This issue has a good antenna article on loop antennas but for some
strange reason the author seems to think it is only a good AMBCB
antenna. This antenna type is excellent for SW also. Depending on what
frequencies you want to the antenna to operate on and the circumference
of the loop you will not need a matching transformer. Closed loop
antennas are generally superior in urban environments with local noise
sources for a number of reasons.

There is also a good article on propagation by Tomas Hood.

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Telamon
Ventura, California