[time-nuts] GPS Antenna Grounding/Lightning protection.

Van Horn, David david.vanhorn at backcountryaccess.com
Tue Jun 19 16:22:20 UTC 2018 

About that "cone of protection"
http://lightningsafety.com/nlsi_pls/cone-of-protection-myth.html



-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Bill Hawkins
Sent: Monday, June 18, 2018 3:16 PM
To: oz at ozindfw.net; 'Discussion of precise time and frequency measurement' <time-nuts at lists.febo.com>
Subject: Re: [time-nuts] GPS Antenna Grounding/Lightning protection.

Oz has some useful observations. 

I've worked on a 50 foot ocean-going fishing party boat that had a 10 foot aluminum mast on top of the wheel house. That will be the shortest path to ground on the open sea. The best you can do is to connect a 4"
wide copper strap from the mast to the keel, with as few bends as possible and none greater than 45 degrees. This seems to work.

I've also worked at a blasting cap plant where 50 foot masts were erected at both ends of an earth-covered powder magazine. They provide a "cone of protection" that prevents a direct hit on the magazine. The mast grounds were measured quarterly with a hand-cranked device specifically made for ground resistance. It had to be less than 100 milliohms.

But if you really want protection from a direct hit, you must disconnect the tower device(s) before the storm hits. The coaxial cable must have only one ground point. The other end should be far from a metallic ground. You'll probably lose any electronics in the antenna, but there are far more expensive things in your lab.

You will also have to deal with the electromagnetic pulse, so all of your equipment, including the computers, must have a common ground point. This provides a ground plane that can change potential relative to the Earth without inducing potentials between devices. Every connection to/from the ground plane must have a surge arrestor. If the risk of nearby direct hits is high enough, isolate the ground plane from all external connections before the storm. You'll need battery backup on the ground plane for all temperature controlled ovens and crystal oscillators. Maybe the Cs and Rb packages also need to keep running.

It would help to move away from Florida, or high hilltops.

Bill Hawkins


-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces at lists.febo.com] On Behalf Of Oz-in-DFW
Sent: Monday, June 18, 2018 2:25 PM
To: time-nuts at lists.febo.com
Subject: Re: [time-nuts] GPS Antenna Grounding/Lightning protection.

Not sure I have much to specific offer, other than some observations.

 1. A path to ground is only a small part of the story.  What's really
    important is the ground reference of all equipment to all other
    equipment. The huge currents and substantial risetimes can cause
    large voltage spikes across even large conductors (>8 AWG.) You want
    everything to stay at the same voltage reference, and you'd really
    like to keep that close enough to ground to prevent arcs from that
    equipment to ground and other equipment.
 2. Long wire runs of even large gauge wire are inductors and can be of
    little value during an event.
 3. 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.
 4. Most of the non-telecom smoking fails I've seen have been power line
    transients. If you took a direct tower hit it's more likely than not
    that your RG-6 would now be plating on a tower leg. An old tower can
    be a pretty poor ground for the microseconds (or sometimes
    milliseconds when you consider return strokes) it takes the
    corrosion in the leg joints to flashover and fuse, or resistance
    heat and weld.
 5. The large currents of a direct strike have predictable but less than
    obvious physical effects like conductor shortening (if they don't
    fuse,) and other significant forces caused by magnetic attraction of
    conductors. One failure case I saw years ago collapsed the conduit
    around a ground conductor. Made no sense until we discovered that
    the conduit was the actual ground path. I'll see if I can find the
    pictures.
 6. Even near misses can induce huge currents (kiloamps) on their own,
    particularly in long vertical cable runs.  I've seen solder joints
    in small empty copper water pipes melt and reflow from a strike a
    100 feet away.
 7. The best coax lightning suppression units I have seen are
    essentially 1/4 wave grounded stubs. These are common is cell site
    installations (and the top /AND/ bottom of the lines.) These are
    always at DC ground and the coax is a the weak point (and ultimately
    the fuse.)  I've seen them surplus and at hamfests and some cover
    GPS freqs.
 8. A near strike will induce some really impressive voltages on
    Ethernet cable runs. Most residential buildings are
    electromagnetically transparent and the protection on most Ethernet
    interfaces is oriented toward ESD.

Oz (in DFW)

On 6/18/2018 1:29 PM, Dan Kemppainen wrote:
> Hi,
>
> I have (or had, I guess) a GPS antenna on a tower that took a 
> lightning hit yesterday.
>
> You can tell it's going to be a bad day when you walk into your shop, 
> and smell burnt electronics. Still have to troubleshoot exactly what 
> got hit, but the GPSDO was flashing no GPS signal, the 5V supply for 
> the antenna to the GPS splitter was dead, the data logging computer 
> had rebooted and the data logging computer monitor was dead. Other 
> network hardware was dead also.
>
> This is a bit surprising since the tower itself is grounded with 4 
> ground rods and bonded to a 150 foot deep well casing near by. The 
> antenna is on the end of 250 ft run of RG6. The GPS antenna cable 
> shield has a grounding block bonded to two ground rods driven down 
> below the basement foundation where it enters the house. I'm guessing 
> the surge ran the coax into the splitter, then through everything 
> connected to it, despite the grounding block.
>
> So, I'm wondering if there are better surge protectors for lightning 
> protection? Maybe something that actually protect the center conductor

> also? Hopefully something that will pass GPS signal reasonably and let

> DC power through. If so, can you recommend some starting points? Other

> suggestions also welcome.
>
>
> Also, If you are considering upgrading your own lightning protection, 
> hopefully this will be some inspiration to get started. As I said 
> earlier, it's a bad day when you smell burnt electronics in the shop.
>
> Thanks,
> Dan
>
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-- 
mailto:oz at ozindfw.net    
Oz
POB 93167
Southlake, TX 76092 (Near DFW Airport) 



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