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].