Fig. 3 The resonator with physical dimensions (unit: mm)
B. Coupled balun: The function of the coupled balun is to achieve the transition from balanced transmission line to unbalanced transmission line. Since the electric dipole at the front of the probe is the balanced transmission, but the output port of CB-CPW is unbalanced port, thus, a balun is needed to achieve the transformation from balanced to the unbalanced. Fig. 4 shows the coupled balun and its specific dimensions adopted by the probe in this paper. The coupled balun is used to transform the differential-mode voltage induced by the electric dipole into the common-mode voltage and the transformation of the input impedance at the resonator to the matched 50Ω impedance is obtained [9].