S21 of dielectric rod antenna shows ripples in CST simulation

Hi There,
I have been working on dielectric rod antennas. In order to check the power received by the antenna i used two dielectric antenna at a distance and measured the S21.
I am using CST 2010 for the simulation.
The S21 response obtained w.r.t. frequency is not smooth curve rather too much ripples are there in response. figure of the response is attached here.
I would like to understand is it because of antenna characteristics or some simulation parameter setting in CST?
please help me understand the cause of non smooth behavior.
with regards,
lkminz

You do not say what the antenna separation is. If they are too close you will get multiple refections between them and that could explain your results
Oedt

I agree with oedt, but to get rid of this effect you should put the antennas in far field. This can become a huge challenge for CST in terms of mesh cells.
You should do it differently and characterize the antenna with antenna simulations: Far field planar wave radiation patterns in Ludwig 3 coordinates are very useful for example. This way you can derive the directivity.
The propagation of a planar wave in between both antennas is textbook material.

thankyou very much for the help
i understood that indeed the ripple response is due to reflection of signal either internal or external. As now i know the reason i am working to reduce this effect, thanks everyone for the response.