Analogue VHF waveforms have been used extensively as signals of opportunity in passive radar concepts. They have time-varying ambiguity properties determined by the instantaneous nature of the broadcast content and correspondingly this results in a time-varying radar performance. In this reported work, the ambiguity properties of high definition (HD) FM signals are examined. It is shown that the digital modulation imposed on these transmissions results in more stable ambiguity properties better suited to reliable and consistent passive radar performance.