Design Pyramidal Horn using CST

Hello, I am amie and new in this forum. i need to design pyramidal horn using cst with the requirement frequency range 10-15GHz. can anyone tell me, what is the formula to calculate the length of waveguide of horn?i still cannot find this formula.TQ

Download the free evaluation of Antenna Magus. The horn antenna is included. Just input your frequency range and click "design".
You can then export the model to CST if you want and import there for further playing around.