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Coil still works great with NO earth ground?

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LAN_403

StevenCaton

Mon May 18 2009, 12:16AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

I recently demonstrated my spark gap coil for some friends. So I started the coil at low power and worked my way up to full power, and everything looked great. (long streamers, no arcing, etc.)

5 minutes later they left and I realized that I forgot to clamp the base of the secondary to earth ground.

I know that in a bipolar TC, or a twin system, an earth ground in not neccesary, but I always thought that in a spark gap coil a ground was absolutely neccesary.

I tried the coil at full power with a good solid earth ground, and then with no earth ground, and the sparks looked exactly the same.

This is really making me wonder if grounding the base of a classic spark gap coil is neccesary at all. (at least for my coil)

Thoughts?

Hazmatt_(The Underdog)

Mon May 18 2009, 12:29AM

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

Grounding helps maintain your tuning and acts as a sink for RF current, we know this. I have forgotten to ground the coil on occasion and typically I have lower performance because the capacitence of the system is now different, so to compensate without connecting to the house ground I can connect a large sheet of metal to the base. The result is the tuning is maintained without a large spike in the ground because the large sheet and toroid act as the capacitor plates. If you use the ground stake then effectively your topload is coupled to the surrounding ground.

For a high power coil you're probably not noticing much of a difference, but for my 1kw coil I would expect to lose 20% of my streamer length due to tuning issues. Now I could retune and probably get back most of that.

A couple of times I couldn't figure out why I had to retune after leaving the primary connected in the same spot months ago, but like I say it was just a matter of forgetting to connect the ground sheet.

StevenCaton

Mon May 18 2009, 02:41AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

I can connect a large sheet of metal to the base

I have heard that this method works with a van de graff too. One can connect the bottom brushes to a metal sheet, and the VDG should still have output.

Well, I guess I'll still use an earth ground, but I was pretty surprised when I realized the thing worked the same without an earth ground.

GeordieBoy

Mon May 18 2009, 11:32AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The base of the un-grounded resonator likely flashed over to ground or to mains wiring somewhere that you couldn't see it. If you consider that several hundred kV is produced across the resonator it doesn't take much to jump a few inches from the un-grounded base to the nearest wiring and then find a way back to earth through the mains wiring.

If you run it for long enough without a ground connection, a blown NST or burning smell may lead you to the location of the arcing!

-Richie,

Sulaiman

Mon May 18 2009, 06:30PM

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

I have used a roll of aluminum kitchen foil as a synthetic 'Earth' with excellent results
lay it in one line, a cross, a star .. whatever.
Using a minimum length of about 10x topload diameter seems ok.

I agree with the idea that the base of the secondary is flashing over to somewhere or other.

StevenCaton

Mon May 18 2009, 07:11PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

The base of the un-grounded resonator likely flashed over to ground or to mains wiring somewhere that you couldn't see it. If you consider that several hundred kV is produced across the resonator it doesn't take much to jump a few inches from the un-grounded base to the nearest wiring and then find a way back to earth through the mains wiring.

Yes, you may be right on that. On a spark gap coil that I built a year ago, I forgot to ground the base of the resonator and it was arcing to the primary coil.
The ground connection on the terminal strip (on the coil that this post is about) is right next to one of the input wires for the NST's, so it could have been arcing through there.

Well anyway, this post actually brings up another question. On my old vacuum tube coil, I ran a mains ground, and then one day I decided to hook it up to an RF ground rod as well. So there were two grounds. When I applied power, the coil wouldn't oscillate. (I only applied a little bit of power and then shut if off really fast) So is there something bad about connecting the base of the secondary to two different grounds? (mains ground AND a ground rod)

Dr. Drone

Tue May 19 2009, 05:40AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

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