Re: VLF: Back on 5.17 kHz / 58 km...

[DK7FC <selberdenken@posteo.de>]

Hi Markus, Paul, VLF,

Markus that looks great! Also the good SNR report from Paul made me most excited. It was uncertain which propagation has to be expected on 58km but from the first impressions it looks like the band is most promising! I'm interested in the diurnal propagation and phase changes on the path to you and Paul. At first it is good to see that someone except Paul can receive the signal at all. So maybe this rises a challenge for a few more people who often thought about doing something on VLF.

Well, the system is in a beta phase :-) It is the first transmission. And it already looks like improvements are necessary: The antenna current dropped during the last evening/night. See the plot on http://www.iup.uni-heidelberg.de/schaefer_vlf/VLF/TX.png
It dropped from 230 mA to 170 mA, i.e. a decrease of up to 2.6 dB. That makes your results even more remarkable.

I think the reason could be that the vertical arrangement of yoghurt cups is now a bit gliding into each other, which would move the resonance frequency downwards. The very good Q improvement from the iron powder cores brings me to the idea to use all 200 available of them. As you can see in the video and image, the lower 4 cups have 3x8 cores inbetween. The upper 5 core stacks are just 2x8 cores. I will use the remaining 24 cores, so i can build 7x 8x3 and 2x 8x2 = 200 :-) This will even rise the height of the stack and thus reduce the coupling and thus reduce the proximity effect, i.e. the losses. So it will rise the Q even more. And it will rise the surface so heat will be even more reduced and the HV withstand voltage will rise! :-) Then the inductance may become to high but then i can remove a few cores on the upper stack (9th floor) and put them on the 8th floor so that the 8th floor also grows to 8x3 (or 7x3) cores. Finally i can then put the cores of the 9th floor onto another by reducing the number of core stacks, so maybe 4x2 (or 3x3 + 2 remaining) cores.
We will see. But i expect that there is more room for improvements. With 3 core stacks like on the 1st to 4th floor, the cores also act as a spacer which prevents the cups from gliding into another...
Probably i will do this modification next monday.

73, and HNY.
Stefan

PS: I'm greatly looking forward to your EbNaut results. Don't hesitate to show the message.

Am 31.12.2016 02:07, schrieb Markus Vester:

Hi Stefan,

during the last hours a good peak apperared here, around 13 dB in 63 uHz. If this is really your carrier (and not just a local GPS harmonic), tomorrow's EbNaut sequence should become an easy decode.

Nice video!

Best 73,
Markus (DF6NM)

 


-----Ursprüngliche Mitteilung-----
Von: DK7FC <selberdenken@posteo.de>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Fr, 30 Dez 2016 5:27 pm
Betreff: Re: VLF: Back on 5.17 kHz / 58 km...

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