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HM_Murdock

Mon Sept 20 2010, 12:39PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

I need a better understanding of the basics of the RF ground used with a TC.

From what I can tell, the RF ground serves as a plate in the capacitor setup of the secondary, with the topload being the other plate. Obviously, the discharge is coming out of the topload...

A few questions...

1- What's going on with the ground side of things...why does the RF ground not arc?

2- If using a counterpoise, will it arc to other nearby grounded objects?

3- When using a earth rod, is there a radius around the rod where the ground is dangerous (need to take care to make it a certain distance from other objects, etc...)?

4- When setting up a grounded strike point to test arc length, do you connect that to the RF ground, or does it need to be a second ground source for better results?

ScotchTapeLord

Mon Sept 20 2010, 01:43PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

In a capacitor, the charge spreads itself on the surface of the conductor. If you look up commercial toroids, you'll see that the bigger they are, the higher voltage they are rated. How can a hunk of aluminum have a voltage rating, you might be asking. Well, most things break down when their rated voltage is exceeded. Toroids, rather than break down, break out! So that's what we're doing with a tesla coil- we have made a lossy capacitor. Now consider the earth as the other plate. Well, if the bigger the toroid, the higher the voltage rating, then what do you think the earth is rated? For our purposes, it is rated for infinite voltage. The charge is so concentrated on the relatively small surface area of the toroid, which is why it breaks out. The charge is spread out much more, as you could imagine, when applied to the earth. So does charge build up on the earth? Yes... but negligibly per a given area. Replace the earth with something smaller, and it will develop a higher charge per given space, and perhaps break out...

Ordinarily we don't want this, but not everybody here is ordinaryl!

HM_Murdock

Mon Sept 20 2010, 02:46PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

That explains a lot...

Makes sense now why I was getting arcing between the primary and secondary with a weak RF ground (small counterpoise). I have noticed the bigger topload I use, the stronger ground I need.

Thinking of running a line out the back door and down to the ground level (kitchen is on the 2nd floor). It will be out of sight when I am indoors.

Any advice on questions 3 and 4 above?

ScotchTapeLord

Mon Sept 20 2010, 03:55PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

3- If it is properly grounded, then it should be safe. Still, there could be some voltage across the inductance of the ground wire, so I wouldn't go too-too near it with anything.
4- In most videos I see with ground strike targets, there is a wire going from the base of the coil to the object, rather than just stuck in the ground. Not sure how big of a difference it makes, though.

RF ground connections must be solid, short, and thick. If it's long it will have significant inductance, and then becomes part of the resonant circuit, like windings on the secondary, increasing risk of primary flashover and general bad stuff.

HV Enthusiast

Mon Sept 20 2010, 06:22PM

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

RF Ground is nothing but a reference point that your Tesla coil will operate in relation too. Nothing more. Earth ground is used because everything else surrounding the coil is typically sitting on top of earth ground, and it also provides a bit of safety as well as the base of the secondary would be sitting at . . . well earth ground.

You could just as easily reference the base of the secondary to 30kVDC or even 14.4kVAC! Of course, you now have a direct connection between output arc and whatever supply or tranformer you're connected too which could make your Tesla coil ultra-lethal. Plus, you have to worry about additional isolation requirements now that your secondary is floating.

So as you can see, earth ground just makes the most sense.

And to why the RF ground needs to be a good, solid electrical connection . . . if you have very high peak currents in your secondary base, if the ground connection has a high impedance, you could have high voltages induced on those ground connections, which could potentially damage other equipment connected to that ground. Hence, why using your 3-prong electrical outlet for RF ground is a bad idea.

But for your 9kV/30mA NST powered coil, i would have no issues whatsoever connecting it to the 3rd prong outlet ground. Just keep your computer and other sensitive equipment disconnected from that same outlet.

HM_Murdock

Mon Sept 20 2010, 07:07PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Ok, this simplifies my life greatly...

I will try this out this afternoon and report my findings!

**EDIT**

Have changed RF connection from my counterpoise over to the mains ground...

Now getting 16" strikes to a grounded wire.

Also, on a somewhat interesting side note, I am doing this plugged into a GFI outlet in the kitchen...when the coil is tuned better, the GFI doesn't trip when I switch off the power strip. When poorly tuned, it trips it every time I kill the power.

Going to take all my functioning parts and assemble them with solid connections in a base...

HM_Murdock

Tue Sept 21 2010, 03:22AM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Finally got my TC in a self-contained platform with properly connected (short) wires. Also reversed my plexi so my primary is underneath it. Looks cooler and lessens the risk of accidental contact (plus, reduces the chance of a strike from the topload I would assume)

Something made a difference (I assume it was the solid connections instead of alligator clips), cause I am now tapped out on turns and am getting some nice, fat streamers to air with my big toroid added.

happy, exhausted, excited...like a kid on christmas eve...

Plus, my wife is happy that I finally have returned the kitchen table to the family...

Pic of (98%) completed unit, and video of it in action with a breakout...I think it's lonely for the fridge personally...lol

How much more output is a 9kV/30mA NST capable of pumping out anyway?

Check out the video and see what you think...

]rebuilt.wmv[/file]

Proud Mary

Tue Sept 21 2010, 11:34AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A good Earth will also act as a virtual 'mirror,' as it does with other vertical resonant elements, improving the antenna efficiency of your coil, so it radiates RFI pollution over a much larger area...

porkcake

Thu Sept 23 2010, 09:11PM

Registered Member #3138 Joined: Tue Aug 24 2010, 05:20PM
Location: Herefordshire
Posts: 7

thats looking great Howling Mad, your questions answered a few for me to, thanks.

i'm using tunsten rods for my SSG i think they're thoriated, tig welding electrodes. more hazards to be aware of!

i'm also using a MMC polyprops for the cap bank, they seem to be forgiving as this is my first build.

i would connect straight to the rf ground, if you connect with two rods a few metres apart, it could be a big potential difference?

nyditch

Fri Sept 24 2010, 06:34AM

Registered Member #3237 Joined: Fri Sept 24 2010, 02:50AM
Location: California
Posts: 1

Hmm, this could be related to mine as well. I've been having trouble getting good streamers. Although I plan to improve my spark gap and tap, could my RF ground be causing a problem?

I thought a 1/16" steel plate as the base of the TC as well as RF ground would work, but could this be hindering the resonance by sheer size?

The secondary has a contacting wire that touches the plate (about 14"x24") in the middle.

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