First off - there's no need to *ground*, as in connect to earth, a TC. You can run a TC in orbit, or hanging from the roof of a building, or...
What you need is a path for the RF currents to flow back to the base of the secondary.
Now, one (poor) way to do this is to drive a stake in the ground, run 100 ft of wire to it, and use "earth" as the return path. Your RF currents leave the topload, go out, touch the earth, flow through the soil, to your ground rod, then travel through your ground wire and back to the coil. (yes, I know RF is actually bidirectional, but the conceptual model is easier this way).
As that RF current flows merrily through the soil and wire, it radiates and couples to other things in the neighborhood, some of which are not going to like high power pulses of 200kHz RF.
The coupling is worst when the current path forms a big loop and when the victim has a big loop. After all that's what tesla coils are, internally: you have one loop, the primary, that's coupled to another loop, the secondary. We have lots of turns, so the coupling is good for a small area.
So, the objective should really be: put a conductive layer directly under the tesla coil, so that the RF path is directly from "topload to conductive layer" and doesn't go meand
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