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

Dr. David Kirkby (Kirkby Microwave Ltd) drkirkby at kirkbymicrowave.co.uk
Wed Nov 26 16:37:03 EST 2014

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.
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)

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