Frequency-variation method (FVM), reported in [1], was further studied for simultaneously measuring the both complex permittivity and complex permeability by intentionally changing the test frequency to obtain different reflections. An enhanced coaxial-probe-based in-situ measurement system has been established. The spectral domain full-wave model is derived to take place of the quasi-static one. A novel coaxial probe is designed so that the one-port calibration could be performed with Agilent-supplied precise cal-kit instead of the liquid standard. Criterions for a right order of the interpolation polynomial used to approximate the frequency-dependent EM parameters; measures to reduce the residual mismatch errors and random error in reflection measurements and to suppress the ambiguities in solving the transcendent equation system were experimentally studied to resolve the problems and improve the accuracy in dispersive absorbing materials' test. Several typical dispersive absorbing coatings have been tested via FVM. The good comparison between the measured results and reference ones validate the feasibility of the proposed improved technique.