Forum Post: Factors affect the Inverterd F Antenna impedance?

Hi all,
I'm having a project related to Bluetooth 4.0. I have decided using CC2540 - 2.4GHz Ble SoC. In some suggested antenna in DN035, i have choice Inverted F Antenna and have make an exact copy of the dimension in DN007. This design note say that " the impedance of the Inverted F Antenna is matched directly to 50 ohm no external matching components are needed". But I too confuse if i copy this antenna and use it with different material, impedance can still equal to 50 ohm? Simulation IFA antenna in ADS 2008 with FR4 material, permittivity is 4.4, substrate height of 1.2mm, tanD = 0.022, simulation result of impedance not equal 50 ohm.
I think impedance of antenna is depend in parameters of material and what should i do to make an exact 50ohm antenna?
Thanks for help and sorry for my bad english.