Conductivity of the antenna, RF module

Dear All,

I have question concerning optical antennas.

I have model which consist of the optical antenna on substrate, and I want to calculate scattering cross section and field enhancement.

My question is about sigma (conductivity). Does any of you know what is the difference between case when we put that antenna is perfect electric conductor (sigma = 0), and case when sigma is depend on wavelength of illumination of that structure?

So actually I need some theoretical instructions if any of you is familiar with that.

Regards and thank you