Planar Microwave Bragg Reflector Resonant Dielectric Sensor

In this paper, a periodic structure is used to design a microwave Bragg reflector with the help of hexagonal lattice, which provides a 5 GHz wide stopband between the low-pass band with cut-off frequency 2.6 GHz and the bandpass response with start and stop frequency 7.8 GHz and 10.5 GHz, respectively. A defect in lattice allows passing a narrowband signal at 6 GHz which is found, from the dispersion relation, to be in the region of backward wave propagation. The maximum electric field intensity is found at the lattice defect, and this property is used to sense the dielectric material. It is found that the material under test does not change the low-pass frequency response, hence, the proposed scheme can be used for the dielectric sensor integrated with a low pass filter. The proposed planar sensor operates in the stopband of the Bragg reflector with a typical sensitivity 22x10³ kHz corresponding to per unit change in the dielectric constant of material under test. The design is fabricated on a 0.5 mm thick RF35 substrate and tested using standard dielectric materials. The numerical analysis of the proposed structure is found to be in good agreement with the measurement results.