## 1 Introduction
Another design procedure for wideband bandpass filters was proposed in [18; 19], where the FSS was fabricated as a completely metal structure, producing a consistent, polarization-insensitive bandpass filtering performance. Nonetheless, 3-D structures, despite presenting extra degrees of freedom in their design, lack compactness, and their fabrication is complicated. To date, the most widespread technique for facilitating the fabrication of a spatial bandpass filter consists in cascading multiple surfaces separated by dielectric slabs. For instance, in [20], a broad bandpass response characterized by a nearly flat plateau and rapid attenuation (with 154% of fractional bandwidth) necessitated the cascading of three FSS layers. However, this method typically yields a bulky structure that presents heightened sensitivity to incident wave angles, especially in low-frequency applications. In contexts such as low-frequency antenna radomes, FSSs with array elements possessing relatively small electrical dimensions are preferred due to their reduced sensitivity to incident angles and compatibility with nonplanar phase fronts, driving the advancement of miniaturized FSS designs [21; 22; 23]. In [21], the proposed miniaturized FSS exhibited a large inductance, and the operating bandwidth could be increased without enlarging the size of the unit cell, demonstrating a wideband behavior with 70% fractional bandwidth and angular stability until 60\({}^{\circ}\). Compared with this study, [PERSON] et al. in [23] validated the application of miniaturizationtechniques for lower frequencies, obtaining a low-frequency bandpass response with good angular robustness. Furthermore, very low-profile topologies were discussed in [24; 25; 26], proving robust bandpass performance, along with cost-effectiveness. As apparent from the aforementioned remarks, angular stability serves as an auxiliary attribute in elucidating the electromagnetic functionality of FSSs in passive radomes [27; 28]. Presently, methodologies in passive FSS radome design predominantly revolve around maintaining stable operational frequencies or mitigating slight deviations when subjected to significant incidence angles. Among these methods, miniaturized structural designs have proven to be the most efficacious, as stated in [29; 30], together with tunable configurations [31]. Since the S and C bands play a crucial role in radome applications due to their significance in various communication and radar systems, low-profile miniaturized solutions were presented in [32; 33; 34; 35; 36; 37]. In [34], a double metallic, single dielectric layered bandpass FSS was examined, exhibiting a 53% fractional bandwidth within the S band. Furthermore, in [32], [PERSON] et al. presented a novel approach employing two resonant miniaturized layers consisting of inductively coupled resonators, yielding a second-order bandpass response. Notably, this structure demonstrated polarization independence and stability across diverse incident angles, boasting a low-profile and compact form factor suitable for potential deployment in very low-frequency applications. Additionally, a low-profile miniaturized-element frequency-selective surface (MEFSS) with a narrowband, second-order bandpass response was introduced in [33]. Its miniaturized-element unit cell integrated a capacitively loaded parallel LC unit, resulting in a narrowband bandpass characteristic. To achieve a higher-order filtering response, the design incorporates five metallic layers forming an FSS, comprising two layers of parallel LC resonators separated by an inductive impedance inverter layer. Remarkably, the proposed design exhibited insensitivity to the polarization and incidence direction of the impinging electromagnetic wave, further enhancing its versatility and applicability. In this challenging scenario, this manuscript features an ultrathin frequency selective surface (FSS) bandpass filter with extraordinary wideband and angular performance designed for operating in the S-C bands.