i study about these kind of antenna a bit previously. i send you this basic paper. maybe it can help you. you should use these basic formula and after that simulate them in ADS simulator. 

Hello Aju,

you find attached a useful document on the equivalent circuit. you can export your design from CST as dxf file.

B.regards

Hi 

Aju Johnkk

Find the pic attached

Hi

Aju Johnkk

I hope that will help you! this design for 13,56 Mhz if you want other frequency just send it to me.

http://www.nxp.com/documents/application_note/M011732.pdf

Hi 

This a document from CST site. I hope this will help you. 

Follow this link to get the information.

https://www.cst.com/Applications/Article/RFID-Reader-Coil-1356-MHz

Thank you all for your kind attention and valuable answers

Aju Johnkk dear hope you will be doing good.

do u get your question resolved?

as i am also interested in same field

looking forward to hear from you soon,