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It is well known that there is no unique solution for complex permittivity determination of medium- and low-loss materials using amplitude-only measurements at one fixed frequency. In this paper, we propose two novel amplitude-only methods for one complex permittivity determination of these materials. The first method uses at least three amplitude-only measurements at different frequencies. For a change in permittivity over frequency, we considered two different approximations (zero order and higher order). The second method utilizes amplitude-only measurements at critical frequencies which result in maximum or minimum amplitudes of scattering parameters. It is shown that at these frequencies, the expressions for scattering parameters simplify and then one permittivity determination becomes possible. We derived two simple objective functions, which depend on only one variable, for the second method. In this sense, the second method greatly reduces the computation times for permittivity determination. For validation of both methods, complex scattering parameter measurements of two polytetrafluoro-ethylene (PTFE) and one soda-borosilicate glass (Corning 7750) samples fitted precisely into two waveguide holders are conducted at X-band (8.2–12.4 GHz), and their complex permittivities are determined by different techniques.

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Abstract-A non-resonant microwave method has been proposed for accurate complex permittivity determination of low-loss materials. The method uses two measurement data of the magnitude of transmission properties of the sample. While the first datum must correspond to a frequency point resulting in a maximum magnitude of transmission properties, the other can be any datum at a frequency different than the first datum and not far distant from the first datum. Two closed-from expressions are derived for a good initial guess using the above data. The limitations of each expression are discussed. The method has been validated by transmission measurements at X-band (8.2-12.4 GHz) of a low-loss sample located into a waveguide sample holder.