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.
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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.
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