Field Containment in Microstrip and Stripline

Microstrip and stripline are some of the most common transmission lines used in high frequency printed circuit boards. Each has its own tradeoffs, but what actually makes them different from one another? It comes down to field containment.

At higher frequencies, the signal on the line isn't just voltage and current but electromagnetic fields that exist between and around the conductors themselves. This introduces a guided mode of wave propagation.

For stripline structures, the line is encapsulated in a uniform dielectric medium. For simplicity, we will assume the line is sandwiched in the middle of two ground planes with an isotropic dielectric medium. The electric field lines are essentially contained inside of this structure. Due to this encapsulation and symmetry, the guided mode of propagation is purely transverse electromagnetic (TEM). When designing a stripline circuit, this becomes important because there isn't a need to calculate an effective dielectric constant. This can change design parameters such as stripline width, characteristic impedance, wavelength, phase velocity, attenuation due to dielectric loss, attenuation due to conductor loss, and the line's wave number.

For microstrip structures, the line rests on top of the dielectric medium and is exposed to air. This means some of the field lines will extend outwards into the air, but also into the dielectric the line is sitting on top of - meaning the fields are only partially contained. The partial field containment/exposure introduces a quasi-TEM mode of propagation and shifts the dielectric constant from just the dielectric to the dielectric and the air surrounding the line itself. The fields are now interacting with two mediums instead of one. All of the parameters mentioned above now change.