RF (radio frequency) absorbing materials are widely used for electromagnetic applications. The uniformity and repeatability are very important for application such as GPR (ground penetrating radar) antenna loading - a method to improve the system performance. There is a demand for a good method to assess the uniformity and reputability of the material. Various methods have been developed for material characterisation. The open-ended coaxial probe is one of them which has been developed for non-intrusive measurements of small samples and was chosen as the best candidate for this study. Commercial probes are very small and not really suitable for our investigation. A large open-end coaxial probe with an outer diameter of 4 cm, which was developed at Liverpool, was employed for this work. Five flat sheet absorbing materials were selected by IDS. A special calibration method and data interpretation algorithm were developed and employed for this work. It is shown in this paper that the results clearly demonstrate that the approach used for this study is accurate and effective. (4 pages)