Fig. 4 The embedded balun with physical dimensions (unit: mm)
Experiments and analysis: In order to demonstrate the performance of the tangential E-probe with resonator and coupled balun proposed in the paper, a conventional broadband tangential E-probe published in [7] is used to compare. Fig. 5(a) is the physical picture of this probe. The measurement setup is shown in Fig. 5(b), and the distance between the tip of the probe and the microstrip line is 1 mm. In the measurement, it should be noted that in order to obtain the maximum electric field component, the probe was placed away from the center of the standard microstrip line. After HFSS optimization simulation, it is found that the peak value of the electric field of the microstrip line is at 2 mm away from the center of the microstrip line. The comparison between simulated results and measured ones is shown in Fig. 5(c). As show in Fig. 5(c), measured results agree well with simulated ones except for a little deviation in frequency between them.The simulated result of the peak value captured by the probe is -24.39 dB at 1.320 GHz, while the measured one is -24.75 dB at 1.334 GHz. The possible reasons is that there are machine tolerances in the manufacturing process and at same time it is difficult to accurately control the gap between the microstrip line and the tip of probe. Compared with the traditional broadband probe, the value of |\(S_{12}\)| is improved by improved by 18.81-dB at 1.334 GHz.
Conclusion: To improve the measurement sensitivity in narrow band, a compact high-sensitivity tangential E-probe with resonator and coupled balun is proposed and tested in this paper. The resonator composed of open-circuited and short-circuited transmission line can enhance the sensitivity. The coupled balun is to complete the transition from differential-mode voltage into common-mode voltage. The |\(S_{12}\)| measured by this probe is improved by 18.81 dB at 1.334 GHz. The compact high-sensitivity tangential E-probe is more suitable for weak signal measurement.