Two methods of improving measurement accuracy of Ka-band wave reflection coefficients close to one due to indirect measurements are described herein. One of them is based on using multiple wave reflections in the open resonator of specific geometry, while another one- on the measurement of S-matrix of system, consisting from two horns with sample between them. Corresponding stand constructions and performance requirements are introduced here. Setup calibration procedures and test data processing algorithms are described. Measurement processes are illustrated by the example of concrete carbon fiber and mesh samples.