On Feb 22, 1:07 pm, "Richard Fry" wrote:
Your description resembles a "Part 15" type installation more than the
broadcast system of your first post. Here are some general statements about
how a compliant Part 15 AM system might perform.

Don't know much about "Part 15" is that something that I can find in
the CFR 47? http://www.access.gpo.gov/nara/cfr/c...le-search.html


Antenna engineering textbooks, NEC calculations, and thousands of field
strength measurements made in the broadcast industry over the last 75+ years
show that the maximum field strength in the horizontal plane that is
produced by a vertical, 1/4-wave monopole with 1 kW of applied power over an
almost perfectly conducting, flat ground plane is about 300 millivolts/meter
(mV/m) at a radius of 1 km (0.62 miles).

A legal Part 15 AM tx that was 100% efficient, and used with the above
antenna system would generate a field at 1 km that would be reduced by the
square root of the power difference, or to a field of 3 mV/m in this case.

How did you get 3 mV/m? I tried 300 mV/m / (sqrt(1kW - 20W)).


But a 3-meter, ground-mounted Part 15 antenna system is only about 1% as
efficient as a 1/4-wave broadcast radiator system. So instead of radiating
100 mW, the Part 15 antenna system radiates around 1 mW. That leads to a
further reduction in the field at 1 km by the square root of 100, bringing
it to about 300 microvolts/meter (µV/m).

Note that all of these fields assume an almost perfectly-conducting ground
over the propagation path. Typical ground conditions are far from perfect,
so the fields at 1 km would not be even this high.

Are there any existing resources that talk about how much attenuation
can each ground type contribute?

By broadcast standards, a 300 µV/m field is very marginal in providing a
usable signal to a typical, cheap AM receiver located inside a home. And
every doubling of the distance decreases the received field by more than 50%
(including ground losses).

From this information it can be seen that claims of "legal" Part 15 AM
coverage extending for a radius of 2, 3 and 4 miles cannot be realistic,
unless the system is not meeting Part 15 limits.

RF

Thanks, Richard!

I don't have the experience background, yet. But in your opinion, is a
field strength of 1mV/m the absolute minimum for decent AM reception?
Also, is the fact that AM more prone to interference the reason why
its field strength requirements are higher than FM?