Is it possible to get the equivalent circuit of antenna being modeled in HFSS/CST. I mean, to extract the equivalent circuit from one of those softwares? any idea to get that in ADS or even within the CST itself.
Yes, it is, but you can extract data from CST/ HFSS. One thing you can do if you want to modeled the same circuit in the same software like CST, you can open that circuit in CST design studio and there you can change anything which you want.
I am no sure what exactly you are trying to do. Finding the equivalent circuit of an antenna is not easy. Most of the models you see are only mathematical model that models a specific frequency range. Some antennas has know models like a series RLC or shunt RLC at one of the resonance frequencies
As soon as, I will publish new article talk about the equivalent circuit of filters, you can use same steps to find the equivalent circuit of antennas (I tested that).
It is possible if you have the HFSS original software. In HFSS there will be HFSS designer where you can design the equivalent circuit of your Antenna.