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Microwave non-destructive testing (MNDT) using free-space microwave measurement (FSMM) system involve measurement of reflection (S11) and transmission (S21) coefficients in free-space. The measurement system consists of a pair of spot-focusing horn lens antenna, mode transitions, coaxial cables and a vector network analyzer (VNA). The inaccuracies in free-space measurements are due to two main sources of errors. 1) Diffraction effects at the edges of the material specimen. 2) Multiple reflection between horn lens antennas and mode transitions via the surface of the sample. The spot-focusing antennas are used for minimizing diffraction effects and free-space LRL (line, reflect, line) calibration method implemented on VNA eliminates errors due to multiple reflections. The time domain gating or smoothing feature of VNA is used to reduce post calibration errors in reflection and transmission measurements. In this paper, complex reflection coefficients were measured using FSMM system for silicon wafers backed by a metal plate. It was observed that the dielectric constants of the silicon wafers are higher than the values reported for intrinsic silicon wafers which maybe due to the presence of highly conductive epitaxial layer doped on the wafers. Results are reported in the frequency range of 8.0-12.5 GHz.