Owen Duffy wrote:
. . .
Fundamentally, wouldn't you expect S/N to degrade in a linear receiver
system as you decrease the Signal+Off-Air-Noise wrt the receiver
equivalent internal noise?
. . .

I wouldn't expect it to change by any discernible amount at HF, until
the antenna gets very short or very mismatched. The noise figure of most
HF receivers is good enough, and the atmospheric noise high enough, that
it takes a poor antenna indeed before the receiver noise becomes
apparent. This is certainly emphatically true on 80 and 160 meters in
Florida.

Roy Lewallen, W7EL