There are undoubtedly official sources from the FCC, etc.

The best hand-held guide I could recommend would be the National
Radio
Club's "Night Pattern Book," a fantastic resource for MW DXers and
broadcast listeners. Basically it is a book of maps of North
America
for each domestic broadcast frequency, with dots representing
transmitting locations, and the night time radiation pattern around
each.

It's available from the following link:

http://www.nrcdxas.org/catalog/books/

The 5th edition is sold out, but the new 2005-06 edition is
scheduled to
be out soon.

Brent Taylor
VE1JH


Dave Pitzer wrote:
Richard,

Both you and Reg have given excellent answers and I thank you
both.

By the way, you mention directional patterns and nulls. Is there
any place I
can find polar graphs of commercial broadcast station's antenna
patterns?

Thanks,

======================================
Dave,

http://www.nrcdxas.org/catalog/books/
Sounds exactly what you are looking for but may take some time to
obtain.

In the meantime, the basic groundwave radiation patterns of mediumwave
broadcast antennas are either simple circles with the antenna at their
centres, or heart-shaped with the antenna at the null.

The first occurs when the antenna is a single vertical mast located
near the centre of a large populated area.

The second occurs when the antenna consists of a pair of masts, which
radiate a very broad heart-shaped beam, located on one side of the
populated area to be covered.

Contour Maps of actual measured field strengths are useful when the
basic groundwave patterns are distorted by the terrain, e.g., the
existence of mountains, forests, rivers, built-up areas, high-rise
cities, or seas, lakes or coastal regions.

Radio frequency Field Strengths are usually measured in terms of
"millivolts per meter" or in decibels relative to one volt per meter.
----
Reg.