Re: LF: Re: MiniWhip antenna, fiber optic

[M0FMT <m0fmt@yahoo.co.uk>]

Hi Stefan and Joe.

Just to say your publishing the results of the tests and ideas is very interesting to me and I guess to just about the whole MF/LF/VLF community.

 

Thinking out side the "Box" is important and you are certainly doing that. Please keep us informed.

73 es GL Pete M0FMT IO91UX

 

From: Stefan Schäfer <Stefan.Schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
Sent: Monday, 8 July 2013, 2:31
Subject: Re: LF: Re: MiniWhip antenna, fiber optic

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