Hi Rik, VLF,
Thanks for the discussion.
Am 29.04.2016 10:40, schrieb Rik Strobbe:
Hello Stefan,
interesting that the cores seem to be more effective (larger
effective µr) for a slim (large l/d) coil.
I think it is just because a long coil without cores is worse then a
short coil without cores :-) It is the relation.
Without the better magnetically conducting center (of a coil having
cores inside) the field lines decide earlier to run back where they
started (just a theoretical imagination). So the flux or B field is
smaller for a long coil and thus the L. With cores inside, this happens
later, depending on the µr.
I'm thinking about a R network like an electrical fence (good
conductor) and some grass touching it. Then there is a current flow
over a short way (grass near the generator). If a constant current
source would be connected to the fence, then it depends on the ratio of
the conductivities (wire to grass) how much current is flowing how far
into the wire...
Just a (maybe stupid) thought: could it be that the cores are more
effective in the coil center?
If you still have the 145mm coil available it might be interesting
to check how the inductance (and thus the effective µr) changes if you
gradually reduce the number of cores while keeping them centered inside
the coil.
Ah i know what you mean. I think if a limited number of cores is
available, the will be most effective in the center.
Another thing: as you noticed for a slim coil the inductance is
almost linear with the number of windings (= coil length). This means
that a bunch of 30 1m high coils (in series) will be as effective as
one 30m high coil.
Athough you will need the same amount of wire and cores
construction and placement is feasible that way.
Yes. So the only way would be to use more of them in parallel. The
ratio L/l does not increase significantly any more at a certain ratio
l/d. If this l/d is choosen, the efficiency is as good as it can be.
BTW it's then still worth to calculate the alternatives to a 35m high
coil :-)
When l/d = 5, the efficiency seems to be maximised. Then, if i use 3
cores in parallel, there is a certain coil diameter. L will rise with
the square of the diameter. The cross section area ratio
core/(core+air) will be a bit lower. There are further mechanical
problems when using stacks of 3 cores in parallel.
Aaaaah, it is all interesting and you won't belive it: I have holidays
for 6 weeks (2 are almost gone) and i have holidays in July as well! I
will continue and there will be a compact coil in the end, i bet!
BTW i have ordered 100 of the 3C85 ferrite cores in the morning! :-)
They have about the same outher diameter. Let me try them into the
stack!
73, Stefan
73, Rik ON7YD - OR7T
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