Multifrequency Measurements of Dielectric Properties Using a Transmission-Type Overmoded Cylindrical Cavity

Characterization of complex permittivity has been performed using TM₀mn modes in an overmoded cylindrical cavity. The Rayleigh-Ritz technique is employed to find a rigorous relationship between the relative permittivity, resonant frequency, and the dimensions of the resonant structure. A functional relationship between the structure´s S -parameters and the system´s unloaded Q factor is established. Complex permittivities of several materials, including polytetrafluoroethylene, high-density polyethylene, cross-linked polyethylene, and sodium naphthalene treated PTFE, are measured at room temperature. Their accuracy is assessed by comparing the results with those obtained from other well-known techniques. An error analysis is also presented to estimate the errors incurred due to the uncertainties in cavity dimensions, resonant frequency, scattering parameters, and the conductivity of the coating layer. In this measurement, an accuracy of better than ±0.5% is attained in measurement of the relative permittivity, while the uncertainty in the measured loss tangent is within a range of ±6%. For properly chosen sample dimensions, this presented method is examined to be very useful for measurement of the complex permittivity over the widely used microwave frequency range, such as the L-, S-, C-, and X-bands.