[TowerTalk] How close does a vertical have to be to

[Jim Brown]

On Fri, 27 Jul 2007 17:58:29 EDT, K7LXC at 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