The symmetric stripline is reliable method for creating a transmission line. The stripline is a TEM (transverse electromagnetic) transmission line. Modeling approximation can be used to design the microstrip trace. By understanding the stripline transmission line, designers can properly build these structures to meet their needs.
A stripline is constructed with a flat conductor suspended between two ground planes. The conductor and ground planes are separated by a dielectric. One advantage of the stripline is that there is an improve isolation between adjacent traces when compared with the microstrip.
As for a microstrip transmission line, the impedance Z0 of a stripline depends on the ratio of the conductor width and the separation between signal and ground planes. In case of a symmetric stripline, Z0,SS can be computed according to the formulas contained in the document IPC-2141A , in particular paragraph 4.2.4.
The characteristic impedance computation can be broken down into two cases, one for a narrow signal conductor and one for a wide signal conductor.
For the narrow signal conductor, that is for w/b < 0.35, the appropriate formula for the characteristic impedance, Z0,SS, is:
b = 2h + t
For the wide signal conductor, that is for w/b ≥0.35, the appropriate formula for the characteristic impedance, Z0,SS, is: