Hi Stephan, well if you are running an optic cable why not run power as well. 
It should be relatively easy to keep the pick out of the power rail. If you are 
worreied about pick up on the power rails use a rechargable battery and just 
use the power feed when you are not receiving for recharging the battery.

Coax is not always the answer, and the old fashioned method of balanced pairs 
is often more effective, If you use a centre-tapped transformer at each end you 
can even feed the power down the "phanton circuit". My old affiliation to a 
telephone company is showing :-))

Alan G3NYK

--- On Wed, 16/12/09, Stefan Schäfer <[email protected]> wrote:

> From: Stefan Schäfer <[email protected]>
> Subject: LF: Noise cancelling by using optic transmission of RX signals
> To: [email protected]
> Date: Wednesday, 16 December, 2009, 16:51
> 
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> 
> Dear Lowfers,
> 
> 
> I
> want to
> start a new subject (for me it's
> new)...
> 
> The
> last weeks
> I still had RX problems, either with the tx antenna or with
> the short E-flied RX
> antenna.
> 
> I
> found out
> that the noise was/is generated by my notebook which made
> RXing difficult, even
> in regions without any else local QRM (what could almost
> bring me to shut down
> the PC and just do nice and easy CW QSOs without anything
> else! (but most
> traffic is in QRSS...)).
> 
> Then
> I have
> done mni tests with many many ferrite cores (that I found
> in an older emc lab
> in our institute), e.g. 20x FT50-77 plus 20x FT50-43 plus
> 20x FT50-61 and e few
> others. I placed them all in series, partly in the near of
> the preamp and/or in
> the near of the RX. Sure, there was a significant reduction
> of noise but lastly
> the noise remains. 
> 
> Accidentally,
> during a test with my VFO connected to the same 12V supply
> of my preamp, I observed
> a strong line in the argo monitor EVEN without an antenna
> (short wire)
> connected to the VFO and when turned the amplitude to zero!
> That meant for me, the
> VFO and its supply is not suppressed enough and so some RF
> comes to the preamp
> by the coax and supply line! (the preamp is supplied by a
> battery). I thought, I
> can use this (normally unwanted) coupling path to test the
> blocking capability
> of the ferrite cores and so I tried it with all variations
> and later with an
> additional BIG ferrite core and many turns of RG174. As I
> meant, surely there
> is a significant noise reduction but the line in argo was
> still well visible!
> 
> What
> I have
> done then (since 4 days) is using a coupling link by an
> optic transmission of
> the 137kHz signal between the preamp and my RX using a SFH750V
> as the TX Diode and a SFH350
> as the RX and abt 20m of a fiber optic cable (yes, there
> are surely more suited
> components but these were available in the moment). These
> are also used as
> optic links into high end audio systems and therefore easy
> to get...
> 
> Now,
> when
> placing the preamp on the same place then during the tests
> before and just
> changing the coax by the fiber optic cable, the line by the
> VFO was completely
> gone!! Just with an antenna connected to the VFO and high
> amplitude (and thus e
> real near-field “connection”) it came back. The
> overall noise level
> decreased to its absolute minimum during all the tests.
> 
> 
> Before
> I used
> this optic cable the only signal that could be received
> (out of the city with
> much much qrm) was DCF39 and DLF and so on. Now, DCF39 came
> out with much
> better SNR and I saw the first LF Stn at all, it was Ossi /
> OE5ODL transmitting
> his 5s in the evening (tnx! very nice SNR in QRSS3
> mode).
> 
> My
> conclusion
> is, that the qrm is always brought to the very sensitive
> E-field antenna by the
> coax, even when doing many usual suppression methods cause
> there always remains
> a residual coupling impedance between both sides of the
> cable. Furthermore a
> long coax cable, say 20m away from the shack carries some
> qrm to the near of
> the rx antenna so the effective distance to the shack is
> always reduced.
> 
> One
> small disadvantage
> of this method is the need for a battery supply. My TX
> diode needs abt 20mA and
> is now working since 3 days with a 7AH Lead Acid battery.
> In the future I plan
> to use a stereo optic cable. Then, one could switch the
> preamp on and off by
> the seconds line doing it the opposite way. So, smaller
> batteries could be
> used, which is necessary when mounting the preamp to a
> shaky fiber mast if one
> does not want to change the batteries each few hours
> ;-)
> 
> Signal
> distortion seems to be no problem, as I can confirm until
> now.
> 
> Now,
> my K2 has
> an optical LF input! ;-)
> 
> Perhaps
> this
> will give some ideas to the local qrm bothered stns who
> tried everything with
> ferrite cores, isolating transformers and so on without a
> satisfying result.
> 
> NOTE: I do not know if anyone has tried
> this before and has
> written a publication about that. My goal is not
> to be the hero in inventions, to become popular and
> especially not to compete with
> anyone (like it seems
> to be usual in career/job to be the one who gets the pay
> rise) but just to
> share my ideas to those who are interested to try something
> new (?). I do not
> say that my solution is the best at all and so on! But, if
> it will help only
> one Lowfer getting new ideas and improving his station, and
> if this improvement
> results in more activity on the band, then it would have
> helped all of us!
> 
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> 
>  
> 
> 73s
> es 55 de
> Stefan / DK7FC
> 
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>