(CST question) transmission coefficient

Hello everyone,
I am a new CST user. I have to simulate the attached structure to obtain the
transmission coefficient S21(dB).
Could anyone please let me know how to get it?
Best regards,
Micasa,

you can use probes along with plane wave excitation to get the transmission coefficients. Probes measure the electric field in time at the point where you insert it. By using DFT the time signal is converted into frequency domain information. You can further improve your result by adding lets say a row of probes to get the electric field at different positions and then you can average the intensity. This will give you close results to the experimental situation where you wold normally measure the transmission by waveguides or horn antennas.