Hello VLF!
The primary coil of the transformer is ready. 10 turns (diameter 2mm) give
9.62mH which gives about xl=500Ohms at 8.27kHz. It is a good idea!
73! Marcin SQ2BXI
-----Original Message-----
From: [email protected] <[email protected]>
On Behalf Of Marcin
Sent: Monday, February 11, 2019 4:55 PM
To: [email protected]
Subject: RE: R: Re: VLF: in VK?
Hello Stefan!
Today, the ferrite cores B64290L82X830 came to me. I'm building a transformer
1:1/1:4/1:9/1:16 and 1/25 on the VLF. I'll check if it works soon.
73! Marcin SQ2BXI
-----Original Message-----
From: [email protected] <[email protected]>
On Behalf Of DK7FC
Sent: Monday, February 11, 2019 2:50 PM
To: [email protected]
Subject: Re: R: Re: VLF: in VK?
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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