coaxial simulation and characteristic impedance calculation

Hi, there
I 'd like to find some reference on simulation of coaxial balun. can anyone give me some advice?
also I am confused about the impedance calculation of coaxial of say a EZ90-25 series semi rigid cable. From the datasheet we can see that its impedance is 25 ohms with outer diameter of 90 mils and that of inner of 40 mils, also we can find the dielectric constant of PTFE is 2.01, I think I can calculate the impedance, which is 25 ohms, from these data using formula

but what I get is something 35 or so, not even close to 25, So anything wrong with my calculation?


Thanks

Julian

Seems like you didn't read the cable specifiation thoroughly. 90 mils is actually the copper tube outer diameter, the inner diameter (= dielectric diameter) is 73 mils. Semi-Rigid Coaxial Cable

Cheers, get the right value,
Thank you FvM for pointing my fault.

Regards