"Jimmie D" ) writes:

I think I remember a test conducted to see how much power it would take to
communicate across the United States coast to coast. They started out making
the contact at near legal limit and were still able to detect the signal at
just a few milliwatts. As I remember a few was less than 10. I am sure this
was part of a ham magazine article from many years ago. perhaps someone else
here knows the details better than I.

Don't forget that the rise of QRP activity forty years ago came due
to the realization that you didn't always need high power.

ANd one subset of QRP is to try to communicate with as little power
as possible. Sometimes that was deliberate, dropping power until
the signal was no longer receivable at theother end.

Michael VE2BVW