In reply to G3CWI:
Best of luck with this Richard, I remember when the 73 KHz band was first made available & some of the antenna designs were remarkable. I remember a photo of a laundry basket,very similar to one I had in the house at the time, virtually covered in copper wire that was the loading coil for that particular Gentleman’s antenna.
At least 500KHz should be easier, with a wavelength of a round 600 Metres.
It appears that there are already several SOTA stations who have used 500KHz professionally & are familar with it’s particular quirks, & I hope they will help you with any questions you may have.
I do not have a 500KHz NOV or any equipment capable of transmitting more than milliwatts that far down, let alone an aerial! But I can listen out for you if I know you are going to transmit. Would you be able to work cross-band with 80 Metres SSB? My CW is very rusty, but it is getting better.