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 P.