Overview

There are three types of losses that occur in microstrip lines:

1. Conductor (or ohmic) losses in the strip conductor and the ground plane;

2. Dielectric losses in the substrate;

3. Radiation losses.

We consider only nonmagnetic dielectric substrates. If a magnetic substrate is used, then a magnetic loss component also will be present. Radiation plays an important role in the design of planar antenna elements. However, this subject is beyond the aim of this written material. In Section 6.1.2, we have discussed radiation loss of open-ended microstrip transmission lines in common microstrip circuits. In most conventional microstrip circuit designs with a high substrate dielectric constant, conductor losses in the strip conductor and the ground plane dominate over dielectric and radiation losses.

We assume that the microstrip line has only low loss. In this case, the distortion of the fields penetrating metal walls are negligibly small. Under this assumption, the current density on the conductor can be directly derived from the known distribution of the tangential magnetic field on the conductor surface. Regarding the theory of weakly lossy transmission lines, the voltage wave in the z direction of the transmission line V( z) reads [1]

(7.1)

where ? denotes the wave propagation coefficient, and ? and ? are the attenuation and phase coefficient, respectively. A similar relation can be written for the current wave propagating along the line. For all the line parameters, we prefer the notation "coefficient" instead of "constant," because in communications these are...