# Parametric Testing of Metasurface Stirrers for Metasurface Reverberation Chambers
## 1 Introduction  
In general, a reverberation chamber (RC) consists of a metal shielded chamber containing stirrers, antennas, a piece of equipment under test, as well as other devices. Formally, an RC can be defined as an electrically large, high quality factor (Q), multimode cavity that uses mode stirring to create changing boundary conditions in order to obtain a statistically uniform electromagnetic field [1, 2]. RCs, also called mode-stirred chambers, are common test facilities widely used for electromagnetic compatibility (EMC) measurements, multipath environment characterization, electromagnetic immunity analysis, power radiation estimation, etc. [3]. In the past, many efforts have been focused on trying to optimize the performance of metallic stirrers in conventional RCs. Mainly through the application of empirical techniques and common sense, heuristic guides, which define and adopt many design criteria, have been proposed. For instance, stirrers should be greater than or equal to the wavelength of the lowest usable frequency (LUF), stirrers should be shaped such that a non-repeating field pattern is obtained in one rotation, etc. In this sense, references [3, 4, 5, 6, 7, 8, 9, 10, 11] provide some practical contributions to stirrer design and stirrer optimization, and there are examples of flexible stirrers that optimize RC performance by selecting the optimum angle of the bend [5, 6], the position of the stirrer blades [7, 8], or the independent number of the stirrer positions [9, 10]. Despite the efforts directed towards stirrer optimization, certain limitations and constraints arise in real-life scenarios (e.g., large stirrers penalize the available test volume; complex stirrer shapes may hinderthe design and implementation, thereby increasing costs). To date, few efforts have been concentrated on alternative stirring methods which may provide a solution to the previous limitative aspects. In this regard, the concept of a metasurfaces reverberation chamber (MRC), with stirrers based on metasurfaces, has been recently proposed [11] as an alternative to conventional RCs. Metasurfaces are the planar counterparts of metamaterials, consisting of a single-layer or few-layer stack of planar structures, which can be fabricated using lithography and nanoprinting methods, and are usually described by effective medium parameters at the macroscopic scale; see [11; 12]. In [11], we used a 1-bit random coding metasurface to realize the MRC. The 1-bit random coding diffusion metasurface was designed in order to obtain all-direction backscattering under normal incidence. By rotating the metasurface stirrer, wave diffusion (at least in the transient electromagnetic field distribution) is ensured. Moreover, such a metasurface is not only a specific mode tuner, but also a phase wall with multiple boundary conditions. Thus, the mode structure changes at each stir state. The main conclusion in [11] was that although the field uniformity was only somewhat improved for the MRC with the rotating 1-bit coding diffusion metasurface (as compared to the traditional one), an extended MRC test zone was achieved due to the lack of mechanical stirrer. In [13], we analyzed the physical mechanism for increasing the number of modes, lowering the operating frequency, and improving the field uniformity in the reverberation chambers, and concluded that a designed sequential metasurface provided better stirring effect than the random one. After dealing with these studies, it is concluded that the coding metasurface stirrer can improve the uniformity of field distribution in the MRC, and can replace the traditional metallic stirrer. Moreover, using metasurface walls in the chamber is useful to increase the number of modes and to lower the useful frequency, especially for the metasurface with unit cells arranged alternatively [13]. In this work, we study and compare three different types of rotating metasurfaces stirrer (also including the mechanical metallic stirrer in the comparison) on the basis of their influence on RC performance.