## 1 Introduction  
Over the last decades, the electromagnetic community has witnessed a significant increase in research endeavors focused on metamaterials and metasurfaces, driven by their wide range of applications including antennas performance enhancement, radar technology, electromagnetic shielding, mmWave, and satellite communications [1, 2]. In particular, frequency selective surfaces (FSSs) have emerged as a pivotal solution for electromagnetic interference (EMI) shielding, providing robust attenuation of both unintentional environmental noise and deliberate electromagnetic threats. By effectively mitigating external electromagnetic disturbances, FSSs contribute substantially to the enhancement in electromagnetic security and operational safety in critical systems including unmanned aerial vehicles (UAVs), electric vertical take-off and landing (eVTOL) aircraft, power substations, and other high-reliability electronic infrastructures [3]. They essentially consist of 2-D/3-D arrays of elements, often referred to as unit cells, meticulously arranged in a periodic structure with subwavelength dimensions. Among this huge class, metasurfaces are particularly important due to their thin profile, making them easily integrable into existing systems. The fundamental configuration of a metasurface involves the etching of a frequency selective surface (FSS) onto a dielectric substrate. Nevertheless, researchers have conceived various multilayer arrangements by cascading FSS layers, achieving superior performance and complex functionalities. Specifically, this manuscript focused on a specific operation of FSSs (i.e., when they act as spatial filters). FSS-based filters can implement almost any desired frequency response, therefore encompassing low-pass, bandpass, high-pass, and band-stop characteristics. Unlike traditional microwave filters that typically consist of a pair of terminals where the response is assessed by computing the ratio between output and input signals, the transfer function of spatial filters is not solely contingent upon frequency. Instead, it also varies based on the polarization and angle of incidence of the impinging electromagnetic wave. Clearly, researchers are continuously developing filters with peculiar features; among them, possessing a broadband behavior is particularly appealing in applications like radomes [4; 5; 6] and electromagnetic shelters [7; 8; 9]. However, despite these emerging demands, the limited operational bandwidth remains a significant constraint for FSS-based filters. The requirement of enlarging the operating bandwidth has led to a number of design methodologies such 2.5-D and 3-D topologies for bandpass and band-stop metasurfaces, wideband absorbers, and absorptive frequency selective transmission structures [10; 11; 12; 13; 14; 15]. In [14; 15], 2.5-D configurations were employed to create frequency-selective rasorbers capable of achieving a wide and highly transparent pass-band situated between two absorption bands. This was accomplished by combining the large inductance of parallel resonators (PRs) and cascading two 2.5-D PRs, which also provided miniaturization. Compared with 2.5-D FSSs, 3-D FSSs exhibit more design flexibility and show potential in wideband band-pass filtering response. In particular, 3-D FSSs can accomplish sharp filtering responses by maintaining, at the same time, a fractional bandwidth superior to 100%. This capability is due to the unit-cell size, which is significantly small compared with the operating wavelength, even in a single layer configuration [16]. Since 3-D structures can form different resonant loops and generate several resonant poles by exploiting the third dimension, the realization of an FSS consisting of three parallel-coupled stepped-impedance slot line resonators (SISRs) was introduced in [17]. This solution resulted in the presence of six transmission poles within the operational band, conferring a high selectivity to the passing band.