Hi Alan.
Phantom circuits are also very useful for signalling, if you use
transformers at each end with two balanced windings connected via a
capacitor you have many combinations of switching and signalling, which we
used in the old Post Office days. You can also graduate to the relms of
double phantoms for music circiuts etc.
73s
Ken
M0KHW
----- Original Message -----
From: "ALAN MELIA" <[email protected]>
To: <[email protected]>
Sent: Wednesday, December 16, 2009 8:37 PM
Subject: Re: LF: Noise cancelling by using optic transmission of RX signals
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
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
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