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Old 19th Nov 2017, 5:21 pm   #1
astral highway
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Default Wireless Self-Resonance Experiments

This experiment may interest you.

(Mods, please can you change thread title to 'Wireless Self-Resonance Experiments'? - thank you. Also is it possible to rotate my sketch please?!)

Some of you will of course be familiar with this, as professionals electronics engineers. But for those of us, who, like me, had only previously investigated self resonance by calculation, or from inductance measurements taken with a bridge or a meter, the method may appear counter-intuitive and even unlikely, since it it involves measuring scattered radiation effects from an open circuit inductor coupled to an induction loop. Yes, you read correctly: without connecting to it at all.

I am directly quoting from the following paper, which inspired the set-up shown in my photos. The author is David Knight G3YNH

The author explains:

''The connectionless measurement method turns out to be straightforward because the electromagnetic field around a resonating inductor is extensive, and the E-field magnification effect is so great that the scattered signal tends to swamp the electric component of the excitation field. Complete electrical isolation is, of course, impossible; but situations involving either minimal disturbance or quantifiable disturbance are not difficult to achieve. The basic technique is that of exciting the coil using an induction loop, and sampling the field using either a small dipole or another loop.''
David Knight expands the theoretical basis of his work at length, and uses various glow tubes to indicate visually the effects of the integer multiples of a half-wavelength.

The author uses a transceiver as a signal source into a coupling loop. I don't have one; see instead my photos showing a a home-made 20W modulator, connected to an induction loop.

This has a low inductance and is non-resonant at the source frequency, which is variable between 190-390KHz. The induction loop is around 20cm away from the inductor under test. It is the outer coil of a variometer, 5 turns of 3mm dia wire, to a length of 5cm. The inner (litz) winding is not connected.

The long red coil (inductor under test) is my main Tesla coil inductor - the secondary coil - and is resting on a reasonable insulator. Ideally it should be suspended totally. Its ends are not connected to anything.

The inductance is approx 25mH. Using the wireless method described, with the coil open-circuit, the measured self-resonant frequency, detected by a scope probe hanging in the air at the distal end of the tube (and again, not connected to the coil) is around 220KHz, which is 100KHz lower than the measurement I made by close-coupling the coil to a tuned circuit, the other day. This previous measurement is mentioned in post no 113 on my thread about constructing a large valve Tesla coil. This may not be accurate as I'm not yet replicating the method of sampling the frequency shown.

The author goes on to explain how:
A coil exhibits self-resonance because a wave travelling along the helix is reflected at the impedance discontinuities that occur at the ends of the wire. Resonance occurs when the wave gets back to its starting point in phase with itself, and a corresponding standing-wave pattern develops. A very strong response is obtained when the wire-length approaches an electrical half-wavelength. This is the fundamental self-resonance frequency (SRF), generally simulated, with moderate accuracy, by representing the coil as a lumped inductance in parallel with a capacitance (the 'self-capacitance'). That this is just a representation, with little to do with the physics of the processes occurring in the coil, becomes obvious when we note that there is also a series of overtone resonances.
I hope to go on to investigate some spectral effects as described by David, as well as to properly replicate his test rig.

Please note, if it isn't obvious, that very large voltages indeed may be induced across the inductor under test, even when excited by low powers of RF (my home-made signal source has about 20W output and runs off a low-voltage bench power supply at 30V)- not a hazard to us, but enough to mess with sensitive measuring equipment - and any capacitors used in the later experiments should be kV rated types.
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Old 20th Nov 2017, 1:23 am   #2
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Default Re: Wireless Self-Resonance Experiments

At work I do a lot of broadband design covering HF through many GHz and I have used similar techniques to investigate inductor behaviour. I use E and H field probes and a VNA for free space measurements. I also use this technique to measure the Q of LC resonators.

However, I find that the most accurate way to model a typical inductor through all its resonance modes is to measure it on a full 2 port VNA and then play with the data on a simulator. In my case this is for inductors that will go on a PCB. The inductors can be anything from 0402 size to a big air spaced solenoid or a handwound inductor on a toroid.

