Characteristic impedance

Characteristic impedance is the voltage-to-current ratio of a traveling wave on a transmission line. It is set by the line’s distributed inductance and capacitance, not by its length.

Telegrapher's equations: Coupled differential equations describing voltage and current along a transmission line. Their traveling-wave solutions yield the line’s characteristic impedance.
Impedance matching: The adjustment of connected systems so their impedances reduce unwanted signal reflection. Matching a load to the line’s characteristic impedance suppresses reflected waves.
Electrical impedance: The frequency-dependent opposition that a circuit presents to alternating current, combining resistance and reactance. Characteristic impedance is a specialized impedance associated with propagation along a line.
Lossless transmission line: An ideal transmission line with zero series resistance and zero shunt conductance. Its characteristic impedance is the real ratio of square-rooted inductance to capacitance per unit length.
Distributed-element model: A circuit model that represents resistance, inductance, capacitance, and conductance continuously along a structure. It explains why impedance depends on per-unit-length line properties.
Reflection coefficient: A complex ratio describing the amplitude and phase of a reflected wave relative to an incident wave. It quantifies the reflection caused by a load different from the line impedance.
Load impedance: The impedance presented by a device or termination connected to the end of a transmission line. It governs reflection at the termination but is not the line’s characteristic impedance.
Lossy transmission line: A transmission line whose conductors and dielectric dissipate signal energy. Its characteristic impedance is generally complex and frequency-dependent.
Inductance: A property of a circuit that relates changing current to induced voltage and magnetic energy storage. Per-unit-length inductance contributes to the line’s wave impedance.
Standing wave ratio: A measure of the maximum-to-minimum amplitude ratio in a standing wave on a transmission line. It indicates mismatch severity when load and characteristic impedances differ.

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