[TowerTalk] Connecting Tower to Ground Rod

[Jim Lux]

On 10/10/13 6:06 AM, Roger (K8RI) on TT wrote:
> On 10/10/2013 8:09 AM, Pete Smith N4ZR wrote:
>> Does this standard apply even if, as in my case, the tower is 190 feet
>> from the house entrance?
>
> The farther the antenna, the less the effect, See the Polyphaser page.
> But you do want to provide a better path to ground at the tower to
> prevent the lightening from coming into the house.
> Many of us also ground the coax shield to the top and bottom of the tower.
>
>  >Does it need to be a direct, dedicated connection, or could a hardline
>  >shield, for example, be used for the tower to house electrical service
>  >ground connection.  In cross section, even LDF4-50A is a lot beefier
> than >#6.
>
> Using the coax is usually not considered good practice unless the tower
> is a long way out.


I'm not sure there's much to be gained by interconnecting the grounding 
systems at the two ends.

What you want is to make sure that the "outside, touchable surface" of 
boxes and stuff in a given location is close to the potential of anyone 
or anything that is nearby (no side flash).  That means, "bond to the 
local equipotential point" (if you're on the 50th floor, who cares what 
"earth" potential is in absolute terms).

So, surmising that you've got two structures some distance apart, each 
with their own earthing system.  Lightning hits near one, raising it's 
potential relative to the other. You've got wires between the two 
structures, and in accordance with good practice, there's either some 
sort of voltage clamp limiting the voltage difference between wire and 
structure OR there's a high voltage isolation interface (optoisolators 
or transformers, for instance)

So now, you potentially have current flowing in your signal wires.

A parallel, lower resistance, bonding conductor (or "drain wire" as it's 
called in a lot of bundled cables) might reduce the current flow in the 
signal wires (or, more probably, in the transient suppression clamp 
devices at the ends)

For coax? As pointed out elsewhere, the size of the shield on coax 
you're likely to run a long distance is probably bigger than any wire 
you might bury in the trench or conduit with it.

There's also the whole exceedingly complex issue of what the impedance 
of those conductors actually is vis a vis the transient potential 
difference. and the earth potential difference.


That's why the practice is "protection and bonding at the entrance to 
the structure"... what happens between structures is sort of a free-for-all.

(and such issues are why optical fiber is so popular...)