The alternative method to model all this is to create a complicated transmission line model for the solenoid and I have a method to model/predict where the resonance modes will be based on the inductor dimensions. I can usually get the first two resonances very close but the third one is harder to model. I don't think David's model goes beyond the first free space resonance. He's demonstrated higher resonance modes but I get the impression these are expected to be at multiples of the first resonance. I don't think this is the case for typical solenoid inductors.

I think David Knight has been looking at this stuff for many years. His methods are really interesting and I'd love to be there at some of his demos. However, I think it's possible to go beyond what he is doing by using a modern VNA and an RF simulator. The resonance modes usually aren't harmonically related although sometimes it appears like this. Often the resonance modes are grouped closer together and my models predict this correctly.

I have to be able to predict/model this stuff because even a tiny resonance in a (power amplifier) bias tee up at UHF can cause a hotspot in it if the reflection coefficient of the load is in just the right place at the right frequency. This can expose the bias tee to a very high RF voltage and if this happens at a resonant frequency then the bias tee inductor can get very hot very quickly. I usually use a thermal camera and a special load I designed to let me explore various VSWR circles at the load whilst looking for hotspots in any bias tee components. Sometimes it's capacitors that can get hot if they have a resonance with a certain load at a certain frequency.
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Old 20th Nov 2017, 4:48 am   #3
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Default Re: Wireless Self-Resonance Experiments

Al,

Interesting to read about these experiments.

I assume that the design of your apparatus, when it is finished, would have a coupling coil around the main Tank coil, and in this case you would be using large valves to drive the coupling coil ?

In any case, one interesting thing about inductively coupling to a tuned or Tank circuit, is the loading of the driver circuitry on the coupling winding, can significantly up-shift the value of the resonant frequency of the Tank coil. But the effect depends on how tightly the coupling between the coupling and Tank circuit is and the impedance of the source driving the coupling coil.

For well coupled coils the effect happens because the damping on the coupling coil neutralizes some of the inductance of the Tank coil and that is what raises the resonant frequency above what you might get with a free resonance test.

So for example the valve's plate resistance driving the coupling coil can play a role in the final resonant frequency value, even if this parameter not officially a "reactive circuit element".

One good example to demonstrate this, is attempting to determine the resonant frequency of a car's ignition coil secondary. If say you drive the primary from a very high Z source, say >100k, you get a secondary self resonance which is primarily determined by the secondary's self inductance and its distributed winding capacitance. However, if you use a low Z source loading the primary, this neutralizes most of the secondary's self inductance and you get a much higher apparent resonant frequency that represents a value of secondary's capacitance now tuning the leakage inductance instead, which is a much lower inductance than the main winding.

With other various loading of the primary and mutual inductance values in between, for other coupled resonant circuits, the secondary resonant frequency will be up-shifted some amount from what the tank circuit will resonate at on a free test, say when its excited into oscillation and you measure oscillatory decay without significantly loading it.

There is a great equation in Terman under coupled resonant circuits that explains this effect well. I'm not sure if it would relate to your setup. Do you have a sketch of what your valve driver, coupling coil & coupling arrangement to your main Tank circuit looks like ?

Hugo.

Last edited by Argus25; 20th Nov 2017 at 5:05 am. Reason: typo
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Old 20th Nov 2017, 12:22 pm   #4
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Default Re: Wireless Self-Resonance Experiments

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Originally Posted by G0HZU_JMR View Post
This can expose the bias tee to a very high RF voltage and if this happens at a resonant frequency then the bias tee inductor can get very hot very quickly. I usually use a thermal camera and a special load I designed to let me explore various VSWR circles at the load whilst looking for hotspots in any bias tee components. Sometimes it's capacitors that can get hot if they have a resonance with a certain load at a certain frequency.
This is very interesting work indeed, Jeremy. Thanks for sharing the specialised techniques that you use at these sorts of exacting frequencies.

