Measurement of reflectivity in free space is a useful technique to quantify the refractive index and associated dielectric constant of materials at millimeter-wave frequencies. The configuration of the target and the incident and scattered radiation fields are often idealized, and the analytic models can be studied numerically to improve measurement precision. A finite element (FE) 2D model of a free space dielectric measurement system at 60-90 GHz is presented with an example target consisting of a partially transparent “disk” on metal with assigned characteristic dielectric values. The simulated data for the reflected signal at the antenna are compared with the predicted reflection spectra and are analyzed to derive the “measured” dielectric constant. The efficacy of using an aperture in the measurement system to control the beam and reduce the required target dimensions is demonstrated. A method to use an array of antenna elements to reduce the illuminated spot size is also explored.
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