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!
73s es 55
de Stefan / DK7FC
