# A Simple AMC Antenna for Liquid Monitoring in an Infusion Bag
## 3 Working Principle  
To study the performance of the proposed antenna models, simulations were carried out with the help of ANSYS HFSS v. 19. In the simulations, the maximum thickness of the infusion bag filled with liquid is set to 10 mm. Figure 3 shows the design procedure of the proposed antenna and the corresponding simulated reflection coefficients. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Value (mm)** & **Parameters** & **Value (mm)** \\ \hline \(L_{1}\) & 38.75 & \(L_{2}\) & 100.00 \\ \(L_{3}\) & 18.00 & \(L_{4}\) & 1.00 \\ \(L_{5}\) & 1.00 & \(L_{6}\) & 16.00 \\ \(W_{1}\) & 7.00 & \(W_{2}\) & 20.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dimensions of the proposed antenna sensor. Figure 3: Design procedure and simulation results in the reflection coefficients of the proposed antenna: (**a**) dipole antenna; (**b**) dipole antenna with standing pole; and (**c**) dipole antenna with AMC and standing pole. Initially, the antenna is designed with a simple dipole configuration, which is printed on a thin polyimide layer and attached to the front surface of the infusion bag, as depicted in Figure 3a. The initial antenna design exhibits poor reflection coefficient values (\(\left\lvert\,S_{11}\,\right\rvert\)), indicating non-functionality in the absence of liquid within the infusion bag. Conversely, when the infusion bag is in the with-liquid state, the antenna demonstrates a resonance exceeding \(-20\) dB at 3.0 GHz. The substantial variation in the S-parameters between the two states provides a reliable means to determine the presence of liquid in the infusion bag. To evaluate the impact of the standing pole on antenna functionality, simulations were conducted with a copper pole positioned adjacent to the infusion bag. Figure 3b illustrates the dipole antenna hanging close to the metal standing pole. It indicates that proximity to the copper standing pole adversely affects the antenna's \(\left\lvert\,S_{11}\,\right\rvert\). Specifically, within the 1.5 to 3.5 GHz frequency range, the \(\left\lvert\,S_{11}\,\right\rvert\) resonance depth remains less than \(-10\) dB in both the without-liquid state and the with-liquid state of the infusion bag. Consequently, the antenna's impedance matching is poor and its ability to monitor liquid is compromised by this interference. AMC structures can enhance the reflection coefficient, phase, and gain of antennas [33] and isolate the effects of nearby environments on antenna performance. Figure 3c shows our proposed design with a square loop AMC structure and its reflection coefficients. The presence of the AMC surface enables the antenna to mitigate interference from the copper standing pole and enhance the sensor's sensitivity and reliability. The antenna operates at 2.4 GHz for the infusion bag in the without-liquid state, while in the with-liquid state it operates at 2.0 GHz. The status of the liquid in the infusion bag can be determined by these changes in resonant frequency. The gain variations of the proposed antenna at the three design stages are presented in Figure 4. In the entire frequency range of 1.5-3 GHz, as illustrated in Figure 4a, loading the AMC results in a significant increase in the gain, ranging from 1 to 5 dBi, when compared with not adding the AMC in the without-liquid state. After adding the AMC, the overall gain exceeds 6 dBi at 2.4 GHz, significantly improving the antenna's performance at its operating frequency. In the with-liquid state, the dielectric properties of the liquid with high permittivity, along with the metal standing pole, destroy the operation of the antenna, causing the antenna gain to fluctuate greatly, as shown in Figure 4b. However, in the presence of AMC, the antenna can still maintain a high gain of more than 4 dBi near the 2.0 GHz resonant frequency. In order to further analyse the specific excitation process, this study also analyses the electric field distribution of the proposed antenna.