The selectivity and response type of the frequency response of a frequency selective surface (FSS) are important factors that determine the suitability of an FSS for a given application. A common application of FSSs is to use them to shield sensitive electronic devices form unwanted interference or jamming signals with frequencies close to the main transmission band of the device. In such situations, spatial filters with highly-selective and narrowband transmission windows are required. FSSs with higher-order bandpass or bandstop responses acts similar to coupled-resonator filters. Therefore, their operational bandwidths are inversely related to quality factors of their resonators. Thus, to achieve a narrow-band response, higher quality factors are needed. For the case of traditional FSSs, these resonators are created using resonant elements within a unit cell. Miniaturized-element frequency selective surfaces (MEFSSs), on the other hand, use the combination of non-resonant reactive surfaces with capacitive and inductive surface impedances to create distributed-type resonators. For both approaches, the minimum-attainable feature sizes used in the metallic patterns of the structures are the bottleneck of achieving high quality-factor resonators. In practice, the minimum gap spacing and trace widths are determined by the minimum feature size that can be reliably fabricated using standard PCB lithography techniques. Therefore, achieving very high-quality factors and accordingly narrowband frequency responses for both configurations is rather challenging.
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