4.9: Power Losses and Parasitic Effects

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Four separate mechanisms lead to power losses in microstrip lines:

Conductor losses.
Dissipation in the dielectric of the substrate.
Radiation losses.
Surface-wave propagation.

The first two items are dissipative effects, whereas radiation losses and surface-wave propagation are essentially parasitic phenomena. The reader is directed to Chapter 8 of [12] for an extensive treatment. Here, summary results are presented.

Conductor losses greatly exceed dielectric losses for most microstrip lines fabricated on low-loss substrates. Lines fabricated on low-resistivity silicon wafers, however, can have high dielectric loss. These wafers are most commonly used for digital circuits, and the interconnect transverse dimensions are generally very small so that line resistance is very high, and again, resistive losses dominate.

Radiation from a microstrip line results from asymmetric structures. In particular, discontinuities such as abruptly open-circuited microstrip (i.e., open ends), steps, and bends will all radiate to a certain extent. Such discontinuities form essential features of microwave circuits and therefore radiation cannot be avoided. Efforts must be made to reduce such radiation and its undesirable effects. In most cases, radiation can be represented by a shunt admittance.

Surface waves, trapped just beneath the surface of the substrate dielectric, will propagate away from microstrip discontinuities as TE and TM modes. The effect of surface waves can be treated as a shunt conductance.

Various techniques can be used to suppress radiation and surface waves:

Metallic shielding or “screening.”
The introduction of lossy (i.e., absorbent) material near any radiative discontinuity.
The utilization of compact, planar, inherently enclosed circuits such as inverted microstrip and stripline.
Reducing the current densities flowing in the outer edges of any metal sections and concentrate currents toward the center and in the middle of the microstrip.
Possibly shaping the discontinuity in some way to reduce the radiative efficiency.