My coil used a 5 foot copper pipe as the main ground. It should be at least 3/4" in diameter. It is hammered into the ground with about 2 to 4 inches visible above ground. The pipe will get a bit crushed at the top, but that is not a problem. The main grounding strap is attached to this end with a round screwdriver adjust type clamp.
The following parts of the Tesla Coil need to be attached to the grounding strap: the bottom end of the secondary coil; the metal transformer case (if there is no case, connect the iron core to the ground strap); the middle electrode in the safety gap. You connect the transformer and the secondary to the ground strap by attaching a section of 8 gauge wire to these components, and then solder these wires onto the grounding strap. This does not need to be done for the safety gap, since the grounding strap is connected directly to it.
You will need a way to protect the visible part of the copper pipe from weathering between uses. I did this by putting a piece of saran wrap tightly over the end, and securing it with a rubber band.
When you are ready to run the coil, pour a little more than a gallon of water around the pipe. (Unless, of course, the ground is already soaked from rain.) This will ensure maximum grounding of the Tesla coil.

Cost of Grounding
1. The grounding strap was bought from American Science and Surplus. I found that two pieces of the 18 gauge wire from Electronic Goldmine worked fine.

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