Problem with negative imaginary impedance when simulating RFID tag on CST

Hello,

When i simulate an RFID tag on CST i get a negative imaginary impedance at the 13.56 MHz frequency.
Can anyone explain this to me please?

Too little info you give here. Anyway the most possibility is due to the uncareful simulator usage.

well, what simulation should i be using?
Transient or Frequency ?

Are there any special settings to be done?