Microwave Sensor Based on Microstrip Line Photonic Band Gap (PBG) Structure

A hybrid integration of planar microstrip line and Photonic Band Gap (PBG) structure is proposed for liquid material complex permittivity characterization. The periodic non-through holes drilled in the substrate to generate the bandgap protect also the microstrip from direct contact with liquid under test (LUT). Based on this configuration, two versions are developed to address the measurement constraint. The first sensor exploits the frequency shift of the bandgap center due to the change in the permittivity of the LUT filled in the holes. The second sensor exploits the behavior of the bandgap as a reflector to construct a resonant structure sensitive to the variation in LUT permittivity. The dimensions of the planar structures are optimized to achieve high precision and discrimination capability. The different empirical expressions describing the complex permittivity with the measured parameters for the two sensors were carried out. For experimental validation, prototypes are used to characterize different commercial oils. The frequency shift related to a change of 1 in LUT permittivity corresponds to 300 MHz around 8.3 GHz. The resonant-mode sensor spans a permittivity range from 1 to 9 with a precision better than 6.6 %. The proposed low-cost sensors are simple and reusable, satisfying the requirement of industrial applications.