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newbie question on s parameters

Question asked by fcolleon on Nov 20, 2009
Latest reply on Nov 24, 2009 by fcolleon
Hi all. I have a doubt about s parameters and voltage signals. For example, suppose i measure s21. i set the output power on 1dbm(pna ports). Then, if s parameters are ratios of voltages, i can obtain the voltage in the port 2, if i know the voltage in port 1. But how do you know the voltage in port1? Knowing the power, if you know the impedance, youc can obtain the voltage(Power=voltage^2/impedance*). But what impedance? In my case, i want to set a antenna measurement system, so, the path should be: port1-cable-waveguide transition -transmitter antenna -air-receiver antenna- waveguide transition-cable-port2.
Then, what is suppose to be the impedance? The combined impedance of all elements, or the system impedance(50 ohms).

Thanks for your help.