[time-nuts] Thunderbolt cabling questions

Jim Lux jimlux at earthlink.net
Sun Jun 10 23:43:58 UTC 2012


On 6/10/12 4:24 PM, Chris Albertson wrote:
> On Sun, Jun 10, 2012 at 2:50 PM,<bg at lysator.liu.se>  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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