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4.5 MHz coil

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Sulaiman

Tue Feb 14 2006, 06:33PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

WaveRider
Since you have a working system you could try this;

Wind a new secondary of the same diameter and wire
but three times longer/higher, (5x or 7x would be better)
you should be able to operate at 3/4 wavelengths
with a voltage peak 1/3 the way up as well as the top.
Explain that with a lumped model!
I saw somewhere on the net a long coil like this
with a thin vertical earth wire parallel to the coil
corona on the thin wire displays voltage gradients !

Steve Conner

Tue Feb 14 2006, 07:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The resonant frequency of a bare coil does indeed depend on a bunch of things, and it does tend to be higher than if the same length of wire was laid out in a straight line.

This is somewhat counter-intuitive. However, Paul Nicholson's Tesla Secondary Simulation project created a numerical simulation code that will work it all out and usually gives the correct answers to within a few percent. I use his code for designing all my coils nowadays. I guess this will be up WaveRider's street

and if he is confused about the physics behind the Tesla resonator I highly recommend reading Nicholson's papers.

You can access a Java version of the simulator online at

Marko

Tue Feb 14 2006, 07:17PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It is true that there are more variables than just the wire length. However, given a fixed wire/insulation thickness and winding radius, fres is clearly strongly influenced by the wire length. Also, the inter-turn spacing and shield spacing (or lack of shield) is indeed important and is one of the things I wish to understand better... What I understand so far is that traditional "lumped" models are not very good at describing what is happening here...

Horse-hockey! The length of the wire is everything! Just how would you define C? L? I have verified that the current at the base of the coil is high and is nearly zero near the top. How do you explain this with a lumped LC model? There is no "topload" in the traditional Tesla Coil sense. The resonance is indeed a "self-resonance" of the secondary coil. This is a distributed effect.....inter-turn mutual inductance and capacitance as well as distributed capacitive effects with the cage. Capacitive loading on the top of the coil will indeed lower resonant frequency, however. End-loading of helical and coaxial resonators is indeed a way to tune them. Helical resonators can be thought of as a helical slot transmission line of about 1/4 wavelength (hence my question about the resonant frequency in my previous post).

Calm down man, if you like starting flaming this is not forum for you. O_o
Obivously I tought the same thing as you did.

I just wanted to answer your question:

A nagging question remains: Why is the resonant frequency 4.5MHz for a 25 m long winding? 1/4 wave at 4.5MHz is 16.67m. I expected the resonance to fall somewhat below 3MHz... Need to give this some thought!

And tried to explain that just length of coil wire does nothing in reality with resonant frequency , coil length is actually very rarely 1/4 wavelength, maybe I got question wrong??

Frequency is determined by interturn capacitance and 'topload' capacitance (here capacitance to ground and surroundings from surface of coil as there is no topload) and we just made that capacitance less lossy by adding cage.

As you ad more wire you increase both inductance and capacitances, and of course that length of wire influences fres.

Cheers too...

HV Enthusiast

Tue Feb 14 2006, 07:44PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I think we all know what parameters affect the resonant frequency of a secondary coil and the discussion is getting cyclic. Please keep all future posts to the original subject of the thread.

Thanks!

Dan

ragnar

Tue Feb 14 2006, 07:53PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

[quote]

Calm down man, if you like starting flaming this is not forum for you. O_o
Obivously I tought the same thing as you did.

[/blockquote]

Hey, be nice. Saying stuff like that isn't your job. He might have been chased out of town by ham enthusiasts with pitchforks and torches, but WaveRider has definitely earned the right to say what he did. Minor confusion, meh, don't worry =)

My oscilloscope might even ship this week, which means I can try this circuit for myself... =)

Or alternatively I can push higher into maybe 10MHz with the method I meticulously described in the old forum. The FETs I'll rig up are 2N7000s, IIRC. FETs driving FETs, cute. :P

Marko

Tue Feb 14 2006, 08:22PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yes im acting stupid, sorry

but that before seemed offensive to me, and I wanted to point on that.

