THE Plastic Holder effect on antenna Performance

Hi All,

I would like to ask about the effect from plastic holder on antenna performance. Practically, i put ABS plastic under the planar antenna, the resonance frequency shifted slightly. However, if i assigned ABS plastic (RELATIVE Permittivity = 3) in HFSS, then frequency shift severely. I am so confused.

I would appreciate if someone could give his/her hand to help me.

Best Regards,
LE