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Stefan
Suggestion ... in the interest of better dipole
balance mount battery so that it isn't taped to one half of the dipole.
Suspect it's quite easy to upset the balance on such a short
antenna.
Jay W1VD WD2XNS
WE2XGR/2
----- Original Message -----
Sent: Monday, July 08, 2013 9:51 PM
Subject: Re: LF: Re: MiniWhip antenna,
fiber optic
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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