Intersting is that coil is driven only by feedback directly to mosfet gate, actually only fast enough mosfet is required :) ?

I have some mosfets like IRFZ24 with turnon about 40ns, could I in reality push that mo Mhz or two without too much overheating. O_o?

cheers..

WaveRider

Tue Feb 14 2006, 08:31PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Again.. sorry if I caused offense. I was having a bit of a bad day today..if that is any excuse for setting a bad example....

On to the interesting bits... Thanks Steve. I have read some of the papers you mention on the topic.... As far as I can gather, closely wound helices will exhibit somewhat higer resonant frequencies than their "unwound" counterparts as a result of inter-turn coupling "stiffening" the system. (Of course, there is the whole discourse surrounding the treatment of helices as "periodic structures" which gives a very general...but highly mathematical description of the problem.... This interests me a lot, but may make Forum members yawn.

)

Marko

Tue Feb 14 2006, 10:15PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

On to the interesting bits... Thanks Steve. I have read some of the papers you mention on the topic.... As far as I can gather, closely wound helices will exhibit somewhat higer resonant frequencies than their "unwound" counterparts as a result of inter-turn coupling "stiffening" the system. (Of course, there is the whole discourse surrounding the treatment of helices as "periodic structures" which gives a very general...but highly mathematical description of the problem.... This interests me a lot, but may make Forum members yawn. )

Not a bad topic, im interested too.
Forget earlier misunderstanding...

I just did some CAD calculations for my DRSSTC secondary, and proved that estimated LC frequency is really (almost twice) higher than estimated quarter wavelenght frequency.
To adjust for that i would need a 10pf toroid.

Some people take lots of care to tune their coils to quarter wavelength frequency, I understand that this is needed if we want to make a good radio antenna but here I am curious how this does influence preformance of TC (any) as point here is not transmiting radiowaves but voltage gain?

Some people just ignore it, go bigger toroid - > bigger sparks and just care to keep it in LC resonance O_o and still get impressive results.(?)

Otherwise lots of texts deny LC circuit as high quality resonator in TC and just say that voltage is gained by quarter wavelength resonance itself (?)

All I wanted to point is that this two things are completly different, as in answer to wawe rider...

Lots to be discussed here?..

I'm also wondering one thing, how to calculate aproximate interturn capacitances, TCCAD says 7pf for my coil but how can it be roughly calculated 'by hand' ?
I need surface of wire, dielectric constants of varnish, there are lots of fluctuations in distance between that cannot be ignored.. (?)

For that 4,5mhz coil maybe a very small toroid could be added as well??

Hazmatt_(The Underdog)

Tue Feb 14 2006, 10:58PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I'm no expert on TC's, but to try to force it to be 1/4 wave resonant I think is just silly. There are capacitences present which de-tune the system from a purely 1/4 wave function. Without the capacitence, sure it could be 1/4 wave. Approximating this would be an unloaded TC, which I like to do because I like the streamers at the end of the wire.
Otherwise, All I see is a sub 1/4 wave resonant system.

I would think that you could approximate the capacitence of the wire pretty closely if you knew the variables you mentioned, but also, the varnish is inconsistant, so that would be a factor as well.
But also, the coil at low potential is different from operating. Why? Well consider this, you measure your coil with generator and scope, this finds your capacitence. Okay great. But when you go to operate, you're not operating at the 10 or so volts that the generator spat out, you're putting much much higher dielectric stresses on the varnish with your 100,000 V+ at the top, so you still have to tune.

Marko

Tue Feb 14 2006, 11:09PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Coils actually can be very well tuned to 1/4 wl.
That frequency is lower than resonance of LC, and with apropriate toroid that two frequencies can match.

But the general question: what is the point in that? I dont see how 1/4 w.l. resonance would dramatically affect Q or gained voltage, if we want to transmit radio wawes then i get it...

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