Non–destructive complex permittivity measurements can be realized using different free space methods. Method selection criterions are pre–request implementation, uniqueness of obtained results, precision of obtained results. In the paper method of two frequencies and movable metal–backing method for one plane dielectric slab are considered. Method of two frequencies is easy realizable, but it has pre–request – considered material should have constant complex permittivity in concerned frequency range. Movable metal–backing method has no pre–requests, but it is more complicated – additional equipment is needed. In both methods reflection coefficient’s absolute value’s square function of two arguments (permittivity and loss tangent) for two different measurement obstacles is analyzed. In case of exact measurement complex permittivity can be found as cross–point of two curves obtained for different measurement obstacles. In case of measurement with defined precision this cross–point spread out to region. Region analysis can be used for measurement error analysis.