"Art Unwin" wrote in message
...
What is the main factor that prevents HF radiation from focussing
for extra gain?

HF can be focused for gain. Main factor is that it takes a big antenna to
focus a signal on 3.5 MHz or so. Even a 2 or 3 element beam is too large
for most people , but it has been done.
To get any gain from a dish at that frequency would take a dish larger than
400 to 500 feet in diameter, maybe much larger. Hard to put that up a
tower.