"Richard Clark" wrote
If all would review the standard FCC groundwave propagation curves,
they would notice that they offer low AM Band signal strengths in
terms of "conductivity" and that the differences in strength for the
5,000 millimhos per meter and that of 40 millimhos per meter (125 fold
difference) DO NOT achieve the same proportional difference in
received signal strength. In fact, the difference is so narrow you
could shave with a razor as sharp as it. Even at the high end of the
band the difference has to be out 700 miles to show the "conductive"
ratio. Of course, over that range of transmission ONLY Sea Water
would support that forecast as continental soil varies vastly in
smaller spans - hence the reputation of the Sea.
_______________

For a reality check, here are the approx distances to the 1 mV/m contour for
1kW of radiated power from a 90 degree vertical with a good radial ground
system. The values were determined from the FCC's standard curves.

Freq Conductivity/Miles

540 kHz 8/66, 40/124, 5,000/140
1,600 kHz 8/22, 40/56, 5,000/126

The average ground conductivity in the U.S. is fairly low, probably
somewhere between 8 and 16 mS/m. The difference in ground wave propagation
over such paths is dramatically poorer than over sea water.

It is also clear from the above values how much better the low freq MW
broadcast channels perform.

RF