This work proposed a non-calibrated free-space materials characterization by vector network analyzer (VNA) and spot focusing lens. Compared with traditional quasi-optical measurements, it has no use of high gain antennas. A smaller spot of terahertz wave at the focal point means that smaller samples can be characterized. In measurements, time-domain transformation of S21 is carried out. Locations of the maximum value correspond to the fastest transmission path. Time-domain gating is applied to isolate the source mismatch error and multi-path effect. After that, samples were placed at the focal point, and frequency domain results of sample signals were recorded and calculated. Phase ambiguity is solved by linear fitting and extrapolation. The method we proposed can extract electromagnetic parameters of materials with no limitations on frequency. Results have higher precision than those applied Gate-reflect-line calibration method, which is considered the most accurate one.
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