CST ： How to simulate a coaxial cable properly

Dear all,
I'm trying to do some measurement of coaxial cable on CST.
I found out that if i change the backgrounds conditions or simply the background material, results will be completely different.
I want to simulate a coaxial cable with these dimensions :
-inner conductor outer diameter : 1.52 mm
-outer conductor inner diameter : 3.5 mm
The inner conductor is PEC , the outer conductor is PEC , the dielectric is AIR.
The frequency range is 1 - 32 GHZ.
I'm using these settings:
- waveguide ports on the faces of the dielectric
- Time domain solver ( using the default settings )
- Background material : Normal ( without changing any properties )
- Boundary conditions : Open add space on every plane.
The problem is that if i change one of the setting above, i'll achieve different resaults.
So, could you explain me how to simulate this coaxial in the right way?
Thank you in advice
...
Using the time domain solver I also see differences if i simulate in the frequency range 0 - 32 GHz or 1 - 32 GHz, (so, including or not the 0 ) and it was totally unexpected from me.
I really need your help.
...

Hi
CST takes center frequency for meshing. so if you are using 0-32 GHz it takes 16 GHz for meshing, if you use 1-32 Ghz it takes 16.5 GHz as meshing frequency.You shouldn't change background material if you are gonna use it in vacuum space or in air(not submerged in water,sand or closed in a metal box,dielectric.) You should use big waveguide ports(like 4-5x of the diameter of coaxial cable) Reducing the range of frequency is also better for simulation(CST takes linearly 1001 samples between frequencies as default). Also use air as a material between inner and outer conductor.