Hi Stefan,
> These high power audio transformers are interesting. I wonder how well
> they work below 1 kHz. But they are to expensive, as you say. You don't
> need them. For any impedance transformation above 1 kHz i use ordinary
> N30 ferrite toroids. There may be slightly better core materials but i'm
> getting very good results and they are not expensive and you can find
> them at many distributors (min quantity = 1, not
> 50000000000000000000000000). Especially at VLF they are well suitable
> and not expensive.
> One of my preferred cores is that one:
> https://www.buerklin.com/en/ring-core-outer--x-inner--x-height--63-x-38-x-25-mm-n30/p/84d258
> It works easily for a few hundret watts on all VLF frequencies.
I do have some ferrites that may be suitable for impedance
transformation at audio over a kHz, perhaps I should stop being lazy
and I should wind a transformer for testing. I keep forgetting that
audio transformers are large because they have to transform 3 orders
of magnitude in frequency - 20 Hz to 20 kHz. At this stage, I am only
interested in the top order of magnitude (2 kHz to 20 kHz) which
implies I can aim for a proportionally less winding inductance and
still have good results. The cores I have are N27 meterial E shape and
when you put to E's together you get a square of about 45 mm by 45 m.
N30 has better initial permeability by about a factor of two but has
worse losses (u" is higher). So I would probably require a few more
turns but I am generally on a higher frequency than you therefore it's
not a big deal :-)
> Yes you don't need to match the cable impedance but the losses on your
> cable are caused by the current and resistance of the wire and so, if
> you transform from 4 Ohm to 50 Ohm, then your losses drop by 92 %,
> regardless if it is necessary to match the cable impedance.
There is another solution to this problem which is more win-win :-) I
have another roll of 3-core cable I can put in parallel to the first
one, then my losses will go up by a factor of two but factoring that
in the calculations, so does the radiated power (for the same TX
power). Then my impedance is higher and as a percentage, the
transmission line loss has less effect. Only problem with this is that
it was a pain to get the wire up and I suspect it will be a 2^2 times
pain to put the second one in parallel :-)
> I'm taking part in your experiments by imagining that i would be in your
> position :-)
Your role as the motivator is quite important in getting anything done!
> My latest thought is that a few (wooden?) poles of 10m height or so
> would be ideal. One is close to the house, and then two more in 100m
> distance from the house, e.g. here:
> http://k7fry.com/grid/?qth=QF44OX00PV&from=QF44OX00LN
> The 2x 100m dipole could be connected using a good HF ladder line down
> to the shack.
The only difficulty with this is the 10 m wooden poles. It's not
something trivial to transport home and install. But I am always on
the lookout for supporting material! A few years ago, I went to a
large country fair and I was negotiating with a company that makes and
installs woden poles for mains power. They would be happy to come and
install a few around my place but the prices were absolutely
unrealistic for me. I also have to respect the wishes of my wife who
is a conservation ecologist and doesn't necessarily like huge
machinery drilling holes and making a mess in pristine native
Australian landscape :-) Wires on trees are fine though and I do have
a few eucalypts that potentially can be used for this.
> Aluminium wire is light weight and cheap and the lower conductivity
> could be compensated by using a higher diameter, something like this,
> https://www.amazon.de/dp/B004UA7XT2
Wow, I have never seen aluminium wire used for fencing, this must be a
really posh version. Also I am surprised to see fencing wire stating
its resistance (0.01 ohm/m) :-) How nice of them! Maybe it's for
electric fence but the resistance wouldn't be all that important in
that case I guess. May even be desirable for the poor animals :-)
> This antenna would have the advantage that you can use it from MF to DC
> and the kangaroos would not be able to destroy the antenna.
> For VLF experiments, the ends of the wire can be grounded, just by
> adding another 100m of wire at the ends and let them slope down, maybe
> with another pole at 4m height.
Fair enough! I think you have convinced me to try the grounded ends
experiment and I will start small with my 120 m loop. I'd be
interested to measure the impedance of that, my suspicion is that it
could be hundreds of ohms given the very rocky ground. I then have to
make appropriate impedance transformers. What sort of impedance range
have you observed with your ground antenna experiments on VLF? Also,
was the reactance part of the impedance positive (inductive) or
negative (capacitive)? I generally use my oscilloscope and signal
generator to measure these parameters using a home made bridge and my
capacitor box. This gets a bit tricky on the field. How do you do it?
Maybe we should start writing a 'techniques' manual for the beginner
VLF enthusiast? Perhaps I can start writing things down in my page and
you can contribute your trade secrets? ;-)
73, Dimitris
|
