When a lightning flash is propagating in the atmosphere it is known that especially the negative leaders emit a large number of Very High Frequency (VHF) radio pulses. It is thought that this is due to streamer activity at the tip of the growing negative leader. In this work we have investigated the dependence of the strength of this VHF emission on the altitude of such emission for two lightning flashes as observed by the LOFAR radio telescope. We find for these two flashes that the extracted amplitude distributions are consistent with a power-law, and that the amplitude of the radio emissions decreases very strongly with source altitude, by more than a factor of 2 from 1 km altitude up to 5 km altitude. In addition, we do not find any dependence on the extracted power-law with altitude, and that the extracted power-law slope has an average around 3, for both flashes.