I had already made another Slim Jim using 2mm bare copper wire on the
sides of a black "electric" plastic tube

The velocity factor of the insulated twinlead is muchlower than bare
wires. If you don't understand velocity factor, please study up on the
subject.

I know what is velocity factor. It is determined by L and C. In the
"other" Slim Jim a plastic tube was between the 2mm wires, but performances
was similar to the 6mm tube antenna, in which only air is between the tube

--
Gianluca