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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