Hello.

I took some measurements of an AM broadcast from multiple points up to
about half a mile from the transmitter. The transmitter was supposed
to be operating at 20W. I don't know what kind of pattern the AM
antenna had, but can I just use the field strength readings that I got
to predict how far the signal can go out? I only was able to measure 3
points, but I want to know if that is sufficient to make an estimate
under the same conditions.

If so, how do I go about starting this? Are there any formulas? Also,
on average AM receivers, what would be considered a minimum field
strength value that would yield an audible sound?

I also read that frequencxies less than 2MHz rely on surface waves. Is
there a site that I can visit that will give me an idea on how to take
this into account given that there are different ground
characteristics? Thanks!