Hi Dimitris, VLF,
Am 11.02.2019 01:07, schrieb Dimitrios Tsifakis:
I have an idea about impedance transformation and that is to use high
power 70V/100V audio distribution transformers at both ends. I don't
have to go to 50 ohm as I don't use the coaxial cable as a
transmission line (it's only a tiny percentage of the wavelength!), so
I could use a 4 ohm to 400 ohm transformer or similar. I think in
general the impedance transformation ratio goes down as the power
rating goes up and we can live with that. THe audio transformer should
work fine on the higher frequency but that will have to be tested.
Perhaps something like this but not at that price!!
https://www.altronics.com.au/p/m1160-300w-100v-line-pa-transformer/
Another option is to get a PA amplifier of suitable power output that
has a 70V or 100V output. You only need one transformer at the antenna
end to get down to 4 ohm to reduce losses. I have a 50 W amp with such
output but that's not enough power.
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.
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.
I can easily try the ground loop on the 120m long antenna as I do have
ground rods at both ends. It would be interesting to compare the
signal strength in both cases. THat's why I am building a network of
VLF stations by giving away the large ferrite rods to interested
amateurs! They can help doing comparisons like this.
Yes, sure. Good luck in getting people motivated to build up a permanent
VLF RX station. It would also be good to have someone building up a
grabber site or maybe even providing a stream. Your activity can cause a
world wide motivation for people who are principally interested in VLF
reception and/or transmission, even if they won't be able to receive you
from so far away.
The ground is very rocky and there is almost no soil at the top.
Typical Australian native vegetation is the only thing that can
survive on it. I have thought about the dipole on the ground and I
have the perfect spot for it which is putting the feed point at the
top of the hill therefore making an inverted-V :-) Only concern with
that is the high voltage at the end of the dipole wires and the
possibility of a bush fire ignition. Definitely a project for winter!
I have tried to tune my ~120 metre wire to VLF in the past with mixed
results. Worth revisiting this. The wire basically starts at my house
and follows the hill resulting in the top of the wire being maybe 20
metres higher than the bottom.
I'm taking part in your experiments by imagining that i would be in your
position :-)
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.
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
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.
73, Stefan
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