Effect of Nearby Metals on Electro-Quasistatic Human Body Communication

In recent decades, Human Body Communication (HBC) has emerged as a promising alternative to traditional radio wave communication, utilizing the body's conductive properties for low-power connectivity among wearables. This method harnesses the human body as an energy-efficient channel for data transmission within the Electro-Quasistatic (EQS) frequency range, paving the way for advancements in Human-Machine Interaction (HMI). While previous research has noted the role of parasitic return paths in capacitive EQS-HBC, the influence of surrounding metallic objects on these paths-critical for EQS wireless signaling-has not been thoroughly investigated. This paper addresses this gap through a structured approach, analyzing how various conducting objects, ranging from non-grounded (floating) and grounded metals to enclosed metallic environments such as elevators and cars, affect the performance of the body-communication channel. We present a theoretical framework supported by Finite Element Method (FEM)-based simulations and experiments with wearable devices. Our findings reveal that metallic objects within $\sim$20 cm of the devices can reduce transmission loss by $\sim$10 dB. When the device's ground connects to a grounded metallic object, channel gain can increase by at least 20 dB. Additionally, the contact area during touch-based interactions with grounded metals depicts contact impedance-dependent high-pass channel characteristics. The proximity to metallic objects enhances variability within a critical distance, with grounded metals having an overall higher impact than floating ones. These insights are crucial for improving the reliability of body-centric communication links, thereby supporting applications in healthcare, consumer electronics, defense, and industrial sectors.