Broadband Characterization of Materials Using a Dual-Ridged Waveguide

A transmission/reflection material characterization technique that uses dual-ridged waveguides is presented. The proposed dual-ridged-waveguide system combines many of the positive aspects of traditional transverse electromagnetic-mode (e.g., coaxial, free space, and stripline) and rectangular waveguide systems, i.e., broadband measurements and accurate calibration. A brief discussion on the derivation of the theoretical scattering parameters, required for the extraction of permittivity and permeability of a material under test, is provided. Two methods for computing the cutoff wavenumber of the dual-ridged waveguide-essential to the material characterization process-are also discussed. The first, which utilizes the mode-matching technique, is applicable to dual-ridged-waveguide apertures composed of right-angled corners. The second uses the surface equivalence principle and a magnetic-field integral equation formulation to find the cutoff wavenumber. This approach is applicable to dual-ridged waveguides with rounded corners, which often result from the dual-ridged waveguide manufacturing process. Thus, for the first time, the effect of rounded dual-ridged-waveguide aperture corners on the measurement of permittivity and permeability is assessed. Experimental material characterization results of a magnetic absorbing material are presented and analyzed to validate the proposed technique. An extensive error analysis on the extracted values of permittivity and permeability is also performed by taking into account manufacturer-specified dual-ridged-waveguide design tolerances as well as uncertainties in sample position, sample thickness, sample-holder length, and measured scattering parameters.