Altium: impedance controlled line on 2 layers PCB

[SG_34]

Altium : impedance controlled line on 2 layers PCB

Hello,

I'm just routing a simple board with SigFox TD1207 module and antenna.
PCB 2 layers, thickness=1.6mm, copper layer=0.018mm (top and bottom)

I need to control impedance line for antenna, must be 50 Ohms, on the bottom layer GND Plane.

I'm using Altium, specify a PCB Rule (impedance = 50Ohms), but it seems doesn't work, trace width = 0.254mm
I try to modify impedance, and width is alway 0.254mm even if I specify 200Ohm!!!!

Why it doesn't works?

Thanks for your help

Steph