Hi Graham,
Am 07.10.2012 12:52, schrieb Graham:
Series tuned L/C cct 1.1 Mhz to shunt the probe ae ?
Yes, that's what i want to do today. It's a very nice sunny day and i'm
looking forward to climb on the roof now again :-)
I thought about a series LC across the drain-source of the first JFET,
to force it not to saturate on the output.
.but is the probe saturated ?
Yes, that's the question. Maybe the saturation happens in the 2nd stage
only. One idea may be to rise the supply voltage from now 11V to 14V or
so (the input JFET is a BF862 which makes a high gain!) which may help
the first stage not to saturate, if this is the case at all.
If IM is generated in the 2nd stage (currently a 2N3053 (is that a good
idea??? ), a simple LC LPF would be OK. I'm not interested in
frequencies above 600 kHz.
... but with high signal levels any rusty bolt in
any metal structure is a noise source
GRRR, once again ;-) But soon they are QRT...
Tnx and 73, Stefan
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
|
|
