dipole antenna in HFSS

Hi,

I'm new in HFSS and I'm trying to simulate a simple dipole antenna over a PEC ground plane,but the rosonant frequency of structure,in return loss curve,is not true according to my reference paper(about -3.5dB at 12.7GHz).
I think it's because of the excitation(I defined a lumped port excitation),I have some questions:

1-Is this difference(about 0.8GHz),normal between HFSS and FDTD?
1-how I can define a diffrential port feed for this dipole in HFSS?
2-I defined height of the air box lambda/4 of solution frequency from the dipole,and also put a PML over it,is it true?
3-how can I get the reflection phase curve of PEC surface(180 deg. for all frequencies)?

I have attached my simulation file.it's in HFSS v10.
can anyone help me?

thanks