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.

Then, what is suppose to be the impedance? The combined impedance of all elements, or the system impedance(50 ohms).

Thanks for your help.

http://cp.literature.agilent.com/litweb ... 2-1087.pdf

It's an old note, but it has lots of basic info.

S21 is the square-root of the power gain. This gain is independent of actual source and load impedance, as long as you agree on a reference impedance (typically 50 ohms). We use "error-correction" to re-normalize the value of the S21 if the source and load impedance are not exactly 50 ohms.

Once error correction is applied, you can get to voltage gain (though almost no one uses voltage gain in any RF work), by computing the voltage at the input port (from S11) and at the output port (from S21). You use S11 to compute the input impedance, and from that you can compute the voltage divider assuming the source has 50 ohms impedance. The voltage of the voltage source is the voltage that would give you the Pin or source power value if the input impedance were 50 ohms.

Now, the S21 squared gives you the power gain. You can compute the power delivered into a 50 ohm load by multiplying the power of the input, by the S21^2. Then you can compute the voltage given this power and the ref impedance of 50 ohms. Now you have the input voltage and output voltage and you have the voltage gain.

But, as I said, no one in the RF world uses voltage gain, they just use power gain, and that is just S21^2.

Now, so you don't get confused, when I say S11 or S21, I mean the linear complex number. When we compute "Log Mag" we use 20*log10(S21) which is the same as 10*Log10(S21^2) so you can see that the dB value of S21 is exactly the power gain. This was done for convenience.