For my part, I'm glad to have an insight into a way of looking at resonances that hadn't occurred to me, and that is within my capacities to replicate to some degree here (likely spectrally, if I hunt around for suitable tubes.) Of course, I am operating at rather low frequencies with a lot of headroom.

It did occur to me that at the frequencies you are describing, even an open circuit track on a circuit board is now an unwanted resonator at some frequencies. Of course I already knew about parastic inductance on metal strips connected to something, but had no insight into the condition under which even a completely open circuit conductor could be a problem.
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Old 20th Nov 2017, 12:31 pm   #5
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Default Re: Wireless Self-Resonance Experiments

Hi Hugo


Quote:
Originally Posted by Argus25 View Post
I assume that the design of your apparatus, when it is finished, would have a coupling coil around the main Tank coil, and in this case you would be using large valves to drive the coupling coil ?
Exactly. A coupling coil(tank coil) around the secondary coil, with the added complexity of a grid feedback coil and RC time constant elements.

Quote:
Originally Posted by Argus25 View Post
In any case, one interesting thing about inductively coupling to a tuned or Tank circuit, is the loading of the driver circuitry on the coupling winding, can significantly up-shift the value of the resonant frequency of the Tank coil. But the effect depends on how tightly the coupling between the coupling and Tank circuit is and the impedance of the source driving the coupling coil.

For well coupled coils the effect happens because the damping on the coupling coil neutralizes some of the inductance of the Tank coil and that is what raises the resonant frequency above what you might get with a free resonance test.

So for example the valve's plate resistance driving the coupling coil can play a role in the final resonant frequency value, even if this parameter not officially a "reactive circuit element".
Interesting finding. Indeed, in my tests so far, a square wave signal into a coupling loop that is part of a resonant circuit, even when well below the apparent self-resonant frequency of the secondary coil, but coupled to it, will produce resonance. I think it is typically 0.7 times lower. Coupling is very tricky indeed.

I can produce a sketch of the generic set up of my apparatus but perhaps not on this threadm, as it takes away slightly from the emphasis on David Knight's paper, if that's ok?
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Old 20th Nov 2017, 2:40 pm   #6
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Default Re: Wireless Self-Resonance Experiments

Al,

ok another thread sometime.

Just an Idea:

There is one very good way to couple energy into or out of a Tank circuit. It is called "Inductive link coupling" The beauty of it is that you can create a link between two circuits, or say just a plate load and one main Tank resonant circuit with a correctly set up link.

This can be done with a tap on the cold part of the main tank circuit (or a coupling coil there) and a similar inductance coil in the plate load, linked together, twisted wires are usually ok, over some distance which can be helpful too, getting your driver valves physically away from the main coil area where all the high voltage action will be.

The trick though, is to tune the link, between the two identical coils with a series variable capacitor between them, to neutralize the reactance of these coupling coils at the operating frequency.

This way maximum current flows in the link circuit (as it does inside a resonant circuit at resonance) and allows energy transfer from the anode of your output stage into your main Tank/Tesla coil and the energy into that coil is maximized. It could be worth looking into this for your application.
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Old 20th Nov 2017, 8:01 pm   #7
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Default Re: Wireless Self-Resonance Experiments

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Originally Posted by G0HZU_JMR View Post
I think it's possible to go beyond what he is doing by using a modern VNA and an RF simulator. The resonance modes usually aren't harmonically related although sometimes it appears like this. Often the resonance modes are grouped closer together and my models predict this correctly.
Interesting... That's exactly what I found when I put my modest 'sparker' Tesla coil on a VNA. The first resonant frequency is 531.38kHz (sorry, MW DX-ers), then they increase with a diminution in return loss as follows:-

1): 531.38kHz
2): 1470.34kHz
3): 2122.48kHz
4): 2842.9kHz
5): 3409.48kHz
6): 3936.16kHz

Haven't worked out any mathematical correlation as yet.
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Old 20th Nov 2017, 8:54 pm   #8
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Default Re: Wireless Self-Resonance Experiments

It is a very interesting subject...

