials was frst underlined by V. G. Veselago et al. [4], who proved the existence of negative permittivity and permeability theoretically. Later, Landy et al. demonstrated metamaterial-based EM wave absorption [5]. However, it took them some time to provide their analytical and experimental reasoning. Today, metamaterials exist in different forms such as multi- functional [6], cross, and circular polarizers in refection [7,8] and transmission [32], and many more. A well-known class of metamaterials, i.e., metamaterial absorbers, exists in a vast variety including, single-band [9], multi-band [38], and wide-band [10] ab- sorbers [11], designed using passive elements [12] or active elements- based structures [13,15,16]. Active or lumped-element-based struc- tures have their pros of compensating the losses, however, it comes at the cost of design complexities, non-linearities, and power issues. Recently, most of the research is focused on achieving multi-band and broadband absorption using simplifed designs [15–27]. However, the absorptivity in most of the designs available in the literature is less than 90%, and the designs are polarization sensitive as well as angularly unstable. A detailed performance comparison of the proposed MA with different recent designs available in the literature is provided in Table 3. To further simplify the designs, single-layered MA structures have been vastly experimented and applied with, owing to the obvious ben- efts of the inherent fexibility in design due to thin structures, low cost, and less computational complexity. In [28], a Jerusalem-cross shaped MA is proposed for X and Ku bands applications at 8.6, 10.2, and 11.95 GHz. From their proposed design, a maximum of 84% absorption with polarization insensitivity up to 60 ◦ and angular stability up to 50 ◦ can be achieved. In [29], a hybrid metamaterial absorber has been proposed for E-mail addresses: ayousaf.msee-26mcs@student.nust.edu.pk (A. Yousaf), awakeel@mcs.edu.pk (A. Wakeel). Contents lists available at ScienceDirect International Journal of Electronics and Communications journal homepage: www.elsevier.com/locate/aeue https://doi.org/10.1016/j.aeue.2022.154329 Received 11 March 2022; Accepted 9 July 2022