Hi Dimitris,
Am 11.02.2019 22:44, schrieb Dimitrios Tsifakis:
[...] and I should wind a transformer for testing.
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.
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 :-)
It will work very good.
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.
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...
Your role as the motivator is quite important in getting anything done!
Oh thanks.
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.
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?
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.
What sort of impedance range
have you observed with your ground antenna experiments on VLF?
I am using guide rails near a street to ground my antenna. This is very
comfortable and the resulting earth resistance is extremely low, lower
than i ever expected! My cable has about 30 Ohm DC resistance, about 1
km long. Before doing an experiment i usually measure the DC resistance
by applying 1 A DC. I have a adjustable DC/DC converter that is gives
0...150 V, homemade stuff. There is no electricity so i am transmitting
from batteries. I bought a 120 Ah LiFePO4 battery that provides > 500 W
for more than 2.5 hours, key down! In one of the last experiments i
measured 64V at 1A, so i have 64 Ohm. And that means that both of the
ground electrodes + the R of the ground is 34 Ohm. So one of the ground
electrodes must have less than 17 Ohm! Unbelivable, isn't it?! At 500 W
RF power i recently got nearly 3 A antenna current. I did a night
transmission attempting to leave a trace near Moscow. There was no trace
of my signal which confirmed that the antenna actually seems to radiate
like a loop, at least in 2000 km distance this was clear. So it was not
a negative result, it was information and confirmation of the theory (i
don't care to much about theory, you know). BUT, Paul Nicholson detected
the carrier and prooved that it was so strong that a 1350 character
message could have been transferred in those 2 hours!
You know what that means? It means that we could do a QRP experiment:
With just 5W TX power from one of my old 7 Ah lead acid gel batteries we
could exchange a message, like callsign + locator or some funny stuff
like 'I have a cat' (history). One day we have to try that, if he likes
;-) And our distance is just as high as the distance between you and
Edgar. OK, Paul will have a better RX system, but you have a mains
connection and so you are not limited by time.
Also,
was the reactance part of the impedance positive (inductive) or
negative (capacitive)?
Inductive. So you need a series C to tune the antenna. I built a C
network using WIMA FKP-1 capacitors and WIMA MKP-10 capacitors, each one
in series with a switch. The smallest has 22 nF, the largest has 10 uF.
With this network i can tune to frequencies lower than 1 kHz. At 8.27
kHz the spectrum already looks quite good with that series C only.
There, i need 169 nF (100+47+22) for the resonance. So the L seems to be
about 2.2 mH.
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.
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.
73, Stefan
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