Adjustable Resolution for Localized FSS-Based Sensing by Synthetic Beamforming

Recently, frequency-selective surface (FSS)-based sensors have shown potential for structural health monitoring due to their sensitivity to changes in element geometry, interelement spacing, substrate properties, and local environment. In addition, these sensors are remotely interrogated and are planar in design, thereby providing a wireless sensing solution that can cover large areas. Traditionally, FSS sensors are analyzed assuming a uniform (plane wave) illumination. However, practically speaking, the sensor will be illuminated with a nonuniform excitation. In this way, the resolution of the sensor is limited to the illumination pattern (footprint) on the sensor. As such, an adjustable illumination pattern is advantageous as it relates to the ability to interrogate the sensor on a localized or comprehensive basis. To this end, this article considers a synthetic beamforming approach to adjust the beamwidth of the focused illumination beam on the sensor as a solution for localized sensing applications. This approach is proposed to generate an arbitrary beam shape with a desired footprint. Moreover, the illumination and spillover efficiencies of the synthetic beam (SB) are defined, simulated, and discussed. The results show that a minimum focal area of 1\lambda _{0} \times 1\lambda _{0} ( 3\times3 unit cells for the FSS sensors considered here) is essential to achieve a spillover efficiency of greater than 80%, thereby reducing the contribution of other neighboring unit cells on the sensor response. In addition, the illumination efficiency when an SB is utilized for illumination is greater than 80% due to the uniformity of the SB. Representative measurements were also performed on a sensor with a simulated strain profile.