Microstrips

An illustration of a microstrip t.l.* is provided in Figure 2.1.1 in a cross-sectional view.** As we can see, this t.l. is composed of two parallel plane conductors, separated by a dielectric sheet with dielectric constant “ε r” and permeability “µ r.” The dielectric sheet is usually called the “substrate.” In this chapter unless otherwise stated, we will evaluate the dielectric material as not ferro-ferrimagnetic, i.e., we will suppose µ r = 1. The case where the dielectric material is also ferrimagnetic will be studied in Chapter 7. One of the two conductors is much wider than the other,*** and it has been indicated as “conductor II” in Figure 2.1.1. The wider conductor is set to the signal ground and for this reason it is also called the “cold conductor” or “ground conductor.” Conversely, the shorter conductor, indicated as “conductor I” in Figure 2.1.1, is called the “hot conductor.” For its physical construction, microstrips are employed as unbalanced t.l. We will indicate with:

Microstrips are the most widely used t.l. in all planar circuits, regardless of the frequency range of the applied signals. Especially at lower frequency, let us say until some hundreds of MHz, microstrips are widely used in multilayer printed circuit boards. In these cases, the ground conductor cannot coincide with the board metal housing or 0 Volt signal layer, but it can be a voltage layer, properly filtered. A possible four layers PCB is indicated in Figure 2.1.2. In this case, conductors “M1” and “M2” are two microstrips with layer “2,” which is a power supply layer, while layer “3” is available for other purposes. Such PCB configuration is widely used in ECL or TTL boards. Of course, in microwave devices, microstrips are always two layers t.l., as indicated in Figure 2.1.1.