The aim of this series of two papers is to propose an original and low-cost tool dedicated to industrial applications and based on the reflection ellipsometry technique for in situ characterization of dielectric materials at microwave frequencies. In this first paper, different theoretical developments are presented that concern first a specific numerical method for calculating the complex permittivity of a single-layer sample from the measured parameters. Based on contour line charts, this method allows obtaining simultaneously the relative uncertainties on the real and imaginary parts of the complex permittivity. Secondly, for experimental comparisons with the classical Fresnel method, a numerical data processing method based on contour line charts has also been developed, which aims at the determination of the reflection coefficients in both parallel and perpendicular polarizations of the material.