How to feed spiral antenna in CST?

Hi,
I'm trying to simulate a 2-arm equiangular spiral antenna in CST.
I'm using coaxial feed to feed the antenna.
Anybody knows where i should place the feed?
Should it be in the middle of the spiral or on one of the arms?
Many thanks

Dear Suraya,
feeding of spirals through coaxial cables is very tricky, as you need baluns (check Balanis book for that).
I would suggest you simulate first with a discrete port between the two arms of the spiral, in order to check the antenna behaviour and then study some proper way of feeding!