## 3 Experiments and Results
### Bidirectional Probe  
The bidirectional probe was also measured for comparison with the modeling done in the previous research [18]. The measurement setup is identical for the two probes, except the absence of the metal plate. The measurement was also carried out in two stages.  
The results obtained for each glucose concentration were compared with the simulated data. Figure 16 shows the dependence of the reflected signal on the frequency during simulation (Figure 16a) and experiment (Figure 16b).  
During simulation, region 4 (6-7 GHz) was most sensitive to changes in glucose in saline. In the case of experimental data, the most sensitive region is region 2 (2.5-3.5 GHz). For a more detailed study of this area, the range was reduced to 3-4 GHz (Figure 17). As can be seen from Figure 16, the difference between the concentrations is extremely small.  
Figure 15: Dependence of the reflected signal on the glucose concentration at a frequency of 2.235 GHz (**a**) and the dependence of the frequency on the glucose concentration (**b**) in saline.  
The amplitude changes for each concentration are shown in Table 4.  
Figure 16: Frequency dependence of the reflected signal of a bidirectional probe on frequency for various glucose concentrations in modeling (**a**) [18] and experimental measurement (**b**).  
Figure 17: Frequency dependence of the reflected signal from the bidirectional probe for various glucose concentrations. Frequency range 3–4 GHz.  
As in the case of the unidirectional probe, the dependence of the glucose concentration on the amplitude of the reflected signal was plotted (Figure 18), the error for each concentration is also shown.  
Thus, according to the experimental data, the sensitivity of the bidirectional probe was 0.136-0.5 dB/(mmol/L). When simulating this probe, the sensitivity for region 2 was 0.66-1 dB/(mmol/L). This area is the least considered due to the small change in the amplitude of the reflected signal. Region 4 in the modeling was considered as the main one due to the large difference in amplitudes with a change in glucose concentration. The sensitivity for this area was 2.69 dB/(mmol/L).  
The results obtained for unidirectional and bidirectional probes were compared with other microwave measurement methods. Table 5 shows a comparative sensitivity analysis.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Concentration Level** & **Frequency** & **Reflected Signal Amplitude (dB)** \\ \hline Blood 0 mmol/L & 3.5 GHz & \(-27.36\pm 0.18\) \\ Blood 4 mmol/L & 3.5 GHz & \(-26.804\pm 0.096\) \\ Blood 5.3 mmol/L & 3.5 GHz & \(-26.43\pm 0.112\) \\ Blood 7.5 mmol/L & 3.5 GHz & \(-26.129\pm 0.105\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Reflected signal amplitude values for different glucose concentrations.  
Figure 18: The dependence of the reflected signal on the concentration of glucose at a frequency of 3.5 GHz.  
As can be seen from the table, the unidirectional probe is superior in sensitivity to all the considered analogs of devices for non-invasive measurement of glucose concentration. The closest to the developed probe is [29]. In this article, the authors used a high frequency signal (50-70 GHz) and considered the deviations of the transmission coefficient. Due to the high frequency, they managed to achieve such high results.