ilam wrote:
I am in a process of understanding the importance of S-Parameter at
high frequencies, please answer my following questions:

1. Why Measuring a wave (voltage or current) is more easier than
volage and current? ( Why measuring S-Parameter is more easier than
other parameters?)

The reflection coefficients are measurable physical entities
and there is a well-defined procedure for measuring them.
Virtual reflection coefficients, such as the ones used by
hams, are not used with S-Parameters. That's an advantage.
The voltages are normalized to the characteristic impedance
of the transmission line and everything obeys the rules of
the wave reflection model.

There are certainly other methods of analysis, h-parameters,
Z-parameters, etc.
--
73, Cecil http://www.w5dxp.com