calculate the parameters S21 of antenna

hi
I want to calculate the parameters of the antenna . I use a transmitter and a receiver antenna

both are attached to port 1 and port 2 of zbv8
so I want to calculate the S21 parameters must be calculated theoretically how
thank you

This is one method to measure antenna parameters: antenna-radiation-pattern-measurment-arpm-1-2-5
Free field is possible to use instead of a anechoic chamber.

A single antenna doesn't have a a S21 parameter.
With the configuration of two antennas, you are measuring the S21 parameter between these antennas. It depends on the antennas, the distance, the alignment and other factors.

The vast majority of antennas don't have an S21 parameter, since they have a single port

Since you have two antennas, you can't measure the S21 of any single antenna, but you can measure S21 of the system consisting of the antennas, the space between them, cable losses etc. As someone else pointed out, what you get will depend on many things.

For something like a pair of half-wave dipoles, it is probably possible to calculate S21 theoretically in free space. By this, I mean you can probably compute not only the magnitude, but also the phase, though I'm not 100% sure if that can be done.

What's usually of more interest with two antennas is the magnitude of S21 - the phase is of less relevance for most applications. Then you can measure the gain. If you know the gain of one (use a dipole), or the two antennas are the same, then its pretty easy to compute the gain of the antenna under test by comparing it to the free space path loss. This of course assumes you can replicate free space conditions properly.

Why are you trying to do this? If its an exam question, you might as well post it, then people could give you some hints when you tell them what you have done. If its for a practical case, describe exactly what you want.

Dave