Planar Transmission Lines

Virtually all electronic circuits, from low-frequency digital to high-frequency RF and microwave circuits, employ transmission lines. Among transmission lines, printed circuit structures, also known as planar transmission lines, are most useful in modern electronics. Planar transmission lines have evolved from the advances of electronic integrated circuits (ICs) and, in turn, they have helped the progress of IC technology, such as making ICs that are more compact and more versatile and that have better interconnections and improved performance. Not only have planar transmission lines fulfilled their most fundamental objective of delivering signals, but they can also be exploited to create various RF and microwave components, such as wideband hybrid junctions, by appropriately combining them. The most well-known and commonly used planar transmission line is perhaps the microstrip line proposed in 1952. It, together with the strip line, forms the first planar version of the coaxial transmission line. Since then, various other planar transmission lines have been developed for use in RF and microwave ICs. Currently, the most commonly used structures for RF and microwave ICs are the microstrip line, the coplanar waveguide (CPW), the coplanar strip (CPS), the strip line, and the slot line. In this chapter, we first discuss the transmission line's static and dynamic parameters. We present a brief discussion of the most commonly used planar transmission lines, including the microstrip line, the coplanar waveguide (CPW), the coplanar strip (CPS), the strip line, and the slot line, along with the closed-form formulas for computing their characteristic impedances, effective dielectric constants, and losses. Although these parameters can accurately be determined using the methods presented in Chapters 5, 6, 7, and 8, closed-form expressions allow convenient and fast computer-aided design and analysis of RF and microwave circuits.