Topband: Vertical array

[Jim Brown]

On Wed, 12 Dec 2007 22:01:17 -0800, Lee K7TJR wrote:

>All the plots in the world of great antennas usually do not take 
>into account the feed lines, power poles, buildings, street lights,
>flag poles, phone wires, and who knows what that may be in the 
>vicinity of a given antenna. 

YES, YES, YES!  It is VERY instructive to build an NEC model of a 
simple antenna like a vertical, then add the feedlines to other 
antennas around it, and study the resulting pattern. I did that, and 
saw skewing of the pattern on the order of 8 dB!  

Your comments re: the effects of surrounding objects on the 
directivity of arrays are also right on target. All arrays provide 
rejection (nulls) in certain directions by producing out of phase 
combininations of the patterns of multiple elements. For those 
cancellations to be deep, the signals from the elements must be 
almost precisely 180 degrees out of phase and almost precisely 
equal. It doesn't take much to upset either the phase or the 
amplitude, which can do anything between shifting the direction of 
the null or killing it completely. And as you've observed, VERTICAL 
angle is just as important is azimuth. 

73,

Jim Brown K9YC