"MRW" wrote in
oups.com:

On Feb 22, 10:58 am, "Richard Fry" wrote:
An estimate is possible, but assumptions will need to be made. If
you can post the readings you got, the path lengths, and the general
location for the transmit site I can give you some idea about that.

Of course if you can post the call letters of the station then the
FCC data for that station will show they should be.

The readings that I got were 3.1 mV/m at 15 meters away. 1 mV/m at
0.32 km away. Finally, 0.71 mV/m at 0.80 km away. We traveled a
straight path that was parallel to the direction of the antenna. It's
not an FCC station, but my professor got permission to test it briefly
at 1.79MHz. I don't know the exact area that we were located, but it
is somewhere east of Magdalena, NM.

The FCC publishes charts that may be used for this purpose. As for
minimum usable daytime field for a typical, cheap indoor receiver in
an urban, residential area, a value of 1 or 2 mV/m typically is
needed.

RF http://rfry.org

Thanks! I was curious about AM stations at one time and looked up some
FCC data. I noticed that some stations have lower power output during
night time compared to daytime. For example, one station had 1 kW
during daytime and decreased it to 300W (I think) during night time.
Does that indicate that night time propagation is better?

Night propagation is complicated by the existence of a strong sky-wave
reflection from the ionosphere's E and F layers. In the daytime, the D
layer absorbs emissions at these angles. Hence the need to reduce power
at night to avoid interference with more distant stations' ground wave
coverage.

--
Dave Oldridge+
ICQ 1800667