VARIATIONAL FORMULATION OF OPEN-ENDED COAXIAL LINE IN CONTACT WITH LAYERED BIOLOGICAL MEDIUM

An open-ended coaxial probe designed to measure layered biological med ia is analyzed with a new method. The probe is considered as an electr ostatic circuit element whose capacitance is solved using a stationary functional. The fundamental transverse electric and magnetic field (T EM)-mode and the series of evanescent wavemodes in the coaxial cable a re used as basis functions. The field-outside the probe is solved usin g a Hankel transform, The capacitance is calculated for homogeneous ma terials and two-layer structures and the results are compared with val ues measured with a phantom model. The method can be easily extended f or structures with an arbitrary number of layers. A practical approxim ation for two-layer cases, originally developed to take into account t he effect of subcutaneous fat in skin measurements, is presented and i ts validity for different combinations of dielectric constants and the thickness of the first layer is demonstrated, The static approximatio n limits the frequency range, but it covers biological measurements up to 500 MHz, The developed method is accurate and easy to adopt in pra ctice.