Hi VLF,
I managed to resonate my antenna on 5170 Hz. It was necessary to
increase the inductance from 1.2 H to 2 H. This was done by increasing
the number of iron powder cores (T106-52) from 100 to 176! It is the
coil out of 10 stacked yoghurt cups that i got from Markus (many thanks
again!). An image from the stacked cores into the coil:
https://dl.dropboxusercontent.com/u/19882028/VLF/20161229_194850.jpg
Although the number of turns of this coil is constant and the stacking
height is even higher (means a lower coupling of the coils and so they
must have a lower L), the inductance is higher now. So this increase
must come from the cores. I clearly saw that the Q is significantly
higher now, so the cores are a good choice for any VLF band. Also i
need less power now. It also means that the proximity effect must be
lower now (due to the lower coupling), because the Q is higher and the
antenna current and the R(DC) of the coils is the same as on 6470 Hz.
Now i'm running 230 mA since 14:40 UTC. I intend to run it until 8 UTC,
then a first EbNaut message follows.
The antenna voltage increased from 11.3 kV to 14.2 kV @ 230 mA. So far
the antenna current is not really stable but it becomes foggy outside
and i did not yet add the transductor to the system. Maybe i'm trying
to build an ALC into SpecLab. This should work well now with the new
interpreter command for the bypass factor of the delay line :-)
Ah there is even a short video (6 sec, 1.2 MB) where you can see and
hear (!) the coil and its internal fan on the roof while running at 14
kV:
http://www.iup.uni-heidelberg.de/schaefer_vlf/VLF/VLF_coil_at_230mA_14kV_5170Hz.mp4
(hopefully the link works).
Lets be optimistic, i'm announcing the first EbNaut message for
tomorrow:
f = 5170.000000 Hz
Start time: 31.12.2016 09:00:00 UTC
Symbol length: 60 s
Characters: 2
CRC 16
Coding 8K19A
Duration: 6h, 08m, 0s
Antenna current: ~ 230 mA
73, Stefan
Am 27.12.2016 12:36, schrieb DK7FC:
Hi VLF,
It's time to come back to 5.17 kHz where i have been in the last kite
experiments 2010/2011. The best result was about 10 dB in 3.8 mHz in
904 km distance, detected by SQ5BPF. His website about this experiment
is still on the web,
http://www.lipkowski.org/~sq5bpf/dk7fc_20101023/index_en.html
In these kite tests my ERP was about 3 mW on 5170 Hz. But this was with
a 300m vertical antenna!
With my fixed antenna i can never radiate such strong signals. The
limiting factor is the coil which can handle 250 mA on that frequency.
The radiation resistance is about 210 uOhm, so the ERP will be up to 23
uW. So this is 20 dB weaker (oh, actually i expected it is even more
weaker). But on the fixed antenna it is no problem to transmit 100x
longer!!
We will see what is possible on that band with such a low power. Who
wants to make a guess? :-)
But i'm not yet sure if i can resonate the antenna down there at all.
The plan is to use the small coil that was used for 6470 Hz. Inside the
coil there are 100 iron powder cores, shown here
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_155753.jpg
and here
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_163021.jpg I
want to replace (a few) of them by ferrite cores, many ferrite cores
"switched in parallel". I need to get the inductance down from 1.2 H to
2.0 H. So first the ferrite cores much provide a higher effective µr
(ur) which is no problem i guess. Secondly the must not saturate (to
much), which is the critical question here. We will see what happens...
It's the right time in the year for such a work and i think i can do a
first transmission on 58 km during this week...
73, Stefan
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