Convert patch antenna into a twoport network...

Hi,

Suppose I simulate a patch antenna. What do you suggest, in order to convert it to a two-port model?

Here's a method:
Using the input impedance of the antenna, and supposing the radiation efficiency equals to 1, we got:

Zin (known from simulation)= Rr+jXantenna + Rl(equals to zero because of radiation efficiency)

On the other hand, we got this formula:
Zin=Z0 * {(1+S11)/(1-S11)}
Z0=50;

So we got mag(S11) and phase(S11).

Now, how to calculate S21? (what is the relationship between S11 and S21, both phase and magnitude)

Cheers.