[time-nuts] lightening protection of a GPSDO system / optical isolated distribution amp

Chris Albertson albertson.chris at gmail.com
Wed Nov 26 17:14:35 EST 2014

You CAN (almost) lightening proof your system.  The trick is to give
lightening a low impedence path to grind at very opportunity.

Start with the antenna mast and call.  Use iron pipe for the mast and feed
the antenna cable down the center of the pipe.  Place two large ground
clamps on this pipe and connect a large diameter wire that takes a straight
path to a group rod.    This will go a long way to diverting energy to
ground because high voltage likes to flow on the outside of a conductor
which would be the pipe and not so much the antenna cable.

The ground rod needs to be bonded to the rest of the building ground system.

Then the antenna cable passes through a metal bulkhead with a bulkhead
connector and all this is also grounded.  After this is might be a high
voltage e on the center conductor.  Use an "lightening arrester that is
bolted to the bulkhead.

At this point you are reasonably safe.  Remember that Ethernet is always
gavalically isolated by transformers

On Wed, Nov 26, 2014 at 1:37 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <
drkirkby at kirkbymicrowave.co.uk> wrote:

> Said mentioned on an earlier thread that if a GPS antenna is used
> outside, lightening protection should be used. This immediately
> reminded me of something that happened about 10 years ago to me
> 1) Lightening damaged my ADSL modem. It because totally dead.
> 2) Every computer and a printer connected to that had the Ethernet
> ports blown up.
> After a hell of a fight with my insurance company, they paid up on my
> household insurance. The total cost was about £10,000, as all were Sun
> workstations, so a bit more expensive than a typical home computer.
> A few years after that, a similar thing happened, but just the ADSL
> modem got destroyed - no computers.
> Clearly if an external antenna is put in a high enough E-field or
> H-field, it can do damage to the antenna, and potentially anything
> connected to it, which would be all your test equipment, in much the
> same way all my computers got their Ethernet ports blown up.
> I would be *very* reluctant to use an external antenna, which is in
> some way connected to a distribution unit into the back of every bit
> of test equipment I have. I can see a potential (excuse the pun), of
> doing a serious amount of damage.
> The only way I would consider doing it, is if there was some optical
> isolation. In principle one could modulate a laser at 10 MHz, pass it
> down an optical fibre, then have a photodiode to recover the
> modulation. Can would obviously be needed not to compromise the
> signal, and that might be impossible.
> I realize the signal strength from an external antenna will be higher
> than an internal antenna, but does that make much (any?) difference to
> the operation of the GPSDO?
> FUNNY, SAD but TRUE story.
> After I got hit by lightening for the second time down my telephone
> line, I decided I needed to do something about it. So I got onto my
> service provider (BT) and asked them what could be done, as my
> telephone is fed via an overhead line. After arguing with them for
> months, they agree to fit some lightening protection to my telephone
> line. The day they came to fit this was a lesson in how incompetent
> some technicians, and their managers can be. Of course BT call them
> engineers, but this guy is not an engineering in my mind.
> BT TECHNICIAN: I need to run an earth wire
> ME: That is ok, so I assume you are going to put an earth rod into the
> ground.
> BT TECHNICIAN: No, I wont use an earth rod.
> ME: So how are you going to earth it? What sort of wire is it?
> BT TECHNICIAN: My manager said to move some earth away with my
> fingers, poke the wire into the ground, then move the earth back with
> my hand. The wire is 1 mm^2.
> ME: That is no good. Let me speak to your manager.
> The BT technician then rings his manager, and puts him on the phone. I
> explain that is not acceptable.
> MANAGER: So how do you suggest we earth it?
> ME: I don't know how to do it. This is not my area of expertise, but I
> know that what you are proposing, with 1 mm wire and poking the wire
> into the ground with your fingers is not acceptable.
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3
> 6DT, UK.
> Registered in England and Wales, company number 08914892.
> http://www.kirkbymicrowave.co.uk/
> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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Chris Albertson
Redondo Beach, California

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