Permittivity measurement technique for a dielectric strip using a rectangular waveguide

A measurement technique for a dielectric strip using a rectangular waveguide is developed for complex permittivity retrieval. The width of the strip is generally narrow, so the sample may not be able to fill the whole waveguide cross section, as shown. When the measured reflection coefficient is too high, or another independent measurement is needed, in addition to changing the size of the sample, it can also be achieved by tilting the sample. A forward scattering solution for the tilted strip in a rectangular waveguide is first constructed using the finite element method as a reference. Then the image theory is applied to make the original configuration as an infinite 2D array of strips. The scattering from the new configuration is solved using the method of moments. Infinite terms of multiple scattering interactions between strips should appear in the complete solution. After a comparison to the finite element solution, an appropriate sample position of the strip in the waveguide can be suggested in which the least terms of the multiple scattering interactions need to be considered. The final forward model is then used in conjunction with a genetic algorithm to retrieve the parameters of interest. Numerical simulation for sensitivity analysis is presented, and the retrieving technique is validated by measuring samples with known properties.