The thermal energy radiated by silicon wafers with different conductivities is characterized experimentally in the mm and sub-mm wave ranges. These samples are heated up, and the energy that they radiate thermally is captured by different horn antennas covering the frequency band between 75 and 500 GHz. The energy radiated (in the order of pW) and collected by the horn antennas is subsequently detected by zero bias Schottky diodes. The measured thermal radiated power agrees with the prediction from Planck's law for the highly doped wafers, corresponding to high conductivities. However, for low conductivities, the measurements show a descending pattern as a function of the frequency, which is not in line with expectations from Planck's law. Parallelly, we have developed a theoretical model providing a classical explanation of these results [1].