When a load doesn't match a line's characteristic impedance, some power reflects back toward the source instead of being delivered — creating a standing wave pattern you can see and measure directly.
A transmission line — coaxial cable, twisted pair, microstrip, or waveguide — carries a traveling electromagnetic wave rather than a simple circuit current, and it behaves that way once its length becomes a meaningful fraction of the signal's wavelength. When the load at the end doesn't match the line's characteristic impedance, part of the wave reflects back toward the source, and the interference between the forward and reflected waves creates a fixed standing-wave pattern along the line.
Every transmission line has a characteristic impedance (Z₀) determined by its physical geometry and the dielectric between its conductors — 50Ω and 75Ω are common standardized values in RF and video systems. A wave traveling down a line only sees Z₀ until it reaches the load; if the load impedance doesn't equal Z₀, the boundary reflects part of the incident wave, exactly like light reflecting at a material interface.
The reflection coefficient (Γ) quantifies how much of the incident wave reflects — Γ = 0 means a perfect match (all power delivered, no reflection), Γ = ±1 means total reflection (open or short circuit). Voltage Standing Wave Ratio (VSWR) is a related, more commonly quoted figure derived from Γ; VSWR = 1:1 is a perfect match, and higher ratios indicate more power being reflected rather than delivered to the load.
Reflected power isn't just wasted — it can also stress the transmitter's output stage, cause standing-wave voltage peaks that damage cable or connectors at high power, and distort the signal through re-reflections back and forth along the line. Matching networks, quarter-wave transformers, and stub tuners exist specifically to transform a mismatched load impedance to look like Z₀ from the source's perspective, eliminating the reflection.
It means the load impedance exactly equals the line's characteristic impedance — all forward power is absorbed by the load, and no wave reflects back toward the source. This is the ideal, fully matched condition.
It depends on the application and power level. At low power and short cable runs, a moderate mismatch (e.g., VSWR 2:1) mostly just wastes some power. At high transmit power, sustained high VSWR can cause standing-wave voltage peaks large enough to damage connectors, cable dielectric, or the transmitter's output stage — which is why most RF power amplifiers include VSWR protection circuitry.
A line only behaves as a distributed transmission line (rather than a simple wire) once its physical length becomes a meaningful fraction of the signal's wavelength — often taken as roughly 1/10 of a wavelength as a rule of thumb. At low frequencies (long wavelengths), even a long cable run is electrically short and ordinary circuit theory is an adequate approximation.
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