On Fri, 27 Jul 2007 17:58:29 EDT, K7LXC@aol.com wrote:
>Some years ago I saw a study done by Bell Labs or RCA or similar in the
>1930's or so in a broadcast engineering handbook that showed some help even a
>mile or so away depending on the frequency.
At that distance, I suspect they were talking about interaction with a
directional array that might fill in a null enough to put the pattern out of
limits. Some background. Directional antennas on the AM broadcaast band are
designed with one objective -- to protect stations on the same and adjacent
channels from interference while still providing coverage to their licensed
community. Those protections are accomplished by NULLS between towers, where
the
radiation from two of the towers cancels. If the null is deep (that is, a lot
lower signal strength than the main pattern, that cancellation must be near
perfect. If it's a deep null, a relatively small change in anything around the
antenna can upset the balance.
It is well known that deep nulls in patterns can be upset by construction of a
new object (a water tower, big power lines, etc.) at some point where it re-
radiation from it is in phase with one of the towers that is part of that
cancellation.
73,
Jim Brown K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|