Hi VLF,
Today the improvements continued.
The goal was to replace the two ferrite rod moving side to side into the
air flow channel in the center of the small VLF coil.
I had the idea to replace them by iron powder cores, T106-25 types. I
recently ordered 100 of them, they were quite cheap:
http://www.pollin.de/shop/dt/Mzk5OTQ3OTk-/Bauelemente_Bauteile/Passive_Bauelemente/Spulen_Filter/Ferrit_Ringkern.html
There are 10 buckets stacked onto another. First i put 3 cores onto
every 2nd bucket, see
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_144048.jpg
This worked very well.
Next i got the idea to put more of them into the coil, 3 on each bucket,
see https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_151736.jpg
That worked even better.
But the whole construction became heavier and sagged into itselfe.
So i put more and more cores into the coil, 2x 6 onto each bucket! See
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_153944.jpg and
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_155753.jpg
I hoped that i can spare one of the buckets to reduce the overall losses
(= heating up) and reduce HV problems due to the higher stack, as well
as further reducing heating due to the higher surface of the higher stack.
Indeed, in the end it was possible to remove 2 of 10 buckets!
That means i got some of the cores back and put them (3 onto another!)
where the highest upheating has to be expected,
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_163021.jpg
Now the is the current final construction for today,
https://dl.dropboxusercontent.com/u/19882028/VLF/20160415_202640.jpg
The weight of the coil is about 3 kg now :-)
I can see now that the Q of the coil is much higher. In a transmit test
i measured a 3 dB BW of 180 Hz. With my 470 pF antenna the L is 788 mH.
So Q is 46 and the overall R (=loss) is about 890 Ohm, if my
calculations are correct :-)
I hope to be able to run more power and current now. 250 mA is hopefully
possible. That is a 2 dB improvement...
73, Stefan
PS: It would be interesting to go on playing the game: Using a bucket
with 40% higher diameter, also using twice the number of cores. This
would end up at a higher necessary stack distance (lower coupling) and
will sometime end up with a single layer coil! There will be a
compromise between: Size, max. power, max. voltage, losses, wire
diameter, weight, costs :-)
I can recommend the idea to those who plan to build a VLF TX coil :-)
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