In reply to M0XSD:
I think that is the answer. Andrew VK3OE has measured the height of ducts supporting VHF propagation and stations above 400 - 800 m will often be above the duct. On New Years Eve I heard VK6BE on 2 m some two and a half thousand km away as I drove up a 1,250 m summit 60 km W of Melbourne. His signal dropped out completely at about 900 m, well before the activation zone.
BUT an hour and a bit later he was 5x2 on 144.1 SSB with my 40 m dipole. The inversion layer had lifted prior to breaking up as it usually does. I could not be heard with my FT817 straight into the 40 m dipole. By the time I got to the car and 50 w into a whip the band had collapsed.
Never leave home without a small 2 m beam and a 40 m antenna. Often I have gone out with just one I have regretted not having the other. Yes I am a slow learner.