Frequency Selective Surfaces (FSS)
Frequency selective surfaces (FSSs) are periodic arrays of conductive elements or patches that create a specific reflection and/or transmission response when illuminated with high frequency electromagnetic energy. A set of example elements is illustrated in Fig. 1. Historically, these arrays have been used as high frequency filters and in radar, stealth, and advanced antenna applications. However, more recently, FSSs have been considered for sensing applications. As sensors, FSSs offer advantages including remote interrogation and passive operation without the need for direct electrical connection (i.e., a wireless sensing solution). FSS-based sensor design can be tailored to the sensing need through selection of operating frequency (which dictates sensor dimensions and hence measurement resolution) and can also be designed to be sensitive to multiple measurands through both polarization and/or frequency diversity. In addition, as microwaves penetrate dielectric (nonconductive) materials, these sensors can be realized in single or multiple layers, depending on the application needs. Moreover, FSS-based sensors can be embedded within a structure during construction or installed on the surface. FSS-based sensors can also be designed to operate in reflection or transmission mode. However, from a practical point of view, FSS sensors designed in the reflection mode are desirable as they require a one-sided interrogation (as opposed to needing access to both sides of a structure to perform an inspection).
Figure 1: Element examples: a) patch, b) loop, c) dipole, and d) cross.
An example of an FSS sensor is shown in Fig. 2. This sensor was designed to measure strain, with the unit cell (single occurrence of the periodic structure) illustrated in Fig. 2a, and the resulting reflection response (|S11|) in Fig. 2b. As seen, when the sensor undergoes mechanical loading (i.e., strain), the resonant frequency exhibits a shift as well. This shift is proportional to the strain and hence can be used for strain measurement. This concept of measured frequency shift proportional to a parameter of interest (e.g., strain, temperature, etc.) remains for other types of FSS-based sensing applications and forms the overall basis for this sensing approach.
Figure 2: Element examples: a) patch, b) loop, c) dipole, and d) cross.