Transmission Lines

A transmission line is a pair of conductors arranged to carry electrical signals from one point to another while controlling the way the signal propagates. Familiar examples include coaxial cable, twisted pair and parallel-wire (ladder) line. At low frequencies a cable behaves like a simple wire, but as the signal frequency rises and the wavelength becomes comparable to the cable length, the line must be treated as a distributed circuit with capacitance, inductance, resistance and leakage spread along its length.

A central property is the characteristic impedance, the ratio of voltage to current for a wave travelling along the line. It depends on the conductor geometry and on the dielectric material between the conductors, not on the cable length. Common coaxial cables are made to standard impedances such as 50 or 75 ohms. The dielectric also sets the velocity of propagation, the speed at which signals travel relative to the speed of light in free space, which is always less than one because the insulating material slows the wave.

When a line is terminated in a load equal to its characteristic impedance, energy is fully absorbed and no signal is reflected. A mismatch causes part of the wave to reflect back toward the source, producing standing waves and reducing efficiency, so impedance matching is important in radio and high-speed digital work. Real cables also introduce loss that grows with frequency, attenuating the signal over distance. The Smith chart is a graphical tool engineers use to handle impedance and matching calculations on transmission lines.

Frequently asked questions

What is characteristic impedance?

It is the ratio of voltage to current for a wave moving along a line, set by the conductor geometry and dielectric. Common coaxial cables are 50 or 75 ohms, independent of length.

Why does impedance matching matter?

If the load does not match the line, part of the signal reflects back, creating standing waves and losses. Matching ensures the energy is delivered efficiently to the load.

What is velocity of propagation?

It is the speed at which a signal travels along a cable, expressed as a fraction of the speed of light. The dielectric between the conductors slows the wave, so the value is always below one.