[TowerTalk] RF Surge Suppressor at Base of Tower or Shack SPG or Both ?

[Jim Brown]

On 10/13/2021 2:19 PM, Gene Smar via TowerTalk wrote:
> I see no need to put a suppressor on the single run of 50 feet of coax 
> that is entirely underground or within steel boxes at the tower base and 
> outside the shack, although my station's SPG is inside the shack box and 
> all coax shields connect to it.  A suppressor on that line would be 
> useless, wouldn't it?  What am I not understanding?

1) That the 50 ft bond between the tower and the shack looks like an 
inductor at RF. Lightning is an RF event, not a DC event.

2) That there can be a very great potential difference between those two 
points in the event of a strike.

3) Protectors clamp the differential voltage to near zero at the point 
where they are installed;

4) RF current on the coax shield induces a differential voltage inside 
the shield that is added to that zero voltage from the protector 50 ft 
away.

5) As to buried coax, the skin depth of soil can be pretty great. I've 
seen estimates from W8JI on the order of 60 ft at 2 MHz, probably a bit 
less at higher frequencies, and highly variable with soil types. I 
wouldn't count on burial a few feet to provide shielding from lightning 
induced currents, which can be monstrous.

Bottom line - protectors protect equipment that they are very close to, 
and their protection is best if at the entry panel, which is bonded very 
close to the house grounds.

73, Jim K9YC