Hi all,
Today i just made some changes on the antenna. Now the dipole legs are
aluminium foil on a plastic tube. Length is 30 cm (29cm on the side
where the battery is connected, GND of the circuit) and diameter is
18mm. The forward current of the TX LED is about 15 mA. Some working
point optimisations were done so the gain is a bit higher (not only due
to the increased electrode capacity). I know, the signal voltage does
NOT rise linearly with the capacity of the electrodes (thinking about
the voltage between 2 parallel plates of a capacitor which is put in an
external electrical field) but it allows to realise a RC lowpassfilter
in a next step.
New antenna:
https://dl.dropboxusercontent.com/u/19882028/LF/20130709_033824.jpg
I can now do some /p experiments in the far field (for my local 630m
transmissions) in a location without trees, houses....
73, Stefan/DK7FC
Am 08.07.2013 03:31, schrieb Stefan Schäfer:
Hi all,
Well, somehow i didn't want to stop, so i now built the RX too:
https://dl.dropboxusercontent.com/u/19882028/LF/DK7FC_fiber%20optic%20active%20dipole_RX.jpg
I made some changes in the RX schematic, using a J310 instead of the
BF862 but the principle schematic is the same. The working points look
as expected. BTW that electrical/optic/electrical converter must be
quite wide in the transferred frequency range, i guess starting from a
few 100 Hz to a few 100 kHz.
When connecting the BNC output to the scope, the noise shown from the
scope does not change. When switching on the active probe there is some
noise visible in the range of 2 ms/div. The probe stands in the lab
where many noise sources can be expected.
And suddenly!!! : It was 1:06 UTC. I am transmitting WSPR-2 each 6
minutes on 630m, from the same building, in some 10m distance. Suddenly
there
was a 472 kHz signal on the scope, although the antenna can't
work :-) The short dipole is completely floating. See another photo
where the floating antenna is placed on a (isolating and low coupling
capacity to sourroundings) cardboard:
https://dl.dropboxusercontent.com/u/19882028/LF/Signal%21.jpg
When switching off the active antenna, the signal disappears completely
on the scope, of course!
When coming closer with my hand to one of the legs of the dipole or the
battery, the signal rises, as expected. I think we can agree that this
antenna IS floating!!
So far for now. It's 3:26 AM and i think i should take some sleep :-)
Looking forward to the tomorrows experiments. Let me know if you have
suggestions for experiments!
73, GN, Stefan/DK7FC
Am 08.07.2013 02:11, schrieb Stefan Schäfer:
3rd: https://dl.dropboxusercontent.com/u/19882028/LF/DK7FC_fiber%20optic%20active%20dipole_TX3.jpg
The optic cable is 9m long.
Am 08.07.2013 02:04, schrieb Stefan Schäfer:
another image of the TX: https://dl.dropboxusercontent.com/u/19882028/LF/DK7FC_fiber%20optic%20active%20dipole_TX2.jpg
Am 08.07.2013 01:59, schrieb Stefan Schäfer:
Hi all,
OK, although it can't work (do you remember?!) i built an active
antenna with battery supply and fiber optic output. The active part is
a BF862.
The
circuit
is
quite
simple: https://dl.dropboxusercontent.com/u/19882028/LF/DK7FC_fiber%20optic%20active%20dipole%20schematic.jpg
It is a first idea for the schematic. Maybe i will change it later so
that the gate is shifted to 9V/2. Just for first experiments. In
various tests with the BF862 i found that it was necessary to add a
ferrite bead directly in front of the gate to prevent VHF oscillations
of the JFET.
The TX is already working: https://dl.dropboxusercontent.com/u/19882028/LF/DK7FC_fiber%20optic%20active%20dipole_TX.jpg
Yes yes yes, it is not truely symmetric and the capacity at source must
be a bit larger than that of the leg connected to gate. Anyway it will
be useful for some basic and interesting experiments.
RX is ready tomorrow.
73, Stefan/DK7FC
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