From another thread: Here's something else to speculate on, I wonder if anyone's tried 100 milliwatts into a full length broadcast antenna tower as a test, and how much range did it get?
A typical 1/4-wave vertical tower system used by AM broadcast stations produces an inverse distance groundwave field of about 305 mV/m at 1 km, for 1000 watts of applied power. This has been proven by thousands of real-world measurements using accurately-calibrated FI meters, and as predicted by mathematics.
If the power applied to that system was reduced to 100 mW, the inverse distance field at 1 km would drop by 40 dB -- to about 3 mV/m.
A MW field of about 150 µV/m can produce a fairly useful signal to receivers in locations with low radio noise levels. If earth conductivity is average (5 mS/m) then the 150 µV/m groundwave from this 100 mW system using 1650 kHz would be located about 4 miles away.
This is more than speculation, but hopefully more useful, also.