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