[TowerTalk] [Bulk] Re: RF Ground is a Myth

[Jim Lux]

On 1/21/15 1:01 PM, Patrick Greenlee wrote:
>
>>
>> Whether the 3 tower leg anchors are sufficient?  I would hesitate to
>> say,  They are close enough to each other that they "probably" act
>> more like a single, large ground, or electrode than multiple units
>>
>>
>> 73
>>
>> Roger (K8RI)
>
> I'm confused...    (still, yet, again...)
>
> There are recommendations to place ground rods (8 footers) 8 ft apart in
> a circle (octagon.) If my 14 ft apart Ufer grounds are too close
> together so as to act as a single point, what about the 8 ft rods 8 ft
> apart?




That wouldn't be a configuration I've seen recommended in any of the 
IEEE literature.

Distance between rods of twice the rod length is kind of a "go to" rule 
of thumb.

At that distance the resistance to "ground at infinite distance" is 
roughly half the resistance of a single rod.

One can (somewhat inaccurately) think of a rod as having a "sphere of 
influence" that is a hemisphere in the soil, with the radius = rod length.

(and yeah, that assumes uniform soil properties, which never happens, 
and identical rods driven identically, etc.)

So maybe driving them closer together accommodates some of the 
variability. Or the guy/gal who did that chose the distance for 
mechanical convenience.

As the phrase has it, this isn't "rocket science".. it's more in the 
10-25% error category.  As Jim noted, the soil resistance varies widely 
in most people's yards in a fairly short distance. And unless you live 
in somewhere like Sacramento, where the soil is basically uniform 
alluvial sediment for 10,000 feet down, there's big differences as you 
change depth.


There's an IEEE standard that has the effective resistance of multiple 
rods for just about any configuration you care to name. 1, 2,3, many 
rods, horizontal, vertical, straight line, angles.

As a result, folks choose a compromise solution that covers the vast 
majority of cases, and if it's critical, they do measurements to make sure.