Message-ID: <4B649F35.6060801@ukonline.co.uk>
Date: Sat, 30 Jan 2010 21:05:57 +0000
From: Peter Dodd <g3ldo@ukonline.co.uk>
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Subject: Re: LF: Noise cancelling by using optic transmission of RX signals
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Is this what you wanted Stefan?

Peter, G3LDO




On 16/12/2009 16:51, Stefan Sch=E4fer 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 wi=
th the short E-flied RX antenna.
>
> I found out that the noise was/is generated by my notebook which made RX=
ing difficult, even in regions without any else local QRM (what could almo=
st 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 plu=
s 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 signif=
icant 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 amplitud=
e to zero! That meant for me, the VFO and its supply is not suppressed eno=
ugh 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 unw=
anted) 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 fe=
rrite core and many turns of RG174. As I meant, surely there is a signific=
ant 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 opti=
c 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/9=
0/486530_DS.pdf>   as the RX and abt 20m of a fiber optic cable (yes, ther=
e 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 ther=
efore easy to get...
>
> Now, when placing the preamp on the same place then during the tests bef=
ore 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. Th=
e overall noise level decreased to its absolute minimum during all the tes=
ts.
>
> Before I used this optic cable the only signal that could be received (o=
ut 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 metho=
ds 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 dis=
tance 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 Lea=
d Acid battery. In the future I plan to use a stereo optic cable. Then, on=
e could switch the preamp on and off by the seconds line doing it the oppo=
site way. So, smaller batteries could be used, which is necessary when mou=
nting 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 tri=
ed 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 pu=
blication about that. My goal is not to be the hero in inventions, to beco=
me 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 shar=
e 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 help=
ed all of us!
>
>
>
>
>
> 73s es 55 de Stefan / DK7FC
>
>
>   =20