Miniaturized-Element Frequency Selective Surfaces

This article presents an overview of the progress made in the design of surface topology, fabrication, and applications of a new class of spatial filters known as miniaturized-element frequency selective surfaces (FSS’s) in recent years at the University of Michigan. The research in designing the new class was initiated with the demand for FSS’s with overall small dimensions for low frequency applications and reducing the dependence of the frequency selective properties of the FSS surfaces on the incidence angle of the incoming plane wave. Traditional FSS’s are usually fabricated from resonant elements, whose dimensions are usually comparable with half a wavelength, arranged in a periodic fashion. Proper operation of such surfaces can be achieved only when a large number of periods are included in a finite surface since the overall response is affected by the interaction among all Bragg modes of the periodic structure. This is also the reason for angular dependence of the frequency response of most old FSS’s. In the new class, these effects can be circumvented through sub-wavelength localization of the filtering phenomena. To achieve such objectives, the use of spatial lumped elements, printed on a substrate surface, are considered which allows for a decrease in the unit cell dimensions to be on the order of a tenth of the wavelength or smaller. It will be shown that by cascading surfaces containing these spatial lumped elements, band-pass, stop-band, or any other filter responses of a desired order can be generated. It is also discussed that the interaction of such surfaces and incoming wave is predominantly in TEM mode, resulting in suppression of all higherorder Bragg modes and elimination of the harmonic responses. As a demonstration, designs featuring highorder, band-pass characteristics with a single and dual transmission zero using only a single substrate are presented. Moreover, producing a frequency response that is tunable or even metamorphic, which can be transformed from a band-pass response to stop-band, by incorporating varactors onto the structure of the new FSS’s is investigated. Next, multi-band characteristics of the new class in a multi-layer arrangement are presented. Investigations are currently under the way to develop a practical implementation of electronically tunable FSS’s using the elements of the FSS itself as bias circuitry of tuning components mounted on the surface.