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Series tuned L/C cct 1.1
Mhz to shunt the probe ae ? .but is
the probe saturated ? ... but with
high signal levels any rusty
bolt in any metal structure is a noise
source
G..
Sent: Sunday, October 07, 2012 12:57 AM
Subject: LF: GRRRRR!! Now i know...
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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