Improved circuit model of open-ended coaxial probe for measurement of the biological tissue dielectric properties between megahertz and gigahertz

This note describes an improved equivalent circuit analysis model for open-ended coaxial probes for measurement of the dielectric properties of biological tissues below the gigahertz level. Some parameters in the conventional model that influence the measurement results were found to be still relative to the dielectric properties of the test sample and the terminal admittance of the probe was found to be dependent on the frequency. This was not found to be the case with the conventional model. According to the simulation results in frequency range from 30 MHz to 1 GHz, a polynomial expression was found to fit the frequency-admittance curve for terminal admittance and the equivalent circuit expression of probe terminal admittance was finally modified. The simulated annealing algorithm was used to calculate the dielectric properties of the new expression. The accuracy of the improved model was validated through a simulation test and experiment based on a series of solutions over 30 MHz-1 GHz. The new model was compared to the conventional model and was found to provide more accurate permittivity estimation over a wider frequency range than the conventional model if said range was between megahertz and gigahertz.