Topband: Ground Conductivity

[Jim Brown]

On 3/25/2020 9:17 AM, Grant Saviers wrote:
> Areas with poor ground conductivity or sites with poor or deteriorated 
> ground systems may see a considerable shift in base or common point 
> resistance with changes in the amount of water in the soil. Similar (and 
> sometimes more dramatic) changes can occur when the ground freezes."

Hi Grant,

The references to "common point resistance" implies a multi-tower array, 
commonly used on the AM band to protect distant stations from 
interference on the same or adjacent frequencies. This protection is 
accomplished by nulls in the direction of the protected station, and 
achieving nulls requires a considerable degree of precision in the 
cancellation of radiation from the various towers. Anything around the 
array, including conductive objects and variations in the ground system 
can screw up the pattern. There are well known examples of things like a 
big water tank mounted on a tower in the far field of the array doing this.

I don't know if it's still required today, but when I worked in 
broadcasting, each station using a directional antenna was required to 
make periodic field strength measurements in their nulls to proof their 
pattern.

These BC arrays are generally far more complex than the 4-squares we 
use, and variations in our radial/counterpoise systems and the soil 
underneath them largely affects nulls, as opposed to forward gain.

73, Jim K9YC