I have been following this interesting discussion, but I have some
perhaps naive observations which could do with some expert answers.
Combining the output from two stations will give double the noise and
double the signal - so no advantage there. I presume the fix is to
make sure the two outputs are in phase, thus producing an improvement
as the signals will be coherent but the noise will not.
Plainly, there will be phase differences between the two receiving
stations because of the different path lengths, and I suppose there
would need to be compensation for this at the combining station.
I can see this being useful for point to point ground-wave links,
where the relative phase between the two received signals can be
predicted and is constant. But how can it work when the incoming
signal is from an unpredictable direction, and the path length (and
hence phase) of a sky-wave signal is varying all of the time?
At best you will have diversity reception (as described by Mal), but
how do you keep two sky-wave signals in synch?