[TowerTalk] Fwd: tower grounding

[jimlux]

HansLG at aol.com wrote:
> Hi Dragonsong,
>  
> I believe the rebar cage will be totally isolated from ground as it is  
> surrounded by concrete. The concrete is a relative good electric insulator and  
> will therefor not be good for grounding the tower. You have to add grounding  
> rods outside the lump of cement holding your tower in place.


This is untrue.  Concrete (unless is specially made to have high 
resistivity, as for electric railroads) has higher conductivity than the 
surrounding soil. Lots of ions, it's hydrophilic, etc.

The relatively high conductivity of concrete is the basis for the 
concrete encased grounding electrode (aka Ufer ground) currently 
required by many jurisdictions as the primary grounding means.  Whether 
you like it or not, the rebar cage is electrically connected to the 
block of concrete it's imbedded in (unless you've taken specific 
precautions against it, which almost nobody does..)

The efficacy and durability of the Ufer ground has, by now, been 
established by over 50 years of field experience, numerous controlled 
laboratory tests, etc.




>  
> There is plenty of information of the "best way" to ground the tower. Two,  
> three grounding rod driven 8 feet into ground is, I believe, considered  
> acceptable grounding for lightning strikes. 

Acceptable is determined by many factors, not the least of which might 
be your local jurisdiction's rules (at least if you want insurance on it).

 From an engineering standpoint, you need to decide what the purpose of 
the lightning ground is (i.e. what sort of impedance are you willing to 
accept, etc.)




When it comes to RF grounding the  field
> is open for suggestions. Little depending on the surrounding ground I,  
> myself, believe in radials stretch just above or on the surface of ground. Other  
> folks believe the radials have to be buried or left several feet  above ground. 
> The length and amount of radial is a science by itself  and I think the 
> advice is:" The more the better", up to some point that varies  between 15 to 60 
> radial.

I would say more a craft or art, than a science.  For most situations, 
the soil properties are not well known enough to actually model the 
behavior in detail. So you keep adding wire until the performance 
doesn't improve, or you run out of money, or you get tired of trenching.