Oh now, i can't belive it. Must tell you that:
All the years i suffered from QRM which seemed to come from DLF at 153
kHz. Now as i installed the new wideband LF/MF probe i got even more
QRM. The design is not so far from the PA0RDT design. After increasing
the antenna by another meter on the top of the roof i got worse signals
than before.
Yesterday i found that it must be a resonance on the cable since i
could reduce the effect dramatically by adding a 1 nF C and a 1 kOhm R
on the RX end.
But i still saw IM. This was the problem with all my previous antennas.
The signals levels are close to the noise floor but when i increased
the probe C it became worse....
Now i checked this oscillation on an oscilloscope. By adding some C or
R i could reduce the amplitude of the signal but never the frequency,
which is strange. Then i noticed that it is AM !!!!! So it must be a
transmitter very close to me since the signal levels were 4V pp here,
and this started to cause noticable IM on LF and MF.
I checked the web with some keywords like "radio" and "heidelberg" and
quickly found this
It is AFN in Heidelberg on 1143 kHz. This is a AM
transmitter running 1 kW in about 1 km distance!
http://www.asamnet.de/~bienerhj/1143-HD.html#1143-HD
It is a 100m tower. I never noticed it since i live here, odd.
Here one can read that AFN Heidelberg will be QRT on MF very soon:
http://www.afneurope.net/Home/ArticleDisplayDD/tabid/649/Default.aspx?aid=25644
So now, that may explain why my RX site often suffered from QRM. And
that explains why a resonated circuit in front of the first active
device in my E field antennas were always the best solution for me!
I don't want to wait until they go QRT but i'm looking forward to see
the difference in the noise background when they go off. For now i need
some kind of filter and i have to take care that the input of the first
JFET does not become overloaded.
Will go on investigating!...
73, Stefan/DK7FC
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