I think the link in the first post doesn't work but here's a link to David Knight's stuff on inductor resonance.

http://g3ynh.info/zdocs/magnetics/ap.../self-res.html

It's a while since I last read his work on this and the latest version of his paper is now over 100 pages long. I think his work dates back a long time. I think it may go back over 10 years at least because I remember seeing the interesting Trio TS430S transmitter experiments he did. I think the last time I read this paper it was much shorter.

This evening I skimmed over the latest version of his paper here:
http://g3ynh.info/zdocs/magnetics/ap...s/self-res.pdf

There's loads of interesting info there and lots of extracts from other researchers' work. However, I think it has become quite bloated now and it doesn't flow very well. I think there is room for a more focussed document that just shows the reader how to model an inductor up through two or more resonance modes. I also notice that this latest version does have graphs to show that the resonance modes aren't always at exact multiples of the first resonance. Anyone with access to basic/modern test gear can do a few simple experiments and spot this in less than an afternoon.

I did a lot of study work of my own many years ago at work on this subject for the reasons I gave earlier. This culminated in various attempts to model and understand what is going on within the inductor itself. I was mainly interested in quite small inductors that go on a PCB and I found that the 'black box' s parameter model of a solenoid is the best one to use in a simulator as it is so versatile and accurate. This was followed by my own (complex) transmission line model.

It generally models the first two or three resonance modes quite well.
I didn't need to adopt any of the complex equations given by the various contributor's to David's paper and just derived my own, based on simple observations and the application of some basic transmission line theory. So it was all very crude but it seemed to give fairly good results for both free space and 'in circuit' use. Note that my research work spanned up into the microwave bands as I was testing for resonances up from VHF into the GHz region. I was working with much smaller inductors
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Old 20th Nov 2017, 9:12 pm   #9
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Default Re: Wireless Self-Resonance Experiments

To get higher resonances on or near integer multiples of the fundamental you usually need some simple symmetry in the geometry (the simplest being a straight thin uniform length of wire). The further you depart from this the more the overtones depart from simple harmonics. What happens at the ends of the resonator also has an effect.
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Old 20th Nov 2017, 9:29 pm   #10
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Yes, that's what I found. Even though a neat solenoid looks like it is a long coil of 'the same stuff' there are end effects and departures from simple transmission line behaviour. But I found that I could still produce a reasonable physical model that behaved well when put through its paces on a simulator. The best behaved model was the VNA 2 port model but it is just a black box model and maybe this is dodging the issue.

If I had to be critical of the David Knight paper, I'd suggest that there is very little there that would help someone make a well behaved inductor model that explores the inductor performance up in the region of these resonances. The goal of the paper seems to be one of pinning the tail on the donkey in terms of calculating resonances. But this is only of limited use for everyday engineering work. A well behaved (wideband) model that doesn't fall apart when scrutinised on a simulator is of much more value I think
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Old 20th Nov 2017, 9:52 pm   #11
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Default Re: Wireless Self-Resonance Experiments

Quote:
Originally Posted by russell_w_b View Post

1): 531.38kHz
2): 1470.34kHz
3): 2122.48kHz
4): 2842.9kHz
5): 3409.48kHz
6): 3936.16kHz

Haven't worked out any mathematical correlation as yet.

Hmmm, ...I can see, having looked at this with some fascination, that the common ratio of the wire lengths (of a theoretical inductor at each listed frequency) tends toward 1 quite rapidly.

If this doesn't make sense, I imagined inductors that would be self-resonant at these frequencies. To simplify things, I kept the length constant (not ideal) and the self-capacitance constant (10pF, again simplifying)
Say we had an inductor self-resonant at each of the above frequencies, with an order to match the order of the list above:

1) 8.9mH, wire length 46m
2)1.172mH, wire length 16m
3)562uH, wire length 11.72m
4)313uH, wire length 8.73m
5)217uH, wire length 7.29m
6)161uH, wire length 6.28m

Then the corresponding common ratios are: 2.87,1.37,1.34,1.20,1.16...

