High angular stability and polarization insensitive optically transparent bandpass frequency selective surface based on micro copper mesh

Frequency selective surfaces (FSSs), as periodic structures that can manipulate the behavior of electromagnetic waves, are widely utilized in microwave and millimeter-wave bands with the rapid development of wireless communications, 5G, and millimeter-wave communication [1], [2], [3], [4]. With the challenges arising from an increasingly complicated electromagnetic environment, FSSs play an essential role in electromagnetic leakage prevention  [3], frequency selection [5], and security protection  [6]. Bandpass FSSs are widely used in the electromagnetic field problems due to their excellent in-band transmission and out-band suppression characteristics  [7], [8], [9], [10], [11], [12], [13]. Nevertheless, the previously proposed FSSs are not available for optical transparency applications due to the materials’ opacity. Therefore, studying optical transparent FSSs (OT-FSSs) has gradually become an interesting topic worth exploring. Compared with traditional FSSs, OT-FSSs can provide optically transparent performance while maintaining microwave frequency selectivity. Owing to this property, OT-FSSs can easily be applied in interesting situations which are relatively difficult to realize by traditional FSSs, such as energy-saving and green buildings [14], [15], [16], [17], medical applications [18], and mobile and wireless communications  [19].

In recent years, some meaningful works have been done to achieve the compatibility of optical transparency and microwave frequency selection. Initially, transparent conductive thin films, such as graphene, indium tin oxide (ITO), and fluorine-doped tin oxide (FTO), were widely used for realizing OT-FSSs. However, since their surface resistances are much higher than that of a perfect electrical conductor (PEC), they suffer from high transmission losses [20], [21]. Meanwhile, the screen-printing technique is an alternative to realizing OT-FSSs. Yang et al. proposed a dual-band optically transparent frequency selective surface (FSS) for GSM shielding [22]. Dewani et al. used silver ink to print the pattern on the PET and realized an FSS window [23]. Like conductive thin films, the conductivity of materials such as conductive Ag NW inks used in screen-printing technology is incomparable to that of metals, which results in high transmission losses in the fabricated OT-FSSs. Besides, meshed metal films made of copper or silver are also commonly used to realize OT-FSSs. Nevertheless, choosing the appropriate mesh grid size and mesh line width is essential, which are important factors affecting the microwave performance and optical transparency of OT-FSSs [24], [25], [26], [27].

This paper proposed an optically transparent bandpass FSS with high angular stability and polarization insensitivity based on the micro copper mesh. The proposed OT-FSS has been designed to operate at 2.07–4.23 GHz, including Bluetooth, Wi-Fi, and 4G LTE bands with low insertion loss. Section 2 provides the design process and simulation results of the proposed FSS. Section 3 presents the fabricated OT-FSS sample and experiment results. Finally, conclusions are given in Section 4.

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