HV loss is hard to estimate. You could make your shunt wire a shunt tube or
rod and basically eliminate corona (A rule of thumb is that 10-15 kV/cm radius
will have virtually no corona.. so for your 6kV, a 1cm diameter tube is in the
right ballpark) Skin depth at 3.5 MHz in Aluminum is .043mm, and the usual
rule of thumb is to make the tubing wall thickness 3-5 skin depths. 0.12-0.20
mm seems about right. Copper could be thinner wall (skin depth is less)

I presume that the corona effect should be visible at dark. So far I have seen
none, but we had not a single day of rain since I mounted this antenna
(incredible summer season...). So, I must verify when rain will come, in a few
days from now they say. Or do you think that corona may not be visible?

I am using a 4-mm diameter wire so, if I will really have corona problems, I
could insert the (vertical) wire into a 1-cm aluminum tube connected at the very
bottom of the wire (i.e. at the antenna feed point). Probably a 2-meter long
tube could be sufficient (RF voltage gradually diminishes getting away from the
antenna feed point).

73

Tony I0JX