On 160m I have an almost useless antenna by accepted standards: just 15m of low wire down the garden and a central heating ground. Not only that, but my Elecraft auto-ATU cannot find a match on 160m. So, I get around this crudely by putting a 1dB pad between the antenna and the auto-ATU and it then manages to find a match. OK, I lose another 1dB but that is a fraction of an S-point.
Now, even with this crude set-up I have had reports from G, GM and PA0 on WSPR running 5W (less 1dB) into the antenna, proving that a piece of wet string is very nearly all you need to get out. Clearly, with a bit of effort I could make at least 10-20dB improvement on my signal - adding loading coils in the antenna, running out more wire and higher up, improving the ground system etc. I shall need to all of these if my efforts on 500kHz are to come to anything.