Hello Peter and thank you.
 
This was my first mail to that subject ;-) But we have found it now, as you can 
see.
Tnx for the efforts!
 
Stefan

________________________________

Von: [email protected] im Auftrag von Peter Dodd
Gesendet: Sa 30.01.2010 22:05
An: [email protected]
Betreff: Re: LF: Noise cancelling by using optic transmission of RX signals



Is this what you wanted Stefan?

Peter, G3LDO




On 16/12/2009 16:51, Stefan Schäfer wrote:
> 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<http://www.datasheetcatalog.org/datasheets/105/486564_DS.pdf>   as 
> the TX Diode and a 
> SFH350<http://www.datasheetcatalog.org/datasheets/90/486530_DS.pdf>   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!
>
>
>
>
>
> 73s es 55 de Stefan / DK7FC
>
>
>   

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