K4GPB wrote:
anyone have a clue how this works?

http://www.v-soft.com/ZipSignal/zip_answer.asp

That *is* a useful site, isn't it?!

I've encountered a number of sites that are able to cross-reference ZIP
codes to geographic coordinates - don't know where they're getting the
data but my guess would be the Census Bureau has a database.

The technical facilities of the radio stations are available in
databases downloadable from the FCC.

The firm that runs the site is a broadcast engineering consulting firm;
their regular work involves the ability to predict field strength at a
given distance, transmitter power, and antenna parameters.

Combine that, and you end up with what you see there.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com