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Date: Thu, 14 Feb 2019 09:33:04 +0100
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Subject: Re: R: Re: VLF: in VK?
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Hi Dimitris,

Am 13.02.2019 00:35, schrieb Dimitrios Tsifakis:
>
> Sadly there are no koalas here, they used to be here but not anymore.
> Also sadly, Australian trees are not very good for climbing, they are
> generally rather brittle. Not a problem though, I have developed other
> techniques to get wires up!
>    
The voltages for such ground loop antennas are rather small, another 
advantage over the usual E field TX antennas. You will stay below 500 V, 
of course depending on the pwoer and loss resistance you will get.

> These are dams, people use the water in them for irrigation. The most
> common crop here is grapes for making wine.
>    
Ideal, just perfect. I wonder why you are the first one in Australia 
doing such experiments. I would...., ah you know...

> Hmm, my understanding is that a DC measurement won't be too useful as
> it will suffer from polarisation problems. When you do a DC
> measurement, try to reverse the polarity and let me know what happens
> to the resistance you are measuring compared to the previous polarity.
>    
I know about the effect of an extra voltage drop on the surface of an 
electrode into soil. Thus i recommend to use a larger voltage. Why do 
you expect a different DC resistance when changing the polarity?
I can do that test you suggested but i don't expect a difference. It 
could be interesting to rise the voltage from 0 to say 50V in small 
steps and note the current and then draw a graph. Then you would see the 
nonlinearity. I would expect that it is only visible below 10 V.

> I think the impedance should be measured with AC and in fact at the
> same frequency you are planning to use.
Yes, sure. It is interesting to measure the frequency response of the 
feed point impedance. That tells something about the loop. I didn't do 
it carefully enough so far.

> I will start such a page and once it has enough material in it, I will
> let the mailing list know. Then I will be open to contributions of
> content :-)
>    
Looking forward to that. Pictures are most interesting for many people. 
An important part of such a page. Looking forward to that.

> Just finished making a pair of impedance transformers. I used some
> junk box large (OD=61.5mm, ID=37.5mm, L=12.8mm) ferrite toroids of
> unknown material and I stacked two on each transformer. I then made an
> autotransformer with a total of 52 turns tapped at 15 turns making
> roughly a 4 ohm to 50 ohm transformer (48 ohm to 4 ohm to be pricese).
> I wasn't aiming for 50 ohms it just happened to be the number of turns
> that neatly fitted on the toroids!! A=3.2cm^2 and the calculated Bmax
> for the expected power levels is less than 500 gauss so we are in
> business.
>    
Nice.
I had to learn that 1 Gauss = 1E-4 Tesla. So then you have 50 mT, which 
is not much. I think that you can go 3x higher even.
In some cases, a separate primary and secondary winding is helpful, for 
example to avoid EMC issues, ground loops and so on. Also for safety 
considerations. And especially when the impedance ratio is very high, 
i.e. the currents are very different.

> I then did some tests with the amplifier, I connected the two 50 ohm
> windings back to back and place the amplifier on one end and a 4 ohm
> high wattage resistor at the other end. I was pleased to see that the
> resistor got super hot and the transformers remained cool as a
> cucumber. I was able to get on the resistor more than 100 watts from
> f=3 kHz to 20 kHz, which reduced to 80 watts at 30 kHz. I can live
> with that. The amplifier is nominally 100 W output per channel.
>    
Very well. Cucumber, a word i had to learn. Do you know how to bridge 
the PA to get another 3 dB of output power?

> Fingers crossed now I will get a chance to fire it up on the antenna
> this weekend!
>    
Good luck and fun.

73, Stefan