I've built a little 80 meter transmitter that seems to be putting out
about
7 milliwatts. By my caculation, that should be good for about 40 miles.
Anyone have any experience with low a power output?

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I once worked 10 miles on CW, with 10 milliwatts on 160 meters, in daylight
with my 20 feet long antenna lying on the ground, and a local noise level of
S6. The other fellow had a half-wave dipole running 40 watts. He copied me
OK.

Propagation must have been by groundwave (on account of time of day) partly
over densely populated industrial areas and partly farm land.

But to translate from this information the possibility of acheiving 40 miles
on 80 meters with 7 miliwatts requires some extra information from your end.

It's best to begin calculation with some sort of standard - some facts
which you already know by experiment or hearsay and can reasonably be
depended upon.

Suppose it is already known that with 100 watts on 80 meters, between two
not necessarily identical antennas, of unequal efficiencies, with a radio
path length of 100 miles, gives an S-meter reading of S-9 + 12 dB at one end
of the path.

At 6dB per S-unit, this reading is 2 S-units greater than S-9. If you like,
call it S-11.

Suppose the power of the transmitter at the other end goes QRP and is turned
down to 10 milliwatts. This is 1/10,000 of the original power, or 40dB down,
or 40/6 = 6.7 S-units smaller in received signal strength.

S-11 minus S-6.7 = S-4.3

So provided the noise level is less than about S-4, the 10 milliwatt
transmission will be fairly easily copied at a distance of 100 miles. Which
is now not so ridiculous as it may first sound without some thought being
given to it.
----
Reg, G4FGQ