We draw circuit diagrams with “lumped”components: ideal R's, C's, L's, transistors, etc., connected by lines that represent zero-length wires. But all real wires, if not much shorter than the shortest relevant wavelength, are themselves complicated circuit elements; the current is not the same everywhere along such a wire, nor is voltage uniform, even if the wire has no resistance. On the other hand, when interconnections are made with transmission lines, which are well-understood circuit elements, we can accurately predict circuit behavior. In this section we will consider two-conductor lines such as coaxial cables and open parallel wire lines. “Microstrip lines” (conducting metal traces on an insulation layer over a metal ground plane) behave essentially in the same way, but they have some subtle complications, which are mentioned in Appendix 10.1.

Characteristic impedance

The first thing one learns about transmission lines is that they have a parameter known as characteristic impedance, denoted Z0. How “real” is characteristic impedance? If we connect an ordinary dc ohmmeter to the end of a 50-ohm cable will it indicate 50 ohms? Yes, if the cable is very long, so that a reflection from the far end does not arrive back at the meter before we finish the measurement. Otherwise, the meter will simply measure whatever is connected to the far end, which could be short, an open circuit, or a resistance.