From: <hvanesce@comcast.net>
To: <rsgb_lf_group@blacksheep.org>
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Date: Fri, 14 Apr 2017 06:43:13 -0400
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Subject: RE: VLF: Carrier on 6470.005 Hz / iron powder cores in high electric fields
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Hello Stefan,

It is good to see your recent (8270 Hz) 34 dB at 31.6 uHz in Todmorden,
21.7dB 31.6uHz on Jacek's grabber, and (6470 Hz) 18 dB in 46.3 uHz in
Todmorden, and a textbook DFCW-6000 spectrogram from Paul. Compliments to
all, including fine work on receivers and processing. 

Your observations on cores (in your message below) were most intriguing;
including the many interesting tradeoffs that you identified, and I was
basically hooked.  I looked at gapped and ungapped METGLAS and
polycrystalline cores vs T106-52, using a 2-D vector modeling tool for the
magnetic field inside of the coil. A first pass (not to be trusted without
experimental verification) with the 2-D field solver and other modeling
tools suggested (best case) up to 10x reduction in hysteresis + eddy current
losses, and 11x increase in core electrical resistance (65 ohms vs 6 ohms)
using 2605SA1 METGLAS, with the same core weight, but excessive core cost
(even with low surplus prices) with spatially even distribution of cores.
But the surplus 2605SA1 METGLAS cores (30,000 available) would, according to
simulations, not give degraded performance near the coil walls (simulations
did show degraded performance with T106-52 near the coil walls), and
placement of 2605SA1 closer to the coil walls could theoretically reduce the
cost of low-loss low-conductivity METGLAS to less than the cost of T106-52.
These simulation findings are not worth much without basic validation by
experiment followed by corrections to the models, but a basic qualitative
validation with 10 cores and an improvised coil would be would be
inexpensive and easy; so I ordered ten of the surplus 2605SA1 METGLAS cores.
The improvised coil is connected to a sweep generator, power amp and storage
scope, ready to go when the cores come in next week. 

If a low-cost low-loss high-Q option is not indicated, at least some
additional information regarding parallel and series Rs and Cs in the coil
model may emerge. Thank you for the ongoing new findings; it seems they
provide good opportunities for continued learning.   

73,

Jim AA5BW


-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of DK7FC
Sent: Thursday, April 13, 2017 7:43 AM
To: rsgb_lf_group@blacksheep.org
Subject: Re: VLF: Carrier on 6470.005 Hz / iron powder cores in high
electric fields

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 皴 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 皴?

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.
>
>