Desperately need HELP! (MATLAB simulation)

Dear all

I am on a final year project rectenna system for power scavenging. I am currently lagged in the simulation of the Archimedean spiral to study its gain and power radiated at different frequency.

I am using the coding that I got from a book titled Antenna and EM Modeling with MATLAB. I would like to seek for advice.. how come when I change the antenna size, it does not give any effect on the output(impedence, gain and power radiated)?

Thanks a lot