Analysis of the Impedance Behaviour for Broadband Dipoles in Proximity of a Body Tissue: Approach by Using Antenna Equivalent Circuits

Ultra wideband (UWB) antenna operation close to tissue is examined by using lumpedelement equivalent circuits in the present paper. The impact of tissue within the reactive near-field of the antenna is introduced in terms of efficiency, impedance and matching to 50 Ω. The parasitic components for the seriesand parallel-resonant stages of the equivalent models are proposed for taking the impact of tissue into account on the antenna design. The first time the antenna impedance behaviour is presented in terms of capacitance, inductance and resistance as a function of the radiator distance on the tissue surface for UWB antennas. The capacitance was observed to increase with the distance on the tissue surface by achieving the maximum value close to the reactive near-field boundary. The inductance has the maximum on contact the tissue, decreasing strongly with the first millimetres and remaining constant with the higher distance. The maximum value of input resistance was seen to clearly increase with the distance, having the maximum value in the first third of the studied range, descending close to the value in free space at the boundary at the end. The results are achieved by realising electromagnetic simulations for the antennas and comparing the performance with the operation of the equivalent models.