LAB wrote:
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

The plastic tube had mostly air between the conductors.
The insulation on 300 ohm twinlead or window line is solid
dielectric around and between the two wires. VF is easy
enough to measure with something like an MFJ-259B.

I measured the VF of 300 ohm window line at 0.8
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73, Cecil, IEEE, OOTC, http://www.w5dxp.com