RF and Microwave Circuit Design

2Planar Circuit Design I: Designing Using Microstrip

2.1 Introduction

Microstrip is the most common form of interconnection used in RF and microwave planar circuits. The concept of microstrip, together with a discussion of its advantages, was introduced in Chapter 1; for convenience, a diagram showing the key dimensions of a microstrip line is reproduced in Figure 2.1.

Also presented in Chapter 1 were design equations that enable the characteristic impedance and propagation constant of a microstrip line of given dimensions to be calculated. In this chapter, the practical issues associated with microstrip design will be discussed in more detail, and examples given of the design of some common microstrip components.

2.2 Electromagnetic Field Distribution Across a Microstrip Line

Propagation along a microstrip line is quasi-Transverse Electromagnetic (quasi-TEM). TEM refers to propagation where the electric and magnetic fields are entirely directed in a plane that is perpendicular to the direction of propagation. It should be noted that microstrip cannot support a pure TEM wave since the wave is travelling in a mixed dielectric medium, i.e. substrate and air. It is important in microstrip design to choose line parameters that minimize unwanted modes; this issue is discussed in more detail in Section 2.5.

A sketch of the electric field surrounding a microstrip line is shown in Figure 2.2.

There will also be a magnetic field, not shown in Figure 2.2, where the magnetic field ...