Characteristic Impedance

There is a rather exotic property of certain electronic components that you’ll sometimes hear or read about. It is called characteristic impedance or surge impedance, and is symbolized Zo. It is a specification of an important property of transmission lines. It can always be expressed as a positive real number, in ohms.

Transmission Lines

Edge-on views of coaxial transmission line
(A) and parallel-wire line
(B). In either type of line
Zo depends on the conductor diameters and spacing, and on the nature of the dielectric material between the conductors.
See text for discussion.
When it is necessary to get energy or signals from one place to another, a transmission line is required. These almost always take either of two forms, coaxial or two-wire (also called parallel-wire). Cross-sectional renditions of both types are shown in above figure. Examples of transmission lines include the “ribbon” that goes from a television antenna to the receiver, the cable running from a hifi amplifier to the speakers, and the set of wires that carries electricity over the countryside.

Factors Affecting Zo

The Zo of a parallel-wire transmission line depends on the diameter of the wires, on the spacing between the wires, and on the nature of the insulating material separating the wires. In general, the Zo increases as the wire diameter gets smaller, and decreases as the wire diameter gets larger, all other things being equal.

In a coaxial line, as the center conductor gets thicker, the Zo decreases if the shield stays the same size. If the center conductor stays the same size and the shield increases in diameter, the Zo increases. For either type of line, the Zo increases as the spacing between wires, or between the center conductor and the shield, gets larger. The Zo decreases as the spacing is reduced. Solid dielectric materials such as polyethylene reduce the Zo of a transmission line, compared with air or a vacuum, when placed between the conductors.

An Example of Zo in Practice

In rigorous terms, the ideal characteristic impedance for a transmission line is determined according to the nature of the load with which the line works.

For a system having a purely resistive impedance of a certain number of ohms, the best line Zo value is that same number of ohms. If the load impedance is much different from the characteristic impedance of the transmission line, excessive power is wasted in heating up the transmission line. Imagine that you have a so-called 300-Ω frequency-modulation (FM) receiving antenna, such as the folded-dipole type that you can mount indoors. Suppose that you want the best possible reception. Of course, you should choose a good location for the antenna. You should make sure that the transmission line between your radio and the antenna is as short as possible. But you should also be sure that you purchase 300-Ω TV ribbon. It has a value of Zo that has been optimized for use with antennas whose impedances are close to 300 + j0.

Impedance matching is the process of making sure that the impedance of a load (such as an antenna) is purely resistive, with an ohmic value equal to the characteristic impedance of the transmission line connected to it. This concept will be discussed in more detail in the next topics.