To answer your question and fit your futur problems regrading coplanar waveguides, I strongly recommned you Rainee N. Simons book entitled "Coplanar Waveguide Circuits , Components, and Systems". More concretly, the Chapter 1. Hope it could help.

Thank you very much, Mr., Alvaro Rosa,

But i cant't find a useful form it it.

Can you help me more?

Hi Ahmed,

There are a few different ways to find the gap capacitance of a coplanar waveguide. If you are looking for an analytical expression for the capacitance per unit length, you could use conformal mapping. In this technique, the coplanar capacitance geometry is mapped onto that of a parallel plate capacitor, taking into account the symmetry and dielectric properties of your substrate. You can find the expressions in Appendix C (page 95) of my phd thesis: Thesis The Josephson parametric oscillator - From microscopic studi...

Good luck!