Hi Edgar,
Of course you were right.
I thought i don't need a completely shielded box because i am in the
garden and there is no electricity.
But, the gate of the FET is quasi-floating and there is always some
stray-C between the antenna wire and that gate.
Even 0.1 pF coupling C and 100 MOhm means a HPF cut off near 16 kHz !!
I run a test, disconnecting the antenna from the preamp and placed the
end of the wire in 0.2m distance. The SNR of DHO38 on 23.4 kHz was 30
dB, despite connected circuitry (diodes, filter, stray-C to ground).
Now i will again move the circuit, into a shielded box :-)
More results tomorrow.
Nice stuff to play with! Still some warm autumn days here...
73, Stefan/DK7FC
Am 06.10.2014 00:51, schrieb edgar:
Hi Stefan,
I suggest that you cover the LPF and FET with a piece of tin to stop
any external signals entering the circuit after the LPF.
A thin piece of metal from coffee tin or similar food container can be
bent in a U shape to cover the components and then soldered at a few
points to the copper ground plane.
Just leave a small clearance holes at the ends for the wires to pass
through.
Regards, Edgar
Moonah, Tasmania.
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