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Here’s a quick sketch to hopefully clarify the idea..
For the sake of discussion, let’s assume the intentional radiating element of the antenna is a copper pipe 1 inch (25 mm) in diameter and 3 meters long. It would be represented in the sketch by the black cylinder.
The ground is shown by the green cylinder, and let’s assume it to be approx 5 mm.
The ground is enclosed by the radiator except for the amount at the bottom which is necessary to take it into the “dirt”, and wouldn’t be longer than is necessary to keep the copper pipe from being shorted to ground by grass, weeds, rain, etc.. Probably a foot or less, so for the sake of argument lets call it one foot or approx 30 cm.
In practice it probably wouldn’t need to be even 30 cm, but I’d guess that would be typical for the actual distance between the ground terminal and the point it enters the actual soil if a transmitter is mounted at ground level. Obviously we don’t want the copper radiator touching the soil, but basically we’d be trying to use the minimum reasonable and practical distance to accomplish that.
For the sake of simplicity, I have not shown power supply or audio wires or etc. Those would vary depending on the particulars of the transmitter, and would be inside the green cylinder and using the green cylinder as their ground (and audio shield if necessary) to avoid having multiple ground wires each taking a separate path to the transmitter. Which makes sense for avoiding ground loop effect “hum” as well as minimizing any contribution such connections otherwise might make to antenna length or radiation.
PS (added on the edit).. all leads would be kept as short as possible. Since transmitter designs and connector choices vary, it can be ignored for the moment or whatever amount one feels the practical minimum can be used.