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

Jim Lux jimlux at earthlink.net
Wed Nov 26 17:07:22 EST 2014

On 11/26/14, 1:37 PM, Dr. David Kirkby (Kirkby Microwave Ltd) 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.

Here's what we do at JPL for spaceflight equipment: reradiators.

Antenna with preamp on the roof... long coax to wherever the signal is 
needed, amp(maybe), DC bias T (minicircuits has them, as do others, or 
build one), and a variable attenuator (in case the system has too much 
gain), feeding a passive antenna.  Another passive antenna (or your GPS 
receiver or your whatever) is a meter or so away.

The inside antennas can be pretty crude.  I suspect stripping back 1/4 
wavelength of coax shield would work.   We use ones that resemble a 
hockey puck and that have the required bandwidth (L1,L2, L5).

You could, if you like, build some sort of shielding box with absorber 
around the two antennas which would cut down on multipath reflections, 
etc.   But that box would require some design so that it doesn't become 
the path for the lightning to your gear.

The entire path from roof antenna to reradiator is sacrificial.

> 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.

Sure, there's all sorts of RF over fiber stuff available. Some is even 
designed for GPS signals specifically.

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