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High-precision permittivity measurements of liquids are essential for specifying the specific absorption rate (SAR) of radio frequency power into human tissue. At VSL, a commercial unit is used for these measurements, but for reaching a low uncertainly some inherent disadvantages need to be overcome. In this paper, a multi-step approach is proposed using a combination of curve fitting, nonlinear progressive technique and an improved transmission line model. This approach is applicable even without a priori knowledge of the liquid properties. Using this approach, the applicable frequency range of the commercial unit is successfully extended from 200 MHz 3 GHz to at least 100 MHz-4 GHz, and the accuracy is improved from 2 % to 0.2 % 0.3 %.