Transmission Lines

A transmission line is a pair of conductors, such as a coaxial cable or a parallel-wire line, used to carry a signal from one point to another while keeping the electromagnetic energy guided along its length. At high frequencies a line cannot be treated as a simple wire, because the signal takes a measurable time to travel and behaves as a wave. The line is characterised by its characteristic impedance, a value set by its geometry and materials that relates the voltage and current of a wave travelling along it.

When a wave reaches the far end of the line it is fully absorbed only if the load impedance equals the characteristic impedance. Any mismatch reflects part of the wave back toward the source. The forward and reflected waves combine to form a standing wave, whose peaks and troughs are described by the voltage standing wave ratio (VSWR). A VSWR of one indicates a perfect match with no reflection, while larger values signal increasing mismatch and wasted power. Matching networks, including short lengths of line called tuning stubs, are used to cancel reflections and present the source with the impedance it expects.

Reflections are also the basis of a useful measurement technique. Time-domain reflectometry (TDR) sends a pulse down a line and times the echoes returning from impedance discontinuities, locating faults, breaks or connectors along a cable. Animations are especially helpful for this subject because they can show a pulse moving along the line, reflecting from a mismatch, and the resulting standing-wave pattern building up, making the otherwise abstract idea of forward and reflected waves concrete.

Frequently asked questions

What is characteristic impedance?

It is the ratio of voltage to current for a wave travelling along a line, set by the line's geometry and materials. Matching the load to it prevents reflections.

What does VSWR measure?

The voltage standing wave ratio quantifies the mismatch on a line. A value of one means a perfect match and no reflection; higher values mean more reflected power.

How does time-domain reflectometry work?

It launches a pulse down a line and measures the timing and size of echoes from impedance changes, which reveals the location and nature of faults along a cable.