A wire antenna illuminated by plane wave or using port

Hi all,

I ran a simulation of a wire antenna (PEC) and wanted to see the RCS polar plot using a far field monitor.

I did it in two ways:
1. Used a port as my excitation source.
2. Used a plane wave as my excitation source.

Now,
When I set the antenna's lengths above one wavelength the results became different between the two cases.

Can someone tell me why is that?

Which software do you use? Could you attache corresponding projects?

I'm using CST. I will try to upload the files later on.

My question is basicly this:
Are the two cases I stated in 1,2 similar in any way?
Or should I expect to see totaly different results?

Thanks.