In space RF communications payloads, it is increasingly required that high-power multi-channel transmitters and broadband receivers have shared, or closely adja-cent RF feeds. Because of the large power level difference between the transmit and receive signals and the limitation of frequency allocation, harmonics genera-tion due to passive non-linearities in the high-power transmission path can be a se-rious problem. This paper describes how to determine the signal levels and dominant mechanisms associated with non-linear dielectric behaviour in this con-text. A novel measurement system for testing dielectric samples is described and measurement results are provided for commonly used microwave dielectrics.