LAB wrote:
I have been surprised for the twin lead problems because i had already
made another Slim Jim using 2mm bare copper wire on the sides of a black
"electric" plastic tube (d=about 2cm) and I get the same good performances
of the copper tube...

The velocity factor of the insulated twinlead is much
lower than bare wires. If you don't understand velocity
factor, please study up on the subject. Velocity factor
can make as much as a 20% difference in element lengths
between bare wires and insulated twinlead. The
characteristic impedance is also different causing the
50 ohm feedpoint position to change.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com