3.3. Planar Transmission Lines
The most important planar TL types are microstrip, stripline, and coplanar waveguide (CPW). Microstrip and CPW forms are more widely used in comparison to the stripline due to their simplicity and ease of fabrication. Microstrip and CPW involve EM field propagation in two media: air above the conductor and dielectric below. This results in two different velocities of propagation and leads to an EM field which is only an approximation to the expected transverse EM (TEM) called quasi-TEM. While in the stripline, pure TEM propagation takes place. The difficulty of the dual propagation regions is resolved by defining an equivalent uniform medium surrounding the strip conductors with an effective dielectric constant εe.
Microstrip Line
Coplanar Waveguide (CPW)
The usefulness of the microstrip technology is limited to frequencies below few 10’s GHz. Above that, electrical performance degrades as a result of increasing radiation loss from the structure. CPW is a good solution to this shortcoming of the microstrip. Ideally, a CPW is composed of a central metal strip symmetrically flanked by a pair of coplanar half-planes serving as the signal (current) return. Not being practical, in actual CPWs the two half-planes are replaced by a pair of finite-width metal strips, with the central strip at a separation from each.
For a sufficiently thick substrate, i.e., h/w and h/s > 2-3, the electrical characteristics are solely determined by the lateral dimensions. Furthermore, for w1/w ≥ 3, the w1 dependence is negligible.
In microstrip, most of the E-field is in substrate, while in CPW only about half is in the substrate, hence dielectric losses are less significant in CPW. Also, since the fields in CPW are tightly confined locally to the separations, the radiation losses in CPW is significant lower than in microstrip.
