[time-nuts] GPS Antenna Grounding/Lightning protection. [Cone of protection and other opinion]
lists at ozindfw.net
Wed Jun 20 18:45:02 EDT 2018
The "Cone of Protection" is a thumbrule. It's not a myth, but it's also
not an absolute law of nature. It's a self-fulfilling prophecy to some
degree because it drives component design guidelines.
Bunker spacing is also a thumbrule, based on a lot of horrible
experience. It's also a function of many variables and differing
requirements, largely the explosive involved. It's also self-fulfilling
prophecy because it enters into design guidelines for explosives
packaging. There's a multitude of reasons those bricks are in a foil
bag. This is one of them.
I'm not a lightning protection expert, but I do a bit of work where I
have to be concerned about it. I look at that thumbrule, but I'm also a
belt and suspenders guy and apply many others. And I build shipping gear
as if it's going to South Africa (much worse that Florida.)
Back to my original statement early in this thread (rope?) "No matter
what you do, it's unlikely you can do anything within economical reason
to survive a direct strike and the 10's to 100's of kiloamps involved.
The real question is how close of a near miss can you survive." for a
small business/residential install, as a minimum I would:
1. Make sure your tower is grounded.
2. Make sure your tower is electrically sound (a good electrical
conductor - all sections are well connected electrically. this is
*/NOT /*a given with used or old tower.) If you are going to clamp
copper straps to the tower or antennas, use some stainless shim
stock as a barrier.
3. Mount your antenna(s) with grounding as specified by the manufacturer.
4. Use a lightning/drip loop at the top of the tower as a absolute
minimum. Better to use a real suppressor, preferably of the 1/4
wave stub type.
5. Ground the coax shield to the tower at the top and the bottom.
6. If you have control lines (power, rotator controls, etc.) use
shielded cable, ground, and suppress as with coax.
7. Use a master ground bar entry into your building, and make sure you
have effective lightning suppression on */everything/* that enters
the building: Coax, AC power, phone lines, ethernets, hopes, wishes,
8. Make sure the master ground bar is well grounded to the tower. See
Commercial installs are copper bus bar. I use it where I can, but I
also have ham installs that use unistrut and copper strap. In an
ideal world this is next to the electrical service panel and ground
there with suppressors on the AC lines into the building. The world
is rarely ideal.
9. I /try/ to bury at least 8 AWG ground wire around the building, but
I'm usually lucky to get it between the AC panel, coax entry, and
tower. One time I used scrap 1/2" copper water pipe.
10. Get a good suppressor on the input feed to the AC panel. The /vast/
majority of failures I've seen come in the AC line. Or phone line if
you still have copper.
11. Make sure your grounds really are. A six foot 14 AWG wire is not a
ground, even for a cable TV entry block. Use wide braid, strap, or
welding cable size wire. And be careful how you route it. There is
a lot of literature on this alone. don't run it across the attic ;-)
12. If you want the buried stuff to last, look at some form of cathodic
Of course you can do all of this and take losses from a hit, or none of
it and never see a problem. This is, as the Polyphaser paper mentions,
Shutting up now, sir.
On 6/19/2018 10:56 PM, Bill Hawkins wrote:
> Seems to me that lightning protection for timenuts who put things on
> masts keeps this from being completely off topic.
> People who store explosives in earthen bunkers have learned from many
> years of experience how far away bunkers have to be spaced so than an
> explosion in one bunker won't affect others. That same body of
> experience came up with the cone of protection.
> It is not a myth.
> Note that lightningsafety.com sells lightning protection. The scrolling
> set of pictures on the home page shows a picture of four masts
> protecting a rocket launch site.
> Bill Hawkins
> P.S. Lightning can enter a home in other ways. A neighbor had a direct
> hit to a tree 15 feet from the house. After generating enough steam in a
> 2 foot diameter tree to split the length of it, a side strike hit an
> outdoor light and did considerable damage in the house. The tree was not
> the highest thing around. That same strike produced an EMP that took out
> one of my two GPS antennas, about 100 feet away. The time from flash to
> BANG was about 100 milliseconds. No, I didn't measure it - I experienced
> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of
> Van Horn, David
> Sent: Tuesday, June 19, 2018 11:22 AM
> About that "cone of protection"
mailto:oz at ozindfw.net
Southlake, TX 76092 (Near DFW Airport)
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