how to measure impedance in RF circuit

i had this query as to how is impedance measured in RF circuits. in RF we generally specify power and not the voltage and current levels, so how do we measure the impedance?

Use a Network Analyzer. Use the Smith Chart display mode.

hi,
do it like Element_115 says. Measure the S11 parameter in the Network Analyzer, which shows the ratio between the incident wave and the reflected wave. The network analyzer computes the input impedance and plots it on the Smith Chart (Zin=R+jX)
Regards