Antenna Efficiency in CST MW STUDIO

In CST MW STUDIO, for my antenna design observed that negative efficiency values in dB? Can any one suggest me what might be the problem?

yes, in general efficiency will be less than 1 if you talk about absolute value.......so if you take the log you will get negative value.

Hi Sandeep,
I am trying to design and transmitter and receiver loop antenna in hfss. I am getting a proper reflection and transmission behavior of loop antenna but how to extract the loss and efficiency of antenna using S parameter I dont know. I am new in this field. from theory I now, squre of mod of S21 is directly propotional to the efficiency but how this relation works I dont know. If you could help me it will appreciate that.
Thanks
Krishna