# Oblique Wide-Angle Multi-Sector Metamaterial Absorber for Space Applications
## 1 Introduction  
Radar absorbing materials have been traditionally used for stealth purposes in the military domain. Recently, they have also become of great interest for solving electromagnetic interference problems in the aerospace domain [1, 2]. In particular, satellites may comprise multiple antennas relatively close to one another, which can affect their performances. A way to mitigate the multipath crosstalk between antennas is to cover the satellite platform with an absorbing material. For this type of application, the radar absorbing structure is mounted in the vicinity of the antenna and the absorber should operate at oblique incidence for a wide range of incidence angles. This point is important as in this case, classical designs for which the maximum of absorption is obtained at normal incidence are generally insufficient. Moreover, the absorber must be ultra-light and comply with space conditions (a wide range of temperatures, geomagnetic storm, resistance to atomic oxygen and radiation, etc.). The mass and harsh environment conditions constrain the design in that multiple dielectric layers are prohibited and only one metallic layer is advisable due to heat dissipation issues. The most classical approach for designing a radar absorbing structure is the Salisbury screen [3], which consists in a resistive sheet mounted a quarter wavelength apart from a ground plane. This solution can be lightweight and has the advantage of simplicity. The Jauman absorber [4] generalizes this principle to multiple resistive layers in order to broaden the frequency band of absorption. This involves thickening and mass augmentation of the structure, which is not desirable for space application. A classical way to achieve a more versatile design is to replace the resistive sheets of Salisbury or Jauman absorbers by a lossy frequency selective surface (FSS). This can lead to a more compact design, like in the case of metamaterial absorbers [5], or to broadband solutions, like in circuitanalog absorbers [6]. We will see that the presence of the FSS is also well-suited for oblique incidence as it provides additional parameters for the designer to use. Many studies have been carried out concerning the design of wide-angle absorbers in the literature [7; 8; 9; 10]. However, for all these designs, the maximum absorption is obtained for normal incidence and the performances degrade as the incidence angles increase. Article [11] is one of the first articles to address the question of oblique incidence in the literature, but the proposed design requires the use of relatively high permittivity (and high mass) dielectric substrates, which makes them unsuitable for space applications. We should also note that measurements of highly oblique angle of incidence are challenging [12]. In a previous study by the authors, two metamaterial absorbers for space applications based on resistive high impedance surface solution were proposed for the TT&C (Telemetry, Tracking and Control) S-band [2 GHz, 2.3 GHz], optimized respectively for normal and oblique (60\({}^{\circ}\) from normal direction) for both TE (Transverse Electric) and TM (Transverse Magnetic) polarization [13; 14] with experimental validation. These two metamaterial absorbers were designed for a specific and limited range of angle of incidence which was well adapted to the case of an antenna being positioned far away from the structure. However, when the antenna is close, it illuminates the structure over a wide angular range. The design of wide-angle absorbers centered on oblique incidence reveals to be a very challenging task. For a concrete scenario where antennas are mounted on satellite platforms, the position of the antennas is known in advance, which circumvents the problem. Instead of an ultra-wide-angle behavior, it should be enough to cover the platforms with different sectors, each being optimized for a specific angle of incidence in accordance with the antenna positions.