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

[Oz-in-DFW]

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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Oz
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