Hi Markus, Ha-Jo,
Thanks for your schematic. I know it from your recent post. I thought
it could be an adavantage to have some LPF in front of the first stage
due to DLF/153 kHz and so on, just to prevent overloading the
soundcard...
In my recording, neither the PC soundcard nor SpecLab run an AGC or NB.
The spectrogram is taken from L1 and i recorded the sound input. BTW
the recording was taken in the garden, so i expect lower QRM in the E
field at least.
I think it could help to reduce the input resistor on the input of the
antenna. From previous experiments with compact active antennas i
remember that a plastic box arround the metallic electrode
significantly helped to reduce static QRM. Maybe this is the same
effect here.
I'll continue to play :-) Another project is a high voltage trap
against wild pigs in my garden "Wildschweinschreck" ;-) I already
carried some HV caps there (5 kV / 90 uF > 1 kJ) in a first step :-)
73, Stefan
Am 28.09.2014 17:56, schrieb Markus Vester:
Hi Stefan,
here is the simple source follower
circuit which picked up Zevs and Schumann resonances: http://df6nm.bplaced.net/VLF/ELF/source_follower.jpg
. The browsable directory http://df6nm.bplaced.net/VLF/ELF/
also contains some spectrograms, an usr configuration, and a couple of
wav files which were sampled at 250 samples/sec. In a postprocessing
step, the strong 16.7 Hz harmonics have been notched out, and the wav
file header has been modified for replay at 60x speed. What then sounds
like statics are actually "Q-bursts", super strong lightning events
which resonate globally in the earth / ionosphere cavity.
I also wonder why your low frequency
range looks so noisy. I suspect that you might still have the noise
blanker activated, which would be sheer poison! It will modulate itself
onto the usually strong 50 Hz line and also onto a possible DC
offset, generating sidebands that will splatter all over the ELF
spectrum. My recipe was trying to keep everything linear, with the
local 16.7 Hz line only a few dB below fullscale, and Zevs only about -
91 dB.
Best 73,
Markus (DF6NM)
Sent: Sunday, September 28, 2014 4:28 PM
Subject: LF: ZEVS RX ant question
HI VLF,
Yesterday i tried again to receive ZEVS. I didn't know if it was on air
but gave it a try. Recorded about 2 hours of the VLF spectrum received
on my T antenna (i.e. E-field). I also used a source follower (J310)
but added a 3 stage RC filter in front of the gate. I thought i can
afford some pF against ground now with the relatively large antenna
which must have about 150 pF. I used this circuit: https://dl.dropboxusercontent.com/u/19882028/VLF/ZEVS%20test%20ant.jpg
But somehow the spectrum looks not as expected. While the MSKs are
still strong, there is much noise below 2 kHz and the sferics appear to
be quite weak. See: https://dl.dropboxusercontent.com/u/19882028/VLF/Garden%20T%20antenna%20VLF%201.png
I remember from tests with a small active antenna for VLF, where i used
100 MOhm resistors on the gate, that charged humidity caused
much noise.
Could this be the same effect here?
Should i try to use a lower input impedance of the preamp?
Maybe you remember, there were certain transistors, several decades
ago, they were called bipolar transistors, NPN and PNP. Maybe i should
try one of them? :-)
Advice is apprechiated. Where are the experts?
73, Stefan/DK7FC
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