# A Simple AMC Antenna for Liquid Monitoring in an Infusion Bag
## 4 Fabrication and Measurement  
The prototype of the proposed antenna is fabricated and measured for verification. Figure 12 illustrates the scenario where the antenna is affixed to the side of the infusion bag and suspended next to the standing pole when the bag is filled with liquid and in the without-liquid state. When there is no liquid in the bag, the sides of the infusion bag are nearly in contact, creating a near-vacuum state within; however, due to the inability to achieve a perfect vacuum in the test environment, a slight gap remains between the dipole antenna and the AMC layer. When the bag is in with-liquid state, the thickness at the antenna location is approximately 1-2 cm. The Kapton layer in the upper part of the antenna is fabricated using Kapton films. The two dipole antennas are cut out of copper foil and pasted on the Kapton layer. Similarly, the four AMC structures are mounted on the 7 mm foam. The entire antenna is fed through a coaxial line.  
Figure 13 presents the simulated and measured reflection coefficients (\(\mid\!S_{11}\!\mid\)) of the proposed antenna, demonstrating their close agreement. In the with-liquid state, the operating frequency of the antenna is 2.06 GHz, slightly higher than the simulated results, yet generally consistent. At this frequency, the \(\mid\!S_{11}\!\mid\) reaches \(-26\) dB, indicating normal operation. Conversely, in the without-liquid state, the antenna operates at 2.44 GHz with an \(\mid\!S_{11}\!\mid\) of \(-22\) dB, suggesting a broadband working state across the frequency range of approximately 2.2 GHz to 2.6 GHz. The test results reveal a frequency shift of 0.38 GHz between the with-liquid and without-liquid states, underscoring the antenna's high detection sensitivity. Variations between the measured and simulated results may arise from factors such as the adhesives used for attaching the antenna elements, fabrication errors, and losses in the coaxial line.  
Figure 11: Simulated reflection coefficients of the proposed antenna sensor for different infusion bag materials and thicknesses: (**a**) PP; (**b**) PE; (**c**) PVC; and (**d**) EVA.  
Table 2 presents a performance comparison of the proposed antenna sensor with other published microwave sensors for liquid sensing, focusing on frequency, contactless capability, implementation technology, sensitivity, size, and frequency shift. In [17; 18], RFID tag sensors were used to achieve long-distance, non-contact liquid detection, but both operate at around 0.9 GHz. In contrast, the antenna proposed in this paper operates in the 2.4 GHz Wi-Fi band, enabling wireless data transmission and communication with existing Wi-Fi resources and terminal devices to trigger alarms, thereby reducing costs. Compared to the three microstrip antenna sensors proposed in [20; 21; 22], the AMC antenna presented in this paper offers higher sensitivity and supports non-contact detection, which is crucial for maintaining the sterility of the infusion environment [23; 30] proposed using FSS and EBG electromagnetic structures for liquid sensing. Although [23] demonstrates higher detection sensitivity, both approaches face the challenge of a large sensor size, making them difficult to deploy in more complex and extreme environments. The authors in [25] proposed a similar AMC antenna-based liquid sensor. However, compared to the antenna presented in this paper, the sensor in [25] has a lower sensitivity (0.011) and requires contact with the liquid for detection. Overall, the antenna sensor proposed in this paper demonstrates balanced sensing characteristics in terms of sensitivity, operating frequency, and relative size. Our antenna features a simple dipole structure that is cost-effective, highly accurate, and easy to deploy, making it suitable for various infusion bags with economic, accurate, and multifunctional capabilities.  
Figure 12: Fabricated antenna affixed to an infusion bag in without-liquid state and with-liquid state.  
Figure 13: Measured \(\left|S_{11}\right|\) of the proposed antenna.