Re: [amsat-bb] Fiber Optics at RF

["Greg D." <ko6th_greg@xxxxxxxxxxx>]

Hi Galen,

Good work using the wave guide for RFI control.  For a normal
computer room (FCC Class-A) environment, the RF levels aren't
nearly as restrictive as you must have.  I was part of the R&D
department that did the Input/Output subsystem for a new server.
So we were the ones hanging the antennas (cables) on the machine,
and had to make them NOT radiate.  This, on a system with nice
strong digital clock lines running every which way, spraying
harmonics all over the place.  Somehow we had to do our job
without making the whole system fai the regulatory certification.

Seeing a fiber cable cause an RFI problem caused quite a stir, as
the results surprised everyone.  We repeated the test to confirm
the data, and contacted the various vendors.  Sure enough, one
of them had a jacket that was conductive enough to act like a
weak antenna.  This was about ten years ago, and I forget which
one it was, but it was one of the name brands.  Black jacket, I think.
The construction of the fiber's terminating connector was part of
the puzzle too, acting as a pick-up.  I have no idea if they still make
them like this, but back then it was a definite problem for that one
supplier.

73s,

Greg  KO6TH


>From: Galen Watts <gwatts@nrao.edu>
>To: amsat-bb@amsat.org
>Subject: Re: [amsat-bb] Fiber Optics at RF Date: Wed, 23 Oct 2002 08:29:03 
>-0400
>
>
>>Date: Tue, 22 Oct 2002 00:05:47 -0700
>>From: "Greg D." <ko6th_greg@hotmail.com>
>>Subject: RE: [amsat-bb] Fiber Optics at RF
>
>
>>Fiber optics ... hard to shielded well because
>>you need a hole to stick the cable into.  We're talking signals in the
>>microwave band here, typically.  Also, check out the fiber cable itself.
>>Some have a metal runner that goes along side the glass for protection,
>>and others have a high graphite content in the jacket.  Both can be
>>problems with Rf fields.
>
>I work at a radio observatory with a fiber computer network.  Imagine 
>extending the 'hole to stick the cable into' out about a foot so it's a 
>tube and you have what we do to shield our network electronics' fiber 
>connections.  It's a waveguide above cutoff, the smaller the diameter the 
>higher the cutoff. We typically get 60-90 dB of noise attenuation out to 
>15+ GHz, and if one stuffs some copper wool in the end it'll cut off out to 
>100+ GHz.  We're trying to keep the RF in as opposed to out but it works 
>both ways and according to some of the most sensitive radio reception 
>equipment on the planet it works quite well.  As for metal runners and 
>graphite jackets we have both in many places and it makes no difference.
>
>The network switch shielded enclosures look like little pipe organs with 
>orange and yellow vines growing out of 'em.
>
>73!
>Galen, W8LNA
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