Medical telemetry revolutionized modern medicine. What happens inside the human body is translated and transmitted in real time using on-body or implanted devices. It is generally used to monitor patients’ vital signs such as pulse and respiration. These devices facilitate patients’ movement without restricting them to a bedside monitor with a hard-wired connection.
Wireless medical telemetry provides numerous benefits; nonetheless, it faces a few challenges. Placing an antenna near a human body affects its performance due to the dielectric loading caused by the large dielectric constant of biological tissue. Moreover, the output power from the on-body antenna is limited to certain SAR values set by the FCC. Additionally, antenna flexibility and miniaturization are crucial for making comfortable and wearable devices. To address these problems, a slotted dipole antenna backed by an artificial magnetic conductor (AMC) is studied.
HFWorks Modeling of Wearable Antennas
To contribute to this research effort, we used our electromagnetic virtual prototyping software, HFWorks, to study and simulate the slotted dipole antenna, shown below.
Slotted Dipole Antenna Backed by AMC
We investigated two design issues using electromagnetic simulation. The first is the effect of the human body on the antenna performance and the second is the SAR level of the antenna. We used two design configurations. In the first study, the antenna was designed without the human arm to study the effect of AMC. In the second, a human arm was added to analyze the performance of the antenna in the vicinity of the human body.
Slotted Dipole Antenna with AMC
The human body is considered a lossy dielectric, and this degrades the antenna performance. Hence, a reflector is used to minimize the body interference. Traditionally, a perfect electric conductor (PEC) is employed. PEC requires a spacing of a quarter wavelength and this can be large at low frequencies. For on-body applications, a quarter wavelength is too bulky to wear. Alternatively, AMC is used. With AMC, the distance between the antenna and the reflector is reduced significantly. An antenna with and without a Rectangular Patch AMC is simulated, and the following results are obtained.
3D Gain Pattern of Slotted Dipole Antenna with Rectangular Patch AMC
As illustrated in the 3 graphs above, AMC helps improve the return loss, enhance the forward gain, and block the back radiation. The spacing between the antenna and reflector is 1 mm. If we were to use PEC as a reflector, a spacing of 30 mm would be required at a frequency of 2.5 GHz. That’s almost 30 times bigger. This would make the antenna uncomfortable to wear.
The Effect of the Human Body on the Antenna Performance
Placing an antenna near a human body is one of the major challenges that face antenna engineers. The dielectric loading of the body can disturb the antenna's performance and could result in its failure.
The antenna with a human arm was simulated both with and without Rectangular Patch AMC. The results are shown below.
SAR Level of Slotted Dipole Antenna. (a) without AMC, (b) with AMC
2D Radiation Pattern of Slotted Dipole Antenna with and without AMC
3D Gain Pattern of the Antenna with AMC
Using AMC improves the return loss of the antenna. AMC minimizes the influence of the human body on the near field of the antenna and, consequently, its effect on the reflection coefficient. Moreover, the rectangular patch AMC not only increases the forward gain, but also reduces the SAR level from an unsafe level (above 1.6 W/Kg) to a level that is far beyond the SAR limit; AMC lowers the back radiation, and this reduces the amount of RF energy absorbed by biological tissues. Exceeding RF emission standards can overheat and damage body tissues.
Conclusion
The integration of artificial magnetic conductor (AMC) antennas in wearable medical telemetry presents a promising solution to overcome challenges in antenna performance and safety. Through electromagnetic simulation using HFWorks, we demonstrated the effectiveness of AMC in improving antenna performance and reducing SAR levels when placed near the human body. This advancement not only enhances signal transmission but also ensures patient safety, marking a significant step forward in real-time patient monitoring technology.
Reference
Steven Jacobson, 2020, “Wearable Antennas Backed by Artificial Magnetic Conductor for Enhanced Gain and Reduced Back Radiation”
