Calculating a counterpoise
Toasty, Wed Feb 03 2016, 05:46PMHi all
I am almost done with my dual mot SGTC and i encountered the problem of not having a good rf ground at my disposal. Now i heard you can solve the problem by using a counterpoise. This is a conductor, usually a sheet of chicken wire or alluminium, with the same capacitance as your top load. Now i have some questions about a counterpoise.
1. If i have a top load of 17.2 pF would a alluminium sheet of the exact same capacitance be sufficient? or do i need some head room or something?
2. I usually see that peoplehave their tesla coil standing ON the counterpoise? is this necessary?
3. How would i calculate the capacitance of a sheet of alluminium?
4. If i have a counterpoise i don't actually need any ground right?
Many thanks
Re: Calculating a counterpoise
Benjamin, Wed Feb 03 2016, 06:37PM
With my tesla coil I have used all kinds of things for a counterpoise from an aluminum ladder standing next to the coil to a pile of metal on the floor. It doesn't seem to matter too much as long as it's fairly large. It doesn't have to be the same capacitance as your topload, which sounds kind of small for a dual MOT coil by the way, I use a 6 inch by 36 inch chicken wire wrapped one. The coil can be standing on the counterpoise but it doesn't have to and the counterpoise does not have to be grounded. I hope this answers you're questions!
Benjamin, Wed Feb 03 2016, 06:37PM
With my tesla coil I have used all kinds of things for a counterpoise from an aluminum ladder standing next to the coil to a pile of metal on the floor. It doesn't seem to matter too much as long as it's fairly large. It doesn't have to be the same capacitance as your topload, which sounds kind of small for a dual MOT coil by the way, I use a 6 inch by 36 inch chicken wire wrapped one. The coil can be standing on the counterpoise but it doesn't have to and the counterpoise does not have to be grounded. I hope this answers you're questions!
Re: Calculating a counterpoise
Toasty, Wed Feb 03 2016, 09:32PM
Toasty, Wed Feb 03 2016, 09:32PM
Benjamin wrote ...
With my tesla coil I have used all kinds of things for a counterpoise from an aluminum ladder standing next to the coil to a pile of metal on the floor. It doesn't seem to matter too much as long as it's fairly large. It doesn't have to be the same capacitance as your topload, which sounds kind of small for a dual MOT coil by the way, I use a 6 inch by 36 inch chicken wire wrapped one. The coil can be standing on the counterpoise but it doesn't have to and the counterpoise does not have to be grounded. I hope this answers you're questions!
thanks, I guess a good counterpoise would ideally have a bigger surface area than your toroid. But can there also be too much? Or does it have little to no impact if you have a bigger area?With my tesla coil I have used all kinds of things for a counterpoise from an aluminum ladder standing next to the coil to a pile of metal on the floor. It doesn't seem to matter too much as long as it's fairly large. It doesn't have to be the same capacitance as your topload, which sounds kind of small for a dual MOT coil by the way, I use a 6 inch by 36 inch chicken wire wrapped one. The coil can be standing on the counterpoise but it doesn't have to and the counterpoise does not have to be grounded. I hope this answers you're questions!
Re: Calculating a counterpoise
Benjamin, Wed Feb 03 2016, 10:00PM
No, you cannot have too big of a counterpoise. It has no impact if you have a bigger surface area.
Benjamin, Wed Feb 03 2016, 10:00PM
No, you cannot have too big of a counterpoise. It has no impact if you have a bigger surface area.
Re: Calculating a counterpoise
Toasty, Fri Feb 05 2016, 03:49PM
Toasty, Fri Feb 05 2016, 03:49PM
Benjamin wrote ...
With my tesla coil I have used all kinds of things for a counterpoise from an aluminum ladder standing next to the coil to a pile of metal on the floor. It doesn't seem to matter too much as long as it's fairly large. It doesn't have to be the same capacitance as your topload, which sounds kind of small for a dual MOT coil by the way, I use a 6 inch by 36 inch chicken wire wrapped one. The coil can be standing on the counterpoise but it doesn't have to and the counterpoise does not have to be grounded. I hope this answers you're questions!
Did you connect your strike rail on your counterpoise too?With my tesla coil I have used all kinds of things for a counterpoise from an aluminum ladder standing next to the coil to a pile of metal on the floor. It doesn't seem to matter too much as long as it's fairly large. It doesn't have to be the same capacitance as your topload, which sounds kind of small for a dual MOT coil by the way, I use a 6 inch by 36 inch chicken wire wrapped one. The coil can be standing on the counterpoise but it doesn't have to and the counterpoise does not have to be grounded. I hope this answers you're questions!
Re: Calculating a counterpoise
Benjamin, Fri Feb 05 2016, 06:25PM
I did not have a strike rail at the time, but if you have one, you would connect it to the counterpoise.
Benjamin, Fri Feb 05 2016, 06:25PM
I did not have a strike rail at the time, but if you have one, you would connect it to the counterpoise.
Re: Calculating a counterpoise
Toasty, Fri Feb 05 2016, 08:19PM
Toasty, Fri Feb 05 2016, 08:19PM
Benjamin wrote ...
I did not have a strike rail at the time, but if you have one, you would connect it to the counterpoise.
Thanks, will try it out tomorrow I did not have a strike rail at the time, but if you have one, you would connect it to the counterpoise.
Re: Calculating a counterpoise
Dr. Slack, Sat Feb 06 2016, 08:32AM
Can it be too big? An infinite sized counterpoise is called a ground.
Dr. Slack, Sat Feb 06 2016, 08:32AM
Can it be too big? An infinite sized counterpoise is called a ground.
Re: Calculating a counterpoise
Art, Wed Feb 17 2016, 02:14PM
The counterpoise is a capacitor formed between the Earth and the metal sheet,
and provides an AC/RF coupling to Earth where stakes can’t be driven.
The value of the capacitor will vary with the elevation of the sheet from Earth,
whether or not that is a big difference, i could only guess.
In the case of a Tesla coil, you’d have a second capacitor formed between the sheet (in place of the Earth) and the toroid.
Art, Wed Feb 17 2016, 02:14PM
The counterpoise is a capacitor formed between the Earth and the metal sheet,
and provides an AC/RF coupling to Earth where stakes can’t be driven.
The value of the capacitor will vary with the elevation of the sheet from Earth,
whether or not that is a big difference, i could only guess.
In the case of a Tesla coil, you’d have a second capacitor formed between the sheet (in place of the Earth) and the toroid.
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