WBAN Transmission Model for Coils at 10.8 MHz in Free Space and Near a Flat Conducting Medium

In this paper, the transmission between two small antenna coils for wireless body area networks (WBANs) operated at 10.8 MHz is characterized in free space and near a phantom filled with water. In order to investigate the influence of the flat conducting medium, measurements are performed at different separations between the coils and at different heights above the medium. The voltage ratio |V₂/V₁| of the received voltage to the input voltage is studied as a function of the distance between the coils. Also, the influence of the medium and the height of the coils above the medium is determined. Transmission models are derived for the voltage ratio in free space and near the flat conducting medium, and a physical explanation is given. The measurements and models agree very well. Circuit equivalent models representing the physical transmission between coupled coils are proposed. The measurement results are compared with simulations of the presented circuit model and excellent agreement is obtained.