## 1 Introduction  
A study of electromagnetic (EM) shielding absorbers is actively carried out, along with an investigation of the increase in undesirable EM radiation interference from an explosive growth of wireless devices, the high integration of electric equipment in automobiles, and several small radio stations. Furthermore, the EM absorbers can be effectively utilized to reduce the radar cross-section (RCS) in future electronic warfare, where many electronics are intensively engaged in information aggregation, engagement, and operations of surveillance and reconnaissance. There are commonly desired features for EM absorbers in either modern electronics systems or for a military purpose. From a performance perspective, a wide operating frequency range and the orientation independency of the absorber are essential, since the frequency, polarization, and incidence angle of the interference radio wave are generally unknown [1]. In addition, a planar, or thin profile form factor, is desirable compared to the commonly used bulky triangular pyramid shapes, since only a minimum footprint would be allowed for the absorbers due to the miniaturization and high integration development trend of electronics. Such a thin profile is favorable not only for electrical stealth but also for physical concealment in military vehicles [2]. However, it is challenging to achieve all the listed characteristics from conventional microwave absorbers.  
Metamaterial (MTM) is an engineered periodic structure made of sub-wavelength unit cells, and it shows a frequency response that does not exist in nature, such as negative permittivity, negative permeability, or both. Such a feature is now widely used in various applications of antennas and high-frequency circuits, from microwave to THz frequency ranges [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Its artificial characteristic causes the device to show an improved performance, one that has not been possible while using a material with ordinary frequency responses. Additionally, a wide range of research has recently been conducted on MTM-based EM-absorbing structures [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. However, since MTM structures are bound to exhibit frequency-dependent responses, techniques that use a multi-layer pyramid shape [21], lumped elements [22], and resistive films [23; 24; 25; 26; 27; 28] have been proposed as a means of widening the operating frequency range of MTM-EM absorbers. Among these, the multi-layer MTM absorber should have a thick profile, and a lumped elements-based MTM absorber is generally harder to fabricate; these features can be thought of as disadvantages in terms of ease of use and realization, respectively. In comparison, resistive film-based MTM absorbers have a relatively thin profile, and an easier form factor to build and handle. Representative resistive materials that can be used in absorbing structures may include indium tin oxide (ITO) and antimony tin oxide (ATO), with ITO chosen more often due to its comparatively higher transparency [29]. This is because transparent absorbers are more suitable for displays, mobile phones, and the glass windows of military vehicles [30; 31].  
In this work, we propose extremely thin, wideband, ITO-based MTM-EM absorbers using split ring resonators (SRRs). This reveals an improved bandwidth ratio over the structure profile when compared to reported works in the literature. It also achieves orientation independence from the polarization of the incoming EM wave due to the symmetric shape of the used SRRs. The design procedure and working mechanism of the proposed structure are explained by the numerical calculation results using the CST Studio Suite. The prototype is built, and all full-wave EM simulations are verified experimentally.