Technical Papers and Presentations

The Electromagnetic Signatures of Explosives Laboratory (EMXLAB) at the U.S. Department of Homeland Security, Science and Technology Directorate's Transportation Security Laboratory (TSL) supports the test and evaluation of millimeter wave imaging systems through the measurement of the dielectric properties of materials.  Resonant Cavity Measurement Systems have been designed and built at the TSL to perform dielectric measurements of materials through resonant analysis.  In support of the measurements, COMSOL Multiphysics^® simulations of the resonant systems using the RF Module have been developed and used to confirm the experimental results.  This paper presents measurements of very lossy liquids such as water, and the unique resonant modes set up in the Resonant Cavity Measurement Systems as the volume of the sample is varied.  Generally, when an air-filled resonant cavity is perturbed by the placement of a small sample of material, the resonant frequency shifts to a lower frequency.  It is commonly accepted that the perturbation causes the effective permittivity of the cavity to increase, thus reducing the frequency of the resonant mode.  However, in certain cases, an increase in the resonant frequency is observed when high dielectric liquids are used as the perturbation material. COMSOL Multiphysics^® simulations of the systems show that if the volume of liquid is large enough, a separate resonant mode is formed in the dielectric liquid itself.  The mode embedded in the liquid couples to the cavity mode, and effectively reduces the resonant cavity size and causes a shift of the resonant frequency to higher frequency.  This effect is eliminated when the volume of liquid is too small to support an internal mode.  The COMSOL Multiphysics^® simulations allowed the mode structure in the cavity and the liquid to be visualized and verified the theory.  Experimental data along with simulation data are shown for multiple Resonant Cavity Measurement Systems.  COMSOL Multiphysics^® simulations were instrumental for confirming and visualizing the phenomenon.