[time-nuts] Thunderbolt cabling questions

[Jim Lux]

On 6/11/12 10:31 PM, Hal Murray wrote:
>
> albertson.chris at gmail.com said:
>> But you know what?  If you simply place an automotive "puck" type GPS
>> antenna on your roof you have to do the same thing.  It must be grounded the
>> same way, same lightening protection and so on.   So in the end you may as
>> well put up a professional looking and permanent  steel mast.  It is not
>> that much more work.
>
> What about putting a skylight high on the roof and putting the antenna up in
> it?
>
> What's magic about inside vs outside the roof/skylight envelope?
>
> -----------
>
> I have a large pine tree out front.  It's roughly 3x the height of my (one
> story) house.  What are the chances of any lightning hitting my house rather
> than the tree?  What if I put an antenna on the top of my house so the tree
> is only 2x the height of my antenna?
>
> Of course, that depends on how far the tree is from my house.  Not far.  Call
> it 45 degrees from the back of my house to the top of the tree.  An antenna
> on the top of my house would probably be below that sight line.
>
> Is there a good book or URL on lightning vs antennas?  Again, I'm interested
> in both the technical issues as well as the local zoning/legal issues.
>

http://www.amazon.com/The-Art-Science-Lightning-Protection/dp/052187811X

Martin Uman and his collaborator Rakov have probably forgotten more 
about lightning than everyone on this list collectively knows about it. 
  This one is a bit pricey still, but is the definitive tome.

A Dover Press version of Uman's "The Lightning Discharge" is <$20, and 
well worth the investment if you're interested in lightning.

Ronald Standler's book "Protection of Electronic Circuits from 
Overvoltages" is a great source on overvoltage protection in general. 
$20 in paperback. Lots of useful information on how to design/purchase 
transient suppression for all kinds of signals.  And surprising 
information on how certain kinds of techniques can actually make things 
worse.



One wants to be careful about texts published by manufacturers of 
protection equipment.  Yes, they typically have valid information, but 
it *is* coming from a source which wants you to "buy more stuff", so 
they tend to be a bit more conservative (more protection = better, even 
if the physics doesn't support it).  That said, much of the high quality 
peer reviewed research on things like lightning rods (aka "air 
terminals") does come from companies making such things.

Also, there are several pubs out there widely distributed aimed at 
applications like FAA Control Towers or high reliability 24/7 land 
mobile radio.  The recommendations in those books may prove to be 
somewhat of overkill for a couple reasons:  most amateurs don't need 
that level of protection; the suggestions aren't always supported by the 
physics, but are there because "they don't hurt", triggering the "nobody 
got fired for buying IBM mainframes" phenomenon... if it's small 
differential cost, why not do it, because if we don't do it, and 
something goes wrong, we'll be blamed.



Legality wise

Nat Elec Code (aka NFPA70) has bonding and grounding requirements.  Art 
250 on grounding, Arts in the 800s on antennas.  Expensive ($80-100) if 
you buy it, but since it forms the basis for California's Title 24 
(State Electrical Code), a scanned version is online at 
https://public.resource.org/.  The antenna grounding stuff doesn't 
change very much, so an older code found at a used book store might also 
work.

NFPA780 is the lightning protection code.

IEEE 1100 - The Emerald Book - is very useful on grounding, transient 
protection, etc. issues in general. Pretty expensive.. find it in a library.
http://standards.ieee.org/findstds/standard/1100-2005.html