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High "Q" secondary

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ShawnLG

Mon Jul 23 2007, 06:11PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

Below is a pic of my high "Q" secondary for my HF tesla coil. The windings are spaced out evenly to reduce the interturn capacitance. This coil design would be great for ham projects also. I don't understand why no one makes these.

Avalanche

Mon Jul 23 2007, 07:00PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Nice! I once had an idea to double wind a small secondary with copper wire and nylon fishing line to reduce the inter-turn capacitance. It's just one of those things I never got around to.

Hazmatt_(The Underdog)

Mon Jul 23 2007, 07:07PM

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

Quite the contrary shawn, people try to make high Q secondaries, that's where the aspect ratio comes into play so much. By keeping the coil stout you increase the Q and inductance/unit volume.

One reason you're not going to find many coils like yours is, well, look at the coil I wound on the PVC former. I did this to look at the wavelength and Cself of the 18 AWG coil. Its self resonant at 1880 KHz, and that's pretty high up there unless you have a 1 turn primary, small tank capacitor and small transformer. Otherwise you'd have to use a pretty well designed solid state driver. Then the next issue is, depending on input power, will it arc to itself.

The other coils are designed with the lowest Cself possible so that when they're tuned their self capacitence won't interfere with whatever capacitor is used with them. These ones are really designed for radio work.

I wouldn't be surprised if your coil resonates at 5 or 6 MHz, and getting a driver for it is going to be some work, but there would be a lot to learn from that experience.

...

Mon Jul 23 2007, 08:02PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

No one makes coil like that because it is a pain to keep the wires spaced out enough, especially since there are only like 4 people that are smart enough to make coils work in the MHz region

I do remember having seen a large coil (like 12" x 60" or something) wound by using a small piece of Al plate with some slots milled in it, and I think that someone got arround to winding a coil using a piece of thread as a spacer.

Steve Conner

Mon Jul 23 2007, 08:08PM

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

What a lovely coil:
But on a cardboard former?
Hams use ceramic.

ragnar

Tue Jul 24 2007, 03:22AM

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

It's nice. I'd personally estimate your Fres to be about 9-11MHz.

People do make these. I did. The reason they're unnecessary in tesla coil use is because the plasma loading swamps any high Q you may ever have had. As soon as the coil breaks out, its Q turns to crap. Waverider has covered this well.

When running at reasonable powers, you'll find the copper gets stinking hot -- watch out for the cardboard spontaneously combusting at a certain temperature, with the help of a flashover. As I found out the hard way with a toilet roll, the WHOLE thing will suddenly go up. You'll get flames about a foot high from the middle.

ShawnLG

Tue Jul 24 2007, 05:15AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

"No one makes coil like that because it is a pain to keep the wires spaced out enough, especially since there are only like 4 people that are smart enough to make coils work in the MHz region"

Use fishing line to space the wire when wrapping, it was much easyer than I thought. After tieing down the secondary wire, carefully unwind the fishing line back on to its spool. Then seal the form with polyurethane.

"But on a cardboard former?
Hams use ceramic."

Ceramic has a very high dialectric constant! This would defeat my mission for making this coil. I choosed carboard because I wanted to get away from the lossy plasic formers that I had used on my last few coils. The ideal secondary form is nothing at all, no losses, no added intertern capacitance. Black Plasma's former looks very promising in that aspect.

I tested the coil with my target frequency and it worked. I still need to take some measurements. But there is not much coupling with the primery and the secondary and the SWR is high. High SWR would be a big problem on my class E PA I have yet to design.

Dr. Slack

Tue Jul 24 2007, 07:29AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Ceramic has a very high dialectric constant! This would defeat my mission for making this coil. I choosed carboard because I wanted to get away from the lossy plasic formers that I had used on my last few coils.

Get away from lossy plastic to lousy cardboard!

No one makes these for TC use as the way to higher Q is through lower resistance, which means fill all the available space with copper instead of 50% with fishing line. But it will get you higher Fres.

As far as reducing the quantity of lossy material in the former is concerned, this would be the side effect of an idea I had a while ago, through being unable to source a plastic former bigger than 100mm, or in anything other than PVC. Our local DIY shop sells polypropylene (ideal dielectric material) waste pipes in 32mm O/D, so why not use a bundle of these? Build up to pretty much any diameter, with the result being 50% air. A bundle of plastic drinking straws might do for a smaller coil. Just a thought. The main problem that occurs to me is that as the turns go on, the tension from the later ones might shrink the springy form slightly, loosening the earlier turns. You would need to make a low tension high tension coil

J. Aaron Holmes

Tue Jul 24 2007, 05:53PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

For the oil-immersed coil I'm working on, I had a friend with a lathe and threading jig put threads on an acrylic tube. Should be the easiest coil I've ever wound!

The idea in my case, however, was to improve standoff of the coil when immersed, not about Fres, Cself, etc. We'll find out soon if it was worthwhile!

Cheers,
Aaron, N7OE

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