Relation between S11 and VSWR in FEKO

Hi everyone,

I would like to understand the relation between these 2 parameters and the real function in POSTFEKO of the checkbox "substract loading" in a source data graph.

In EDITFEKO, I designed an antenna. I used LZ card on my feed segment (Re part: 50, Im part:0) then A1 card (Magnitude of source: 1, phase : 0) and to finish, OS card (all currents chek).

Here are some graph to present my problem:

Figure 1

Figure 2

Figure 3

So you can see that blue line in Fig1 and Fig2 are the same, which correspond to a S11 courb Mag part without substract loading of 50 ohm. In Fig3, VSWR is quite different if I check the box or not. I have always considered the red line (in FIG 3) as my real VSWR because it is written in the documentation provided this :

" For Impedance and Admittance as well as S11 and VSWR calculations, an arbitrary load can be substracted by checking the Substract loading fild. (This is required, for example, when the user loaded a port to simulate the transmitter / receiver impedance.) The load can be specified as an Impedance or an Admittance. If the source is a voltage source, the load is substracted as if it is in series with the source; for a current source, it is substracted as if it is in parallel with the source. "

I have to say that I'm not at ease with theses notions of impedance so maybe the answer is simple. Forgive my unknowledge and my english (i'm french native).

Any advice would be great.
Thanks

Maybe my problem isn't well described so don't hesitate to ask for more precisions.