Unwanted Spikes In Retrieved Refractive Index

In the process of retrieving Mu and Epsilon from scattering parameter by following D.R Smith's paper Electromagnetic parameter retrieval from inhomogeneous metamaterials i got unwanted spikes in real part of refractive index which is as shown re(n).bmp.I could not able to understand why i am getting this?Is it due to anti resonance?Any suggestion will be of great help.

Thanks