Balanced vs. Unbalanced Transmission Lines: Key Differences

This article explains the differences between balanced and unbalanced transmission lines. It builds upon the basics of transmission lines, which serve as a crucial link in wireless systems.

A transmission line is essentially a two-wire cable connecting the transmitter/receiver parts of a wireless system to the antenna. Its primary purpose is to efficiently carry RF (Radio Frequency) energy from one end to the other. Antennas are typically placed outdoors for optimal signal reception, while equipment is usually housed indoors for protection from environmental factors like rain, wind, and dust.

There are two main types of wire transmission lines: balanced lines and unbalanced lines.

Balanced Line

Figure 1: Balanced Line

As illustrated in Figure 1, a balanced line consists of two parallel conductors separated by a consistent distance, typically ranging from about 1/2 inch to several inches. Insulating spacers, often made of ceramic or plastic, maintain this separation.

The key characteristic of a balanced line is that the same amount of current flows in each wire with respect to ground. The current in one wire is 180 degrees out of phase with the current in the other wire. Crucially, in a balanced line, neither of the wires is connected to the ground.

Unbalanced Line

Figure 2: Unbalanced Line

Figure 2 depicts an unbalanced coaxial transmission line. In contrast to a balanced line, an unbalanced line has one conductor connected to the ground.

A common example of an unbalanced line is a coaxial cable (coax). A coax cable consists of a solid center conductor surrounded by a plastic insulator (e.g., Teflon). Over the insulator is another conductor made of fine wires, often in a braided form. An outer plastic cover protects the fine wire braid shield.

Coaxial cable is unbalanced because the current in the center conductor is referenced to the braid. The braid conductor is connected to the ground. Coaxial cables are available in various sizes, from 1/4 inch to several inches in diameter.

Of the two transmission line types, coaxial cable is the more widely used in many applications.