# A Simple AMC Antenna for Liquid Monitoring in an Infusion Bag
## 3 Working Principle
In the without-liquid state, the antenna resonates at 2.4 GHz with a reflection coefficient (\(1\,S_{11}\)) of \(-\)19.1 dB, an operational bandwidth of 2.34-2.48 GHz, covering the ISM radio band utilized in 5G wireless networks, and a peak gain of 6.45 dBi. When the infusion bag is filled with liquid, the high permittivity shifts the resonance frequency to 2.0 GHz with a reflection coefficient of \(-\)19.2 dB, exciting the TM\({}_{40}\) mode of the antenna and reducing the peak gain to 4.5 dBi. This shift alters the 3-D far-field radiation pattern, introducing additional side lobes, diverging from the dipole's typical omnidirectional behaviour, and resulting in a significantly reduced radiation gain. The AMC structure effectively suppresses back radiation in the without-liquid state, enhancing its performance compared to a traditional dipole. Despite the frequency shift, the antenna maintains high radiation efficiency, exceeding 0.8 across 2.0-2.5 GHz in both states. Its efficiency peaks at 0.98 at 2.4 GHz in the without-liquid state and remains high at 0.93 at 2.0 GHz in the with-liquid state. This frequency shift and alteration in the operational mode enable the antenna sensor to detect the presence of liquid in the infusion bag and communicate this change to a ceiling-mounted Wi-Fi antenna. Figure 6: Simulated reflection magnitude and phase response of the AMC unit cell under normal plane wave incidence. Figure 7: Simulated reflection coefficients, radiation efficiency, and 3-D radiation patterns of the proposed antenna sensor in without-liquid state and with-liquid state. In practical applications, when the infusion bag is suspended on a standing pole, the AMC should be positioned on the side of the pole to shield against its metal effects, as shown in Figure 8. We also investigated the impact of the bag's hanging direction on liquid state detection. Figure 8 presents the simulated reflection coefficients of the dipole antenna facing the metal pole, showing a compromised performance in both states, with resonance depths above \(-10\) dB, indicating the antenna cannot operate properly. Figure 9 illustrates the electric field intensity distributions on the dipole antenna, AMC surfaces, and standing pole in both the without-liquid and with-liquid states. In both states, strong electric field distributions are observed on the AMC surface. However, compared to the configuration where the AMC faces the standing pole, a significant electric field distribution is present on the standing pole itself, indicating that it destroyed the antenna's normal radiation performance. This confirms that in this hanging direction, the AMC fails to shield the pole and suppress its electric field, emphasizing the need to position it on the pole's side in practical applications. In order to verify the sensitivity of the sensor, we simulated the correlation between resonance frequency and infusion bag thickness in HFSS, as shown in Figure 10. The frequency range is 1.8-2.6 GHz, while the thickness of the infusion bag varies from 0 to 15 mm with a step of 3 mm. As the thickness increases, the resonance frequency decreases,  
Figure 8: Simulated reflection coefficients of the proposed antenna sensor with the dipole antenna on the side of the standing pole in without-liquid state and with-liquid state. Figure 3:0 Electric field intensity distributions on the dipole antenna, AMC surfaces, and standing pole when the dipole antenna is on the side of the standing pole: (**a**) without-liquid state and (**b**) with-liquid state. exhibiting a total shift of 0.5 GHz. At each resonance frequency, the antenna sensor maintains \(\left|S_{11}\right|\) below \(-10\) dB, indicating good impedance matching. Figure 10b shows the fitting curve between the resonance frequency and the infusion bag thickness. When the thickness exceeds 0 mm, the fitting curve tends to be stable, and the overall fitting degree achieved is 99.811%. In hospitals, infusion bags vary in material and thickness, typically ranging from 0.1 mm to 0.3 mm.