I want to correct this:
I experimented here with very large arrays years ago by watching signals
and watching phase, and I found skywave 160 signals commonly have phase
level variations between arrays centered about 1-2 wavelengths apart.
is at the root of the very reason we use spatial diversity, and why
diversity works. It stands to reason if phase and level were stable,
spatial diversity would not work.
The spacing I typed could have been worded better. At 1 wavelength spatial
separation phase and level was pretty stable. At 2 wavelengths spatial
separation phase and level started to show issues on skywave.
This is NOT an abrupt transition like flipping a switch. It is a progressive
transition where the likelihood of skywave not being in phase increases, or
the chance of not having the same levels from each area increases. This
increases fading when antennas are directly combined, or when one antenna is
VERY large spatially.
While this varies with propagation conditions and array spacing, and I'm
assuming it is sensitive to latitude and direction of path, it was bad
enough here that I never combined small arrays directly into a "giant array"
or used a really long Beverage antenna.