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Nondestructive evaluation of stratified (layered) composite structures at microwave and millimeter-wave frequencies is of great interest in many applications where simultaneous determination of the complex dielectric properties and thicknesses of multiple layers is desired. Open-ended rectangular waveguide probes, radiating into such structures, are effective tools for this purpose. The technique utilizes a full-wave electromagnetic model that accurately models the complex reflection coefficient as a function of frequency and material properties. While the electromagnetic model assumes an infinite waveguide flange (or ground plane), the measurements are conducted using a finite-sized flange. Consequently, the results of the electromagnetic model and those from measurements may not be sufficiently alike for accurate dielectric property and thickness evaluation. This paper investigates the effect of using an open-ended waveguide with a standard finite-sized flange on the error in evaluating the complex dielectric properties of a composite structure. Additionally, we present the design of a novel flange that markedly reduces this undesired effect by producing very similar electric field properties, at the flange aperture, to those created by an infinite flange. Finally, the efficacy of the design for evaluating the dielectric properties of a layered composite structure is demonstrated as well.

We have improved the measurement uncertainty for rectangular waveguide vector network analyzer (VNA) measurements in the WM-250 (WR 1.0), 750 GHz–1.1 THz frequency band. We developed a new waveguide flange design for precise connections and dimensional measurements to establish traceability to SI in VNA measurements. This is the first such achievement to be reported using a VNA in the terahertz frequency band.

Measurements have been made at 56.2 GHz of reflection coefficients at the junction of two sections of 0.074- by 0.148-in rectangular waveguide, which are offset in the E or H plane. Special precautions were taken to preserve identical internal dimensions of the two sections at the junction. A special fixture was built to allow controlled offset movements. The experimental data show good agreement with theoretical calculations. This data was obtained with the object of setting tolerances on waveguide alignment for acceptable reflections at waveguide flange junctions in rectangular waveguide at millimeter wave frequencies.