[TowerTalk] Lightning Protection

[Jim Lux]

On 7/4/13 2:59 PM, K8RI wrote:
> On 7/4/2013 4:09 PM, les wrote:
>> Is it better to place lightning protecting at the base of the tower or
>> where it enters the house. All cables will be in a buried conduit.
>> _______________________________________________
>
> It all depends, but it is required at the entrance by code.  If the
> tower is mote than a few feet from the house, then I'd also do it there.
>
> I ground all coax braids at the top and bottom of the 100' tower with
> the use of bulkhead connectors. You can reduce the loss "slightly by
> using a grounding kit at each point instead of bulkhead connectors, but
> at the frequencies most of us run, I think it's a waste of time and no
> small effort.  OTOH on microwave frequencies, every little bit helps and
> there the loss of connectors is far more than at HF.  From the

And, in fact, I think those bulkhead connectors might serve as a HV 
clamp of sorts. They'll probably breakdown at 5-8 kV or so. (coax is 
0.4" in diameter, so distance from center pin to shield across face of 
connector is around 0.2".  0.2" breaks down at 14 kV in free air. A rule 
of thumb is that the breakdown on a surface is 1/3 that of free air, so 
you're looking at 5-8kV.  Unless you've flooded the connector with 
silicone or oil or something, and that raises a whole lot of other HV 
breakdown issues.

if you're doing the "remove outer insulation and clamp outside of shield 
to ground" scheme, one advantage is that you're not buying more 
connectors.  For PL-259/SO-239/UHF, that's not a big deal, but if you're 
buying DIN 7/16 or even N connectors, or you're running heliax, the time 
and money to install could add up.




>
> In my installation the coax goes to the grounds at the base, then back
> up about 3 feet to a large NEMA box. They enter the junction box through
> bulkhead connectors as well. there is a 6-pack inside.
>
> from there they go underground in conduits to the house and shop where
> there are grounding panels and polyphasers.  In the shop I have a patch
> panel that aids in disconnecting the rigs.
>
> Remember the loss figures for connectors is at their upper frequency
> limit so when used at HF the loss is miniscule.


Not only that but the loss figure is the "minimum measureable loss 
that's not zero".  SMA connectors have a loss typically quoted as 
0.03dB*sqrt(GHz).  But that's really more like the minimum loss feasibly 
testable in manufacturing.  Most, if not all, connectors will have much 
less loss in real life, if they are clean, and mated properly.