Dear Stefan,
I guess You should add a
high-pass
RC-filter behind your present low-pass filter (so that the antenna
itself
is still dc grounded) to suppress the
lower frequencies you
do not want. This high-pass filter may use the two resistors you need
for
biasing the FET anyway.
HW?
73 Ha-Jo,
DJ1ZB
"DK7FC"
<[email protected]>
schrieb:
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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