what is impendans?

RF input signal will modulate the active device bias point,I mean there is a dynamic load line.At device output termination,is's superimpose of both RF and DC voltage.So it is very important when bias RF device use a costant voltage source.The output RF voltage will be determined by output impedance,in another words,input impedance of output match network.
I think I maybe have also some mistake on this topic.So I want discusse with all guys deeply.

Actually, this is related what it happens today as supply voltage scale down. Imagine an emitter made on a 90nm CMOS technology. 1.2V is a maximum supply limit. In this case, the 50Ohm antenna must be seen by the PA as a lower impedance (by means of a well suited matching network) an so keep the emitted power contant.

as i know
50Ohm is not R
50Ohm is (L/C)^0.5 ( or SQRT(L/C) )