Hi VLF,
This evening i received ZEVS for the first time in the garden :-)
I used nearly the same circuit than in my email below. However, i found
that the J310 was damaged! Remembers me on the old discussion if
protecting diodes are necessary or not :-) But this is a large antenna,
compared to an active E probe like the PA0RDT-antenna...
Now i've replaced the JFET and added two 4148 protecting diodes on the
gate against ground and plus.
Reception looks much better now:
0...24kHz:
https://dl.dropboxusercontent.com/u/19882028/VLF/wide_500msperdiv_732mHzFFT_garden3rd.png
0...4 kHz:
https://dl.dropboxusercontent.com/u/19882028/VLF/0...4kHz_500msperdiv_732mHzFFT_garden3rd.png
0...500 Hz:
https://dl.dropboxusercontent.com/u/19882028/VLF/0...500Hz_500msperdiv_732mHzFFT_garden3rd.png
ZEVS in 7.6 mHz, 40 sec scroll rate:
https://dl.dropboxusercontent.com/u/19882028/VLF/ZEVS_1st.png
It looks like i need some more gain on the lower frequencies. Will
modify the circuit a bit...
73, Stefan/DK7FC
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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