Anyone hazard the terms to unity?

More importantly, why does the series tend to 1 so quickly?
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Old 20th Nov 2017, 11:47 pm   #12
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From my experience, it's risky to think in terms of a lumped self capacitance if you want to explore behaviour up towards resonance. It's best to think in terms of transmission line behaviour but even that is only an approximation unless you add more complexity to the model. I've got some old notes and test results backed up somewhere because I tested various inductors using the near field probes and also the VNA. I used a fixed length of wire and used the same bit of wire to make inductors of various L/D ratio. Then I logged the resonance behaviour.

I also looked at trying to make several inductors of the same inductance but with different dimensions. This meant using wires of different length in each case. I do tend to be a bit lazy when it comes to winding coils neatly. My research work was fairly casual here. I am really more interested in commercial SMD inductors and how they (mis)behave up in the GHz region. But I did get good agreement between the various techniques I used. I make VNA measurements on SMD inductors quite regularly for research work. The same applies to SMD capacitors and resistors. The search for parts that behave well over a wide frequency range is a never ending one

It's interesting that I'm testing at frequencies about 1000 times higher than you and Russell and David Knight. I think the same rules apply but the test coils are much easier and quicker to wind in my case. The big impressive coils you and Russell have wound for your Tesla experiments must have taken ages to wind. I don't think I could wind coils that neatly. Was it done in some kind of jig?
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Old 20th Nov 2017, 11:51 pm   #13
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Al,

Those Knight glow tube experiments are really interesting.

I was wondering, since I'm not a Tesla coil builder, would there be any in value in adding an external capacitance across the coil, which dwarfed the winding's distributed self capacitance, to produce a fundamental resonant frequency with less energy spread into the overtones. Though the the peak voltages across the coil would then be lower of course, but that could be compensated for with more drive/excitation.

Or is this idea just regarded as a practical problem for Tesla coils because of the high voltage rating required for any lumped tuning capacitors ?

Hugo.
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Old 21st Nov 2017, 12:19 am   #14
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Those Knight glow tube experiments are really interesting.
Hey Hugo,

They really are, aren't they! Do you think you might replicate some of them?

By coincidence, my partner's father worked, before a tragic early death, in defence physics /ballistics and analytical geometry when she was growing up - at one point for Ferranti!

And before I even met her and knew this, I owned a Ferranti NSP1 glow tube, of a type that I can use for some of David Kinght's experiments.

It was wonderful when she first saw the distinctive graphic design on the box of the tube and told me what personal significance it had to her past. Now this is a tangible link between her history and my interest in electronics, which she supports wholeheartedly.

Quote:
Or is this idea just regarded as a practical problem for Tesla coils because of the high voltage rating required for any lumped tuning capacitors ?
It is an excellent idea, and yes, that's the limiting factor, spot on. Impedance mismatches and poor tuning can result in a few tenths of uS duration 'violence' in terms of flashovers between closely coupled inductors, etc. In a poorly tuned circuit there can be a 40KV or more voltage gradient between primary and grid feedback coils, for example.

Thank you for your idea, I'll maybe post you the simplified circuit without lumped components, in a PM if that's ok?
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Old 21st Nov 2017, 12:23 am   #15
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I don't think I could wind coils that neatly. Was it done in some kind of jig?
I wound mine on a lathe run very, very slowly. There were two of us at it. I fed the wire at the rear and the lathe 'driver' kept his foot over the stop-bar. We stopped and started a few times!

I'd better say now that I didn't design my coil seriously, or how you're meant to (from the primary inwards). I made it from a few bits and bobs I had, and 'it just happened...' The secondary is a piece of PVC drainpipe, varnished in the lathe then wrapped with as many turns as I could get on it. Then varnished again.

I wound a loose-coupled primary from brake-pipe and I feed it from an old neon mast-lighting transformer via an R-C filter, and resonate the primary with three 15kV ceramic capacitors ganged up and equating to about 4nF. I put a short clip on YouTube here.

