Materials Measurement Phil Bartley Shelley Begley

Weakness of both the NIST and TransmissionModels• Both models computes the wrong solution when the phaseshift of S21 is greater than -360 degrees at the firstmeasurement frequency.• This can often be overcome by computing the group delayand computing an estimate of the permittivity.• An alternative is to provide the model with an approximatevalue of the permittivity.Both models can become “confused” when the length of the sample is long comparedto the wavelength of the measurement frequency. There are several values ofpermittivity that satisfies the measurements. This problem can be overcome by eithermaking a shorter sample, lowering the first measurement frequency or providingadditional information such as group delay or an initial guess of permittivity.44