Hi everybody, for PIN Diode simulation in CST you can applie two method:
1) you can simulate the PIN diode by its equivalent circuit (RLC)
2) Or you can simulate the PIN Diode like this: In the OFF mode, the PIN Diode is removed from the simulation and the antenna should be simulated with a complete slot. When the PIN is ON, it is modeled as a metal tape.
You can see this article, it will be very usefull for you:
https://onlinelibrary.wiley.com/doi/10.1002/mop.27649/full
good luck and welcome for any question.