The signal varied in strength between full quieting and several times faded out to the point where the repeater reset.
The interference was first heard by Allan, VK3BF who could just hear it on the input frequency. Myself and Adam, VK3HDF went in search of the interfering signal at approximately 6PM. We could not hear it from near the repeater site. After heading in the direction of Allan's place we picked up a signal while travelling west along Ferntree Gully road in Mount Waverley.
The signal appeared to be coming from a medical supplies factory in Ricketts Road. Although the signal was quite detectable from outside the two storey building, it was not very strong and was only just detectable on the foxhunt sniffer 600 metres away. We could not reliably hear the signal from the freeway behind the wire fence of the building concerned.
What appears to be the explanation is that there may have been some upper level ducting (inversion layer) active in the afternoon. The company I work for has done a fair amount of research into this phenomena and has measured path gains of between 6 and 16dB with their in-house developed Digital Radio Analyser. The lower levels of gain can be active for several hours at a time.
It is not clear at this stage what the signal still eminating from factory is used for. There were no reports of similar interference on the Sunday.
Bryan Ackerly, VK3YNG.