On Feb 22, 12:10 pm, "MRW" wrote:
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?


With regard just to your last question, atmospheric (mainly from
lightening around the world), galactic, and typical man-made noise are
all much higher--40dB or more, usually--at 1MHz than at 100MHz. You
need signals above the noise for practical communications.
(Bandwidths also enter into the picture, but I believe the noise is
the key issue.)

Cheers,
Tom