On 6/10/12 4:24 PM, Chris Albertson wrote:
On Sun, Jun 10, 2012 at 2:50 PM,<[email protected]> wrote:
... 3m
of antenna cable is no problem. Antenna position is more important than
the exact type of antenna. I'd rather have a decent antenna at a very good
site, than a very good antenna at a slightly worse antenna site
3M is trivial. 30M will work fine too.
I agree about the location really mattering more than anything else. What
I did was drill a 2" hole through the roof up from the attic and push a 10
foot gallanvised iron plumbing pipe up.
you would probably want appropriate flashing around that to prevent
water (and vermin) ingress.
The antenna sits on thop ithe
pipe and is higher then the roof top ridge and then the cable go down the
center of the pipe. I pipe flange on top of the pipe makes a perfect
mounting platform. I used a timing antenna comes inside a white pointed
plastic radome. These sell for just under $30 on eBay. Maybe it is
coincidence or not but the four holes pin the standard pipe flange match up
with the four holes in the bottom of my antenna and there is enough room
inside the hole in the center for an "N" connector. It is worth getting
the antenna "done right" because it is the most important part of the
entire system. Those dome type antenna are worth it. the shape is
designed to shed both bird poop, and snow. Birds can be an issue with a
flat top antenna, no snow here.
You probably get snow every few decades (it snowed in Malibu a couple
years ago, for instance), but I wouldn't worry about snow loads, even
so. <grin>
HOWEVER, your scheme is going to be tricky to pass muster with the
National Electrical Code. Two aspects need attention:
You need to have a ground wire from the mast to the ground point
and
You need to have some form of ground of the coax shield at the point
where the coax enters the building. (a "listed antenna discharge unit"
is the usual way).
While Southern California isn't exactly the lightning capital of the
world, we do get some. A bigger concern (and the primary reason for the
code requirement) is that above ground power lines can come down and
touch your antenna.
And someone living in a more lightning prone area is going to want to
take those precautions.
The installations I've seen typically use the same general "pipe" scheme
(using rigid conduit, which looks a lot like pipe, but has a smooth
inside with no burrs) to a box on the roof, and then regular conduit
running down the outside of the building. Then at the point of
entrance, the ground bonding conductor goes from the conduit to ground,
and there's a coax grounding block in a box at the place where the hole
in the wall is.
Granted, if lightning does hit, everything connected to the antenna is
going to fry, unless you have some sort of reradiation scheme to provide
an air gap. That's what we do when we test GPS receivers destined for
space, where you don't want to take the risk of killing the expensive
flight hardware.
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