# A Simple AMC Antenna for Liquid Monitoring in an Infusion Bag
## 3 Working Principle
Figure 5a-c illustrates the simulated electric field intensity distributions of the proposed antenna in the three design stages. From Figure 5a, it can be observed that the dipole antenna exhibits a higher electric field  
Figure 4: Simulated gain of the proposed antenna: (**a**) without-liquid state, and (**b**) with-liquid state. intensity in the absence of liquid and standing pole interference. When the infusion bag is filled with liquid, the electric field intensity decreases, and four nulls of the electric field appear in the field distribution. At this point, the antenna operates in the TM\({}_{40}\) mode. Figure 5b further illustrates the electric field distribution when the dipole antenna is attached to the infusion bag and placed near the standing pole. The electric field distribution of the dipole remains largely unchanged compared to previous observations. However, in the absence of the AMC structure, a pronounced electric field distribution appears on the standing pole, significantly disrupting the dipole antenna's working mode and causing much of the energy radiated by the antenna to be absorbed by the metal standing pole. This interaction results in substantial interference with the antenna's performance, notably affecting the \(|S_{11}|\). Figure 5c shows the electric field intensity distributions of the dipole antenna as well as the AMC surface and compares these distributions on the standing pole with and without the incorporation of the AMC structure. The antenna consists of two primary components: the dipole antenna and the AMC structure, which form the resonant mode predominantly through their superposition. From Figure 5c, it is shown that the absence of liquid in the infusion bag excites the TM\({}_{20}\) mode, a fundamental resonance of the printed dipole antenna characterized by two electric field nulls. The integration of AMC enhances the electric field distribution around the dipole antenna. Conversely, when the bag is filled with liquid, the high relative permittivity significantly reduces the electric field distribution on the dipole, impairing its effective operation and inducing a mode shift to TM\({}_{40}\). Additionally, strong electric field distribution is observed on the AMC surface in both the with-liquid and without-liquid states, particularly near the region opposite the dipole antenna across the infusion bag. This indicates that the electromagnetic waves emitted by the dipole antenna are effectively coupled to the four AMC units. Compared to Figure 5b, the AMC structure significantly suppresses resonance with the standing pole, effectively eliminating its electric field distribution. As a result, the AMC surface fully reflects electromagnetic waves, acting as a barrier that minimizes environmental interference and enhances antenna performance. The performance of an AMC is characterized by its reflection phase, reflection coefficient, and bandwidth. Ideally, an AMC fully reflects the incident signal with no phase shift at its operational frequency, represented by \(S_{11}=1\angle 0^{\circ}\). This ideal response underscores the AMC's efficacy in preserving signal integrity across its designated frequency range. A study of the reflection responses for different AMC unit cell geometries is discussed in [33], where a square loop unit cell was chosen over a patch configuration. The optimization of the unit cell parameters was carried out using the Floquet modal in Ansoft HFSS. Figure 6  
Figure 5: Electric field intensity distributions on the dipole antenna, AMC surfaces, and standing pole: (**a**) dipole antenna; (**b**) dipole antenna with standing pole; and (**c**) dipole antenna with AMC and standing pole. shows the reflection phase and reflection magnitude of the square loop AMC unit cell when impinged with an RF signal at 2.4 GHz, with values of \(-\)0.01 dB and 0.21\({}^{\circ}\), respectively. Figure 7 illustrates the reflection coefficients, radiation efficiency, and 3-D far-field radiation patterns of the proposed antenna in without-liquid and with-liquid states.