Frequency Selective Surfaces In Action

While composed of sub-wavelength structures, Frequency Selective Surfaces are capable of manipulating electromagnetic waves—bending, filtering, or absorbing them across diverse communication and sensing applications. Is it shaping the future of adaptive electromagnetic environments?

What if one could control the behaviour of electromagnetic waves—bending, absorbing, or filtering them—using surfaces thinner than a human hair? There is a whole fascinating world of metasurfaces, where material science meets electromagnetic mastery. 

A metasurface constitutes a thin, artificial layer of material that modifies the amplitude, phase, or polarisation of light to regulate the way electromagnetic waves travel. Metamaterials are artificial substances made of sub-wavelength building units, usually made of metal. Considering the polarisation response, all metasurfaces can be categorised as perfect absorbers, high impedance surfaces, reflecting surfaces, or frequency selective surfaces (FSS). 

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FSSs are composite material metasurfaces that have an electric response and are made partly transparent within specific frequency bands while reflecting, absorbing, or rerouting to other frequencies. Numerous unit cell types with a range of geometrical patterns, including patches, cross dipoles, loops, hexagons, convoluted shapes, and fractal geometries, can be employed in the design and implementation of FSSs. Within a period known as a unit cell, these elements generate frequency responses that can be dictated by the structure’s form.

Nevertheless, the radio frequency response of the filtering FSSs are separated into the four primary types of low pass, high pass, band stop, and band pass filters based on their material, geometry, and construction. FSSs with a regular number of slots (or apertures), as illustrated in Table 1, are inductive and act like a high-pass/band-pass filter, rejecting waves at both lower and higher frequencies while permitting waves to flow through at the resonant frequency of the slots. FSSs with periodic arrays of patches, on the other hand, are capacitive and work similarly to a low-pass/band-stop filter, rejecting waves at the patch’s resonance frequency while permitting waves at higher and lower frequencies to pass through.

The array element, structural design, and application-oriented can be used to classify FSSs. The general classification of the FSS is shown in Fig.1. 

FSSs – Basic Element Type

Four fundamental groups of FSS elements are depicted in Fig. 1(b): group 1 consists of N-poles or center-connected shapes including dipoles, tripoles, square spirals, and Jerusalem crosses; group 2 consists of looped shapes like circular, square, and hexagonal loops; group 3 consists of solid interior or patch types of different shapes; and group 4 consists of combinations of any of those as mentioned above.

Convoluted Meandered FSSs

FSSs that employ convoluted geometries, such as meander lines, to minimise the size of their unit cell and profile are known as convoluted meandering FSSs. The benefits and applications of this structure have been defined in Fig.2 (a).

Fractal-Based FSSs

In order to achieve multiband characteristics, self-similar fractal patterns are used in the construction of fractal-based FSSs. Fractals have similar shapes that can be produced by using a feedback loop to repeat a mathematical calculation. The classification of this type with structures is given in Fig.2 (b). The types of FSSs with their properties and applications have been listed in Table 2 based on structural design and application-oriented.

FSSs continue to play a pivotal role in shaping the future of wireless and radio frequency technologies. With their unique ability to control electromagnetic wave behaviour, these structures offer promising innovations in antenna design, filtering systems, and beyond.