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
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