INFLUENCE OF HIGHER-ORDER MODES ON THE MEASUREMENTS OF COMPLEX PERMITTIVITY AND PERMEABILITY OF MATERIALS USING A MICROSTRIP DISCONTINUITY

The accurate electromagnetic analysis of discontinuities in a microstr ip device, used for the broad hand measurement of complex permittivity and permeability of materials, is presented, This analysis is based o n the spectral domain approach together with the mode matching method which consists in the electromagnetic field matching at the discontinu ities for each mode of the microstrip line, The study of the influence of higher order modes on the S-parameter measurements enabled us to d etermine the domain of validity of the transmission-line theory that h as been used until now in the processing of the data, The use of the t ransmission-hue theory for the description of the electromagnetic beha vior of the cell discontinuities permits in the 45 MHz-14 GHz frequenc y band the achievement of a good precision (better than 5%) far the re sults on materials with low electromagnetic characteristics (epsilon < 10 and mu' < 10), The improvement in high frequencies of the results on materials with greater permittivity and permeability is provided by the calculation of higher order modes (about 10) in the analysis of t he microstrip discontinuity.