suspended microstrip line from 0.5GHz to 20 GHz

Hi all

i am trying to design an air suspended transmissio line which should work from 0.5GHz to 20GHz.

i have calculated its width dimension using CST built in micro .
but i can not get the good results for the my desired frequency range.

the S21 becomes worse as frequency increases which i think is due to miss matching. i want a smooth horizontal line close to 0dB throughout the range assumng low loss dielectric .

so i am just wondering how to create such a transmission line which is matched for desired frequency band, the cST built in macro gives the width of the line for the desired 50 ohm characteristic impedance but i think this width may not work for my desired frequency range.

anyone having some experience in this area , i would appreciate their comments

thanks