The orange is just a little bit more capacity for fine tuning.
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Old 21st Nov 2017, 12:26 am   #16
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Those Knight glow tube experiments are really interesting.
They would be fantastic to demo and watch in a lecture hall or similar. Not sure I'd want to go too close to any of this stuff but there are various glow tube experiments on David Knight's web page. I remember the early ones with the Trio TS430S transceiver because I have the same radio here at home. It was interesting to see it being used in research work like this.
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Old 21st Nov 2017, 12:41 am   #17
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Quote:
I wound a loose-coupled primary from brake-pipe and I feed it from an old neon mast-lighting transformer via an R-C filter, and resonate the primary with three 15kV ceramic capacitors ganged up and equating to about 4nF. I put a short clip on YouTube here.
The orange is just a little bit more capacity for fine tuning.
Impressive stuff. I remember my physics tutor at school set up a tesla coil experiment and he made the mistake of having the on/off switch quite close to the coil itself. Although he had performed this experiment loads of times before like this the arcs were really long and severe on this occasion. I'm not sure if this was a humidity thing or not but it was very funny watching him try to turn it off without getting zapped. He ended up getting zapped several times and we began to doubt his claim that the arcs were painless. He definitely wasn't enjoying himself on that demo...
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Old 21st Nov 2017, 12:27 pm   #18
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Thinking of an inductor plus self-capacitance only gives the fundamental resonance, and even that only approximately. Treating the wire as a transmission line gives harmonics too - but evenly spaced (as harmonics always are by definition). Adding the cross-coupling between turns then turns the harmonics into overtones, but by now things are getting too complicated for simple models so full wave simulation is needed.

It may be that for some geometries a simple model can give insight. I am thinking of things like a large coil with very few turns (so little interaction between adjacent turns) or a very long solenoid with many turns (so some uniformity - the basic element becomes a turn instead of a segment of wire, and strong interaction between nearby turns).
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Old 21st Nov 2017, 2:21 pm   #19
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I'd better say now that I didn't design my coil seriously, or how you're meant to (from the primary inwards). I made it from a few bits and bobs I had, and 'it just happened...' The secondary is a piece of PVC drainpipe, varnished in the lathe then wrapped with as many turns as I could get on it. Then varnished again.

I wound a loose-coupled primary from brake-pipe and I feed it from an old neon mast-lighting transformer via an R-C filter, and resonate the primary with three 15kV ceramic capacitors ganged up and equating to about 4nF. I put a short clip on YouTube here.
Nice demo, Russell! I like the breakout pattern of mechanically-excited coils, it is distinctively more random than with solid-state modulated or valve-powered circuits, breaking out above, below and to the side of the top-load (toroid, typically). What did you use a a spark-gap?

Everyone: any thoughts on my series based mathematically on wire length, at the bottom of post#11 - and where to go next with it?!! I am intrigued !!
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Old 21st Nov 2017, 4:45 pm   #20
russell_w_b
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Default Re: Wireless Self-Resonance Experiments

Quote:
Originally Posted by astral highway View Post
Nice demo, Russell!
Thanks, Al.

Quote:
I like the breakout pattern of mechanically-excited coils, it is distinctively more random than with solid-state modulated or valve-powered circuits, breaking out above, below and to the side of the top-load (toroid, typically). What did you use a a spark-gap?
The spark gap is three spherical copper RF knife-switch contacts on the end of the 50Hz supply (photo) so two gaps in series, and I put a vacuum cleaner nozzle adjacent to suck out the arc and leave the spark. It's very noisy! I have an old tumble-drier motor and had planned to make a rotary spark-gap, but life (and work) gets in the way sometimes. Manana...

I wound the primary spiral to resonate at fundamental frequency, which is about half-way around, and which allows adjustment for the corona loading the secondary and pulling the resonant point when it goes over. I get corona with a tighter tap on too, which, from bridging, I presume is from the second resonance point.

It was only afterwards I decided to stick the thing on a VNA to see what it looked like.
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Last edited by russell_w_b; 21st Nov 2017 at 4:50 pm.
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