The purpose of this paper is to discuss the significance of FMCW waveform generator spectral purity with regard to its propensity to limit over-the-horizon (OTH) radar performance. The fundamental principle of operation of FMCW OTH radar is first described and then further developed in order to yield results in terms of several parameters of a synoptic database of OTH radar propagation data acquired with the Australian project Jindalee OTH radar frequency management system. The method of data analysis is addressed in detail and followed by the presentation of the results of the analysis of 5 years of data. Finally, the high demands of waveform generator spectral purity are emphasized in the sense of avoiding radar performance degradation due to this potential form of internal noise limitation. ¿ 1998 American Geophysical Union