Assessing the Intrinsic Radiation Efficiency of Tissue Implanted UHF Antennas

Dielectric loss occurring in tissues in close proximity to UHF implanted antennas is an important factor in the performance of medical implant communication systems. Common practice in numerical analysis and testing is to utilize radiation efficiency measures external to the tissue phantom employed. This approach means that radiation efficiency is also dependent on the phantom used and antenna positioning, making it difficult to understand antenna performance and minimize near-field tissue losses. Therefore, an alternative methodology for determining the intrinsic radiation performance of implanted antennas that focuses on assessing structural and near field tissue losses is presented. The new method is independent of the tissue phantom employed and can be used for quantitative comparison of designs across different studies. The intrinsic radiation efficiency of an implant antenna is determined by assessing the power flow within the tissue phantom at a distance of at least λg/2 from the radiating structure. Simulated results are presented for canonical antennas at 403 MHz and 2400 MHz in homogeneous muscle and fat phantoms. These illustrate the dominance of propagating path losses in high-water content tissues such as muscle, whereas nearfield dielectric losses may be more important in low-water tissues such as fat due to the extended reactive near-field

Experimental Investigation of Non-Line-of-Sight Channels in an Intra-Body Network at 2.38 GHz

The characteristics of the intra-body propagation channel between implanted antennas are highly application dependent. Measurements of the forward path gain between identical implant antennas within two multi-layered tissue mimicking liquid phantoms were used to investigate the nature of the intra-body channel at 2.38 GHz. One of the antennas was held in fixed locations in the phantoms and a robotic positioner with millimeter accuracy was used to vary the second antenna's position. The results show that the shortest line-of-sight path is not always dominant and depending on the particular geometry of material layers and their dielectric properties other propagation paths may also be important. This highlights the importance of careful system design in intra-body networks as the link budget between implanted nodes may need to consider alternative propagating paths, depending on the application scenario