Effect of illumination pattern on FSS-based sensor resolution

Frequency selective surfaces (FSSs) are periodic arrays of conductive elements, and when illuminated by electromagnetic energy, have a specific transmissive or reflective response. FSS-based sensing is a relatively new application of FSSs, and recently these sensors have shown promise for crack detection, strain and temperature sensing, amongst others. Generally speaking, when an FSS sensor is illuminated in its entirety, the response of the sensor is related to the entire FSS landscape. In this way, the resolution of the sensor is equal to the FSS dimensions. However, this is limiting as it relates to localized sensing. As such, to improve the achievable resolution, the FSS sensor must be illuminated locally in order for the detected response to be related to a specific region of the sensor. In this way, an FSS sensor can be considered as consisting of many sensor cells which are illuminated individually. To this end, in this paper, the effect of illumination pattern and cell dimensions are studied as it relates to the FSS sensor resolution. For this work, a grounded loop FSS was designed to operate at X-band (8.2-12.4 GHz). Five different FSS dimensions were considered for simulation, with three of these fabricated for measurement. From this, the achievable resolution for a given sensor design and illumination pattern is quantified. Additionally, the performance of these sensors is compared to that of the ideal FSS (infinite dimensions and uniform illumination). The results show that optimum sensor cell size (for a given FSS design and illumination pattern) is determined based on when the FSS response (resonant frequency and Q-factor) is constant (with respect to increasing cell size). In addition, as the cell size is directly related to the illumination footprint, the optimum cell size can be adjusted per sensing applications needs based on the illumination source and distance from the FSS.