The Electromagnetic Signatures of Explosives Laboratory at the Transportation Security Laboratory supports the test and evaluation of millimeter wave imaging systems through the measurement of the dielectric properties of materials of interest. The EMXLAB also actively works towards identification of materials using wideband millimeter wave imaging. Accurate complex permittivity values are necessary to discriminate materials, model their response in MMW imaging systems and create simulant materials.

This work describes the simulation, development and implementation of a resonant waveguide cavity measurement system designed to operate between 75 GHz and 110 GHz and to measure the complex permittivity of sub-microliter volumes of liquid. Unlike other resonant waveguide cavity systems, the liquid is not placed at the center of the cavity in a fixture. Instead, the liquid is placed on the opposite cavity wall to the coupling aperture without a fixture. This requires the standard perturbation theory equations to be modified to allow the sample placement on the cavity wall.