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winding lathe built -> Enamel-coated copper wire on PVC water pipe - PVC cement to hold the windings

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Hazmatt_(The Underdog)

Sat Oct 11 2014, 04:43AM

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

Okay...

1. Why a quarter wave? Simple, draw out a sine wave, 90 degrees is the first peak, this is quarter wave. YOU COULD drive the coil at 3/4 wave, which is the second peak, 270 degrees of your sine, and right at 180 degrees, or HALF WAVE, you have a NULL, just like your sine wave picture.

2. Why is it not exactly one quarter of the wire length? Capacitence. The winding proximity of each turn next to the other is a fraction of a pico-farad. Tesla realized this when he had to tune his close-wound coils to a greater degree than his space-wound coils. Turns have a proximity capacitence which lowers the resonant frequency. A 4.5" coil with H/D of 4 can have 12pF or more of self capacitence from the winding and coil form contributions, I know this, most of mine are of this sort.

That capacitence, which you overlook in your calculations, is why you see a lower resonant frequency then calculated.

3. The full wavelength or transmitted frequency should be 4x or nearly 4x the winding length.
Just for fun I verified the NULL on a coil by exciting one end and probing the other, there was nearly no voltage at the probe! It's neat when the physics proves the theory.

4. A gas dielectric will reach a breakdown voltage sooner than a solid, unless you have some SF6 lying around.

tarakan2

Sat Oct 11 2014, 06:44AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Posts: 179

Quarter of the wave for 1000KHz is about 28000 cm.
Are we talking about the wavelength of electricity in copper at this frequency?

Or do we have Tesla Transformers that run at GHz?

GrantX

Sat Oct 11 2014, 08:55AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

tarakan2 wrote ...

Or do we have Tesla Transformers that run at GHz?

The little stubby output antenna of a microwave oven magnetron (the ceramic thing with a metal hat) is essentially a 2.45GHz "Tesla coil" since it is a 1/4 wavelength antenna. If your microwave oven is particularly old and greasy the magnetron can actually achieve "breakout" like a high frequency VTTC (I think this occurs due to gunk building up in the waveguide and causing the antenna to flashover to ground - my mate's ancient oven went to a fiery grave).

Andy

Sat Oct 11 2014, 04:11PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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If it was B field theory it would be 28000 cm(assuming accurate haven't checked), if its E field it would be smaller, as B is large in amplitude based on time, and E is lower in amplitude based on time.

I think

tarakan2

Sat Oct 11 2014, 06:44PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Not sure what B and E fields are. May I ask you to elaborate, please.

Speed of light in vacuum: 299,792,458 M = 29,979,245,800 cm

29,979,245,800/1,000,000 = 29979.2458 cm

29979.2458/4 ~= 7494cm
Not possible because the bigger the coil gets, the lower the frequency is. For that reason even Wardenclyffe Tower could not function at 1/4 wavelength.

What other theoretical 3D wave forms do we have that may be around 30cm X4 at 1MHz?
So it may work for a microwave wave guide but not for Tesla coils since they work in the range of 10^4 to 10^7 Hz and are impractical above and beyond this range.

Andy

Sat Oct 11 2014, 06:56PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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tarakan2

Sat Oct 11 2014, 07:31PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Posts: 179

Andy wrote ...

Hi

OK.
So I have my frequency.
In this case I care only about the length (1 dimension)
So each of these fields will produce a wavelength that will somehow have a relationship with the frequency (1,000,000Hz)
Are there any formulas that tie E and B fields with a specific Tesla Coil of a specific geometry?

Dr. Dark Current

Sat Oct 11 2014, 08:51PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Its not true with the quarter wave, simply adding a topload will decrease your resonant frequency (relatively a lot) while the winding length remains constant. And the resonant frequency can be even higher than 1/4 wavelength in some cases, but most often its lower.

tarakan2

Sat Oct 11 2014, 10:56PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

Dr. Dark Current wrote ...

Its not true with the quarter wave, simply adding a topload will decrease your resonant frequency (relatively a lot) while the winding length remains constant. And the resonant frequency can be even higher than 1/4 wavelength in some cases, but most often its lower.

So in the f = c/ lambda formula, c is not speed of light in vacuum. It is some other constant. Am I correct?

Numbers get more realistic if we use Fermi velocity in copper instead.

Hazmatt_(The Underdog)

Sun Oct 12 2014, 05:05AM

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

tarakan,

IF YOU DO THE RESEARCH... you will find that transmission velocities in coaxial cables is often 0.86 x c

But this varies depending on the medium.

The propagation is also on the surface of the medium, this is the skin effect, which is another headache!

AND if the copper is silver plated, now you're dealing with an extremely conductive medium plated onto a second medium, whereby most of the RF energy, if not all, is inside the 3 mils of silver, and the copper is just acting as a mechanical support.

AND ALL OF THIS... goes right out the window with Tesla coils because they are NOT IDEAL antennas, so antenna theory doesn't really matter at this point.

If you really want to go down the rabbit hole, the secondary has multiple resonances which approaches a ladder network in simulation.

Honestly, just build your coil and be done with it, and stop worrying about minutia.
It takes about $20,000 in equipment to measure these small problems, and who has the time, seriously.

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