[time-nuts] Thunderbolt cabling questions

Bob Camp lists at rtty.us
Sun Jun 10 20:25:13 EDT 2012


On Jun 10, 2012, at 7:43 PM, Jim Lux wrote:

> 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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