Hi Eddie, Paul, VLF,
Paul, thanks for the report. I remember we were heavily struggeling with
15 characters in 16K25A, 80s symbols, 46 hours! Now it could be done in
8.5 hours or faster!
(
http://abelian.org/ebnaut/calc.php?sndb=18&snbws=0.000046&snmps=&code=16K25&sp=15&crc=14&nc=15&submit=Calculate
)
Eddie, the transmission was stopped by the safety function at 06:59 UTC.
From http://www.iup.uni-heidelberg.de/schaefer_vlf/VLF/TX.png it looks
like it was not necessary. But it was just a short test. I'm planning
further improvement steps.
The resonance is pulled down from 8270 Hz to 6470 Hz by putting 7 tubes
of iron powder cores into the 0.25 m diameter PVC coil body. Each tube
consists out of 33 vertically stacked T106-52 cores. A picture in
attachment.
You see the wooden plate can carry up to 9 tubes. I made measurements of
the resonance frequency as a function of the number of tubes. The
problem is that they are placed in a high electric field, 0 V on the
bottom of the coil, 30 kV on the top. The length of one tube is 375 mm.
The cores are conductive! I a test series i removed the coating from 50
cores or so, easily done with potassium hydroxide and warm water.
When putting a blank T105-52 core between two metallic plates, i can
measure 5 A at 30 V DC, i.e. 6 Ohm. You can build compact dummy loads
out of them! :-) But the temperature coefficient will be extreme. I saw
that the temperature coefficient is positive, after a few seconds :-)
So a tube out of cores has not only an inductance, it also acts as a
capacity switched in parallel to the antenna, even if coated cores are
used. They still have a capacity between each other. This makes the
arrangement a bit complex. In fact it is a RLC circuit, somehow. R is
negligiibileble but C is playing a big role. And L is wanted of course.
There are also saturation effects! When using a single core stack, the
resonance is pulled down to 7815 Hz (delta f = 455 Hz) but the stack
becomes warm. It only becomes warm in the center, not on the ends! The
effect is quite expressed. It must be due to the fact that the flux is
strongest in the center (along the length) of the coil. All these were
interesting experiments.
On the image you can see the core stacks are separated to each other,
just 20 mm or so. I thought this could be an advantage because thermal
convection will be better and so the cores can stay cool(er). However
yesterday i noticed that there is another negative effect: When the
cores are coming closer to the winding, the coupling capacity (lastly
the capacity switched in parallel to the antenna) will increase. The
helps to pull down the resonance frequency. But it is pulled down by the
unwanted coupling capacity, not by the inductance of the cores. And this
lowers the Q significantly! With 7 stacks/tubes separated from each
other, the resonance dropped to 6380 Hz. I was able to bring it up to
6470 Hz just by using a cable tie (3 dB types) which pulls them a bit
closer together in the center of the coil, which keeps the inductance
constant but reduces the coupling capacity! There was a significant
increase of the antenna current by doing this step!
The idea for the next improvement is to use 8 stacks of these cores
(meanwhile i have 400 cores available! :-) ) which are fixed to each
other as compact as possible, one in the center and 7 around it. This
massive rod will then have a minimal capacity to the winding and the
iron core cross section area will rise to 8/7 which further reduces the
flux in each stack/tube/rod... A single compact rod rather than 7 single
dangling rods are also much better to transport and they have a higher
mechanical stability. Fine adjustment can be done by adding a small air
gap here and there. I can imagine that this will increase the Q so that
i can run more antenna current with the same RF power.
I also did a test by using a shield of thin aluminium foil arround one
of the iron powder rods, without building a short cut loop of course. I
thought this could help to reduce the losses inside the cores . It was
just a short test. There was no significant improvement but the test
wasn't done very accurately. Maybe it can be repeated. But i expect the
losses are rather due to magnetisation losses than due to resistive
losses of the RC series arrangement, because R is quite small (200 Ohm
per stack).
Another thing to tell: I build a stack of 33 cores where the coating has
been removed. With one stack the frequency dropped twice as much as with
a normal stack, so the effective µr is 4 times higher. But the stack
heated up quite dramatically. I thought this is due to the missing
isolation between the cores and the galvanic connection between them.
But maybe it was just due to saturation because of the higher µr?
Still some experiments to do :-) : A PVC tube with aluminium foil
arround it, put inside the coil/field: Measuring the Q and the resonance
frequency (which will drop due to the parallel capacity). Then filling
the tube with cores, coated and un-coated. Measuring the Q and resonance
frequency. This will help to separate the effects of a Q decrease and
resonance frequency due to the foil and due to the cores. Maybe the
results will show that it is possible to come down to 5170 Hz with this
coil, by removing the coating of all cores and applying a foil shield
against E fields and currents inside the cores...
Just some thoughts and observations shared with the homebrewers...
73, Stefan
Am 13.04.2017 10:30, schrieb g3zjo:
I see from your Grabber that you ceased at 07:30. Shame because I
needed a little longer to confirm the line in my 47uHz, it looks
promising though.
73 Eddie G3ZJO
On 12/04/2017 22:50, DK7FC wrote:
Now i'm down on 6470 Hz again.
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