Nizar,
What I pointed out in my previous post is that your characteristic impedance measurements were not in accordance with well defined transmission theory.
Your results showed a rise in characteristic impedance and then considerable rolloff especially near 100 MHz. What should be observed is a slow decrease which flattens out at VHF. This can be seen in the graphic I posted.
After a few MHz the equation is Zc = SQRT (L/C) where L and C are the inductance and capacitance per unit length respectively. C can be considered constant with frequency while L decreases with frequency. L consists of internal and external inductance. The skin effect and the fact that internal inductance of the inner conductor decrease to zero at high frequencies are the prime reasons for this.
Roger Need