Hello Jim and all others,
Tnx for your ideas!
I could answer in a long mail, as usual ;-) but today i try to keep it
shorter ;-)
I have done some modifications to the active antenna. The main change is
a source resistor of 100 Ohm, of course without the parallel C. This
reduces the voltage gain of the first stage but anyway. It also improves
the linearity. Additionally i meant that the audio input level was to
high for the soundcard (the software audio level control was just a
little above zero!). Thus i reduced the circuit from 3 amp stages to 2
stages (2x BF862, the 2nd is the LED driver as well). Now the volume
control is set in the middle, fine. But the overall level is abt 20dB
less and so one can see spikes in the ragnge of 5 to 15 kHz that comes
from the PC ( i supply the optical/electrical back converter out of the
12V of the PC). By adding a common mode choke i reduced those spikes
arround 15 dB but they are still visible, anyway. Mostly i want to get a
clear 8,96...8,98 window, anything else is not really interesting for me.
The IM is still available but its influence on the desired frequency
decreased strongly. There is such a strong decrease of QRN on the 600
window, i almost cannot belive it. I am still not sure about that...
This happend from 18 to 20 UTC (still visible on the 2nd grabber). First
i thought the battery gets empty but later some VLF transmitters came up
again so i hope this is really the noise decrease that occurrs each
evening. If this is true, the new arrangement could be much more
sensitive, i hope so.
BTW, isolating the ground from the lightning arrester changed nothing (i
assume still abt 20 pF coupling capacity).
To see if something is wrong with the circuit or if the QRN in the 600
window is still caused by IM (this would be wondering because the noise
appears very homogen now, not like yesterday) i do transmit now a signal
out of my VFO that is connected to a 10m long wire that is in the near
of the main (1st) grabber in the 2nd floor, so the distance is abt 25m
while the antenna voltage is 6,7 Vp. I am interested how the 600
window(s) will look tomorrow in the morning :-) Just for fun i do change
the QRG as if i would transmit DFCW-600 but without switching out the
signal during the letters (or e.g. between 2 dots), just to see a
difference to lines that could be caused by IM (like yesterday). The
color scale is not optimal in the moment but first i want to see if the
QRN comes back in the morning. All pics are staved separately...
73!!
Stefan/DK7FC
Am 21.05.2010 03:10, schrieb James Moritz:
Dear Stefan, LF Group,
----- Original Message ----- From: "Stefan Schäfer"
<Stefan.Schaefer@iup.uni-heidelberg.de>
...>
In Spectrum Lab there are different methods for noise reduction (e.g. of
lightnings). Those were discussed here in the last weeks. While
Jim/M0BMU preferes the clipper (using 0 dB above AVG if i remember
correctly), Markus/DF6NM prefers the noise blanker.
...>
I don't really have a particular preference; both seem to give a
similar improvement in apparent SNR, and both will introduce some
intermodulation, since they are inherently non-linear techniques. Your
spectrograms seem to confirm there is not a great difference... BTW,
the Spectrum Lab clipper, where the clipping threshold tracks the
average signal level, does seem to give better results than simple
fixed-threshold clipping, especially when other signals or narrow-band
noise are present.
...>
I still have to reduce the IM in the first stage (2nd order low pass
filter at the output of the first stage does not better the situation),
either by reducing the wire length or adding a series resistor or...hm,
will see (any ideas?).
...>
At my QTH, there are 4 MF broadcasters within 1km, giving combined
peak FS of 10s of volts per metre, so I get these problems a lot... I
find it is important to keep an open mind about possible sources of
this kind of QRM.
The first thing is - are you sure the intermodulation actually occurs
in your active whip? Any length of wire acts as an antenna, and will
often be connected to some non-linear device (e.g. mains cable
connected to rectifier diodes), so there are an almost infinite number
of places outside the antenna circuit that the 9kHz IM product could
be generated.
A prime candidate is the earth connection - if your antenna is high up
on a building, the earth connection will be carrying considerable
broadcast-frequency current - I have found this often results in
intermodulation, perhaps due to some kind of electrolytic effects
occuring at the actual ground electrodes. This effect makes my TX
antenna unusable for reception at many frequencies. Any mains power
supply connected to the antenna preamp can cause a similar effect if
the internal diodes, regulators, etc. are not well decoupled at
broadcast frequencies.
Another place where intermods can occur is the PC, which will
typically have long cables connected to it (mains, network, sound card
etc...) again acting as antennas and carrying MF broadcast currents.
Any intermod generated in something connected to the PC can be coupled
into the sound card inputs by the ground loops that tend to exist
inside the machine. For example, I found using my 9kHz antenna/preamp
at home with a laptop with AC adaptor gave a similar 9kHz line to the
one you see. I eventually found this was still there with the preamp
output disconnected from the sound card, but with the earth connection
present between the computer and the preamp. I assume the cables
connected to the preamp were acting as an antenna, and flowing to
ground through the PC and adaptor. The cure was to wind several turns
of the AC adaptor lead through an EMC ferrite core to reduce the MF
current flowing in it. For the DK7FC grabber, you could easily test if
the 9kHz comes from the antenna or PC end of the system by un-plugging
the optical fibre.
Then there are usually many other cables, or large pieces of metal,
near the antenna, via which intermods can be occuring. The resulting
9kHz voltages/currents will be picked up by your antenna. Moving the
antenna is the easiest way to check this, or also to solve the
problem, usually.
But then, if the FS is high, it may be that the active antenna is the
problem after all. In your circuit, it is unlikely that IM is occuring
after the input stage due to the presence of the bandpass filter. You
could try adding a source degeneration resistor to reduce the gain and
improve the linearity of the input stage (say 100R, but you would also
probably need to change the gate bias voltage to get a sensible bias
current). I would suggest putting about 47R resistor in series with
the gate, to reduce the likelihood of VHF oscillation occuring, which
can cause strange problems, including intermodulation at low
frequencies. Probing the drain with an oscilloscope would reveal what
large signals are present. Also, the drain resistor in the second
stage needs looking at - 1k is rather too high a value for a 12V
supply and a FET with zero gate bias and Idss of 10 - 25mA.
BTW, problems with high broadcast station field strength were some of
the reasons leading me to favour a loop antenna design, for 9kHz and
other frequencies as well. It is much easier to include filtering
directly at the antenna terminals, since the loop is much lower
impedance. Also, ground connections are less of an issue.
Cheers, Jim Moritz
73 de M0BMU
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