Figure 2.

(a) The field concentration of a traditional microwave ring resonator; note that the field is concentrated between the resonator and its ground-plane and therefore the substrate plays the most important role in determining of the resonance frequency. Also, the field concentration at the region above the sensor, which usually considered as the sensing area is smaller which limits the sensitivity of microwave resonator-based sensors. (b) Perspective view and field concentration of the resonator, it could be seen that the fields around the tag is almost constantly distributed in both on top (where material-under the test is placed) and underneath (where could be considered as its substrate). Therefore, one could expect higher frequency dependency of the tag to the MUT which could be translated to higher sensitivity in comparison with traditional microwave resonator sensors. (c) The simulation setup for studying the effect of the distant of the MUT from the tag on the sensitivity of the sensor. In this simulations, ten layers with the same thickness of 1 mm above the tag have been tested with the same permittivity as the air and only the permittivity of one of them has been changed to 2 at each step determined by layer number in parts (d) and (e) [images in parts (a), (b), and (c) are obtained from HFSS]. (d) S21 spectrum of some of the simulation results. (e) frequency variations versus the layer number (i.e. distance), it could be seen that, as the distance becomes higher, the sensitivity decreases in an exponential manner and therefore variations in the permittivity of MUT at higher distances could negligible in comparison with the same variations at lower distances, therefore we expect the sensor to measure glucose concentration variation in ISF rather than blood, so all the experiments are designed based on ISF measurement and the variations of its components.