Hi Jay,
Thanks for the suggestion.
Well, i also had some thoughts on the balance issue. The battery is
mounted to the dipole half that is connected to source of the FET. The
battery itselfe should have this potential as well. So it's just the
geometry that slightly deforms the symmetry, maybe.
I thought about using a tube with a higher diameter so i can mount the
batteries inside the dipole half connected to source. This
would then allow a maximum of symmetry. Last night i didn't find a
suitable tube. And i thought that the construction does not need a
perfect symmetry for first tests as it is completely floating.
At least i can do some measurements of 630m signals in the far field
and in daytime and in an open field. I can test different heights above
ground, horizontally and vertically. In each position i can use a
separate ground (3 short eath rods and a wire) and connect this to one
of the halfs, just to show the difference in the levels.
Later it will be interesting to do the test on the top of the
institute, i.e. with a noisy ground! Will the S/N suffer from the
ground connection? I assume yes! And that would mean that this concept
is worth to build and use daily!
To my surprise it appears that the dynamic range and noise performance
is much better than i thought. But i'm still not sure if i will see the
(daytime) natural background noise with this construction.
It's quite warm outside now and i will see if i can do some first
experiments today as long as there is no skywave...
73, Stefan/DK7FC
Am 09.07.2013 13:17, schrieb [email protected]:
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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