Dr. Slack wrote ...

That counterpoise looks great.

From a connector somewhere on that mesh, run a wire to the bottom of your secondary, or the bottom of the secondary I sense current transformer, and to your TC power supply mains ground. If you are going to operate the TC in that position, then a radial out to your garage door would be good. Are there any other significant areas of conductor that either might get hit by streamers or present a large capacitance to the topload (like steel joists above our head out of shot)? If so, run a radial to them as well.

Thinking about a detailed page, maybe three illustrations would do it. On one extreme, a TC operating in the middle of a field, with secondary bottom connected to the ground. On the other, operating inside a Faraday cage, with secondary bottom connected to the floor of the cage. Then a third showing how any real situation is usually somewhere between the two. But always, the plan is to get the current back from things that are capacitively coupled to the topload back to the secondary bottom terminal, without undue inductance, and without producing inadvertent large voltages between innocent bystander conductors.

Depending on your operating position, the real earth may, or may not be, a conductor that has significant capacitive coupling to the topload. If it is, then connect to it by the shortest means, whether that's an earth spike, or your mains protective earth. If it isn't, then forget about it.

The nice thing about a counterpoise is that it's an organising principle, as much as anything. It's a cheap and easy to provide terminal that does have much coupling to the topload, and it's in the right position to connect with a short wire to the secondary, and back to the TC PSU ground. You'd connect any arc target to it as well. Then having done that, you look around. What else does the topload see? Garage door? Radial to that.

Sulaiman wrote ...

As you have a steel roof you may want to consider inverting your TC
use the roof as 'earth'

The downside is access to the primary circuitry,
the upside is - a frikin TC in the roof !

(no floorspace consumed, a chandalier of lightning etc.)
(I've not tried, just dreamed)

Dr. Slack wrote ...

Can you confirm that I also need to connect all this steel to mains earth as well

They are already connected to some degree by the capacitance. The question is, do you want to manage that connection, or just let it happen?

The primary is being driven by electronics which are directly coupled (OLTC) or heavily capacitively coupled (if you have an isolation transformer in there) to your mains earth, live and neutral wiring. Consider the secondary bottom terminal (SBT) as your 'star ground', and connect everything to that. However, the distance from the counterpoise to the SBT is pretty small, so it's more convenient to connect to that.

As an exercise in method acting, think of yourself as a piece of charge, leaving the topload in any (or all) directions. What's the first thing you hit? How do you get back to the SBT to complete the circuit? If it involves going through other impedances (a long wire has inductance, an air space between conductors is a capacitor) then it will generate a voltage across that. That's how spikes get into household wiring. Charge lands on a live wire, and if the live has a high impedance to earth at RF, a large voltage is generated live to earth by the RF current flow. That's why (a long time ago, maybe a previous thread) I said have capacitors between live and ground and neutral and ground at your TC power supply. This is to provide a low impedance between these to reduce the voltages launched into the rest of the house wiring.

Your garage would be a Faraday cage if the large conductive elements were all connected together securely. So that would be the case if it turns out you have conductive uprights which the roller door and conductive roof are bonded to. A few radials from the SBT to the bottom of those uprights would be more use than a wire out to a ground spike, would have less inductance. That's assuming you're not going to put a spike through the concrete (which is possible, only needs a small hole and a discretely mounted terminal) which would connect to the ground and your uprights in one go. But that spike in the floor is to connect to your uprights and the conductive layer beneath the concrete, not because it is called 'earth'.

There's nothing special about ground, or the earth, it's just a conductor. If it's near, it's relevant. If it's far, then other conductors are more relevant.