Hi Stefan,
> Yes. First you only need the primary winding to see if it stays cool at
> the full volume level. You know, saturation and so on. Only use as many
> turns as necessary for the frequency, that is ideal. But probably you
> know all that.
> If you know the co
> re material and the cross section area the the B max, then you can do a
> rough calculation as well...
> Here it's a bit like sitting a pub, exchanging some ideas about VLF.
I'd love to participate in a pub outing with VLF gurus! There's
nothing like a good chat over a couple of beers!
I will do a proper test: an appropriate resistor (50 ohm if I decide
to do 50 ohm to 4 ohm transformer) at the output winding and the
amplifier at max power. The only thing that should get hot must be the
resistor!
I made long time ago a web calculator for this:
http://people.physics.anu.edu.au/~dxt103/calculators/toroid.php
Anyway, I should stop talking about it and just try it out.
> > 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
> Go up? I think you mean go down, or switch in series. Ah, yes, put in
> parallel but switch in series. Yes, then, twice the R, 4 times the L and
> so 1/4 of the Cs and only 70% of the current, at the same power level.
> So you can switch the Cs in serial to get on resonance again. But this
> will be even a higher stress for them because the current is 40 % higher
> for each one then. So you need more of them and choose another
> configuration.
Yes, I meant that I will physically put the two cables side by side
and then connect them in series.
> > 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 :-)
> >
> Yes. And just you are not the one with limited space, so it is better to
> use the wire in another arrangement...
Well, that's a compromise I choose to make in order to have all the
gear near the house and be able to use mains power. I have done field
operation on LF and MF before and it's tricky but not impossible. I
have a number of 12 V 65 Ah SLA batterries which are heavy but I can
transport them around with my car.
> > 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.
> >
> How high are such trees? Can you climb them? Are there Koalas sitting on
> them? And are they dangerous? I've heared they are sleeping and eating
> all the time.
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!
> > 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.
> It all depends on the ground resistance you can get when using your
> ground rods or whatever at the end. The dry ground is ideal for getting
> a good radiation resistance but on the other sde you need a high antenna
> current and so you need a low loop resitance. It is a proadoxon.
> I already asked about those small ponds arround the dry landscape. What
> is it? For what are they used?
These are dams, people use the water in them for irrigation. The most
common crop here is grapes for making wine.
> It is no effort to walk arround and lay some cable on the ground and try
> see how much antenna current you will get, and then calculate the
> losses, or the serial resiatance. Make a DC measurement, that is very
> informative and quickly done. Then you have an impression how you have
> to wind the transformer. Use a higher DC voltage than 12 V, maybe 100V
> DC is a good idea, if you have a suitable power supply.
> You will need to contact these small ponds to get the loop resistance
> down. They are ideal for the job, really ideal.
> We also have to consider other conductive metallic structures below the
> ground. If there is a metally water pipe below the ground, it may bypass
> you antenna current, ie.e. it lowers the loop area! Also the electrical
> grounding of your house may be an issue. If you lay a 2000m wire close
> to the house of a neighbour, and your and his mains ground is conbnected
> together, then the efficiency will be poor.
> Assuming a 1000 m distance between the wire ends, and using those small
> ponds (put a plate of 1m x 1m in there, for example and as a test at
> least), and assuming there is no bypass, i estimate you can come down to
> say 200 Ohm? At least it will be an interesting information and a next step.
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 think the impedance should be measured with AC and in fact at the
same frequency you are planning to use. Anyway, there is enough space
here to avoid being close to other houses, which is good. I do have
the permission from the land owner north of me to put antennas on the
ground, but he runs cattle and they will probably interfere with the
wire. There are times though that the paddock is rested, and that's
when I should strike!
> > 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?
> >
> I have a very good scope that's also able to show a spectrum. I modified
> it that it can run on a 12 V battery and i take it to the feed point in
> my car each time. There there signal strengh of the fundamental
> frequency is maximised during the tuning process, using the FFT and auto
> measure function.
Ok, I see. My CRO is an old digital Tek and I don't want to modify it
but it does run with a small 12V to 240VAC inverter.
> > 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? ;-)
> >
> That would be a very good idea and it is good to start at the beginning,
> your beginning. It is also a bit like a diary, you can always look back
> what you've done a year ago.
> Maybe, who knows, a few OM are following the discussion and will join
> the experiments, not only but hopefully in Australia.
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 :-)
73, Dimitris VK2COW/VK1SV
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