Approximation for measuring complex material parameters at λ/2 resonances in transmission-line measurements

We propose an approximation for removing resonant artifacts that arise in permittivity and permeability measurements when using the transmission/reflection (T/R) method at frequencies where the test sample length is an integer multiple of a half wavelength. In order to address this issue, we approximate the input impedance of the transmission-line fixture around the λ/2 resonance with a simple algorithm based on a 1st-order regression. The characteristic impedance of the sample-loaded section of the fixture is computed from those regression coefficients, and the permittivity and permeability can be computed by assuming that the refractive index obtained from the conventional T/R method does not suffer from resonant artifacts. Our approximate results are validated when compared with those from one of the conventional T/R methods, the Nicolson-Ross-Weir (NRW) method.