VSWR & Standing Waves
Voltage Standing Wave Ratio (VSWR) is a measure of how efficiently radio-frequency power is transmitted from a power source, through a transmission line, into a load.
The Physics of Reflection
When an RF signal travels down a transmission line (like a coaxial cable), it expects the load at the end (like an antenna) to have the exact same impedance as the cable (typically 50Ω). If the impedances match perfectly, 100% of the energy is transferred.
However, if there is an impedance mismatch, some of the energy "bounces" off the load and travels backward toward the source. This is the Reflected Wave.
The Standing Wave
As the forward (incident) wave and backward (reflected) wave cross paths, they interfere with each other. Because the waves are moving at the same frequency in opposite directions, the resulting interference pattern appears to "stand still" in place.
The Math
Γ = (Z_L - Z_0) / (Z_L + Z_0)
VSWR = (1 + |Γ|) / (1 - |Γ|)
Return Loss (dB) = -20 * log10(|Γ|)
Simulation Controls
Legend
Transmission Line View ANIMATING AT 60FPS
Antenna Resonance Profile MAGNITUDE & RETURN LOSS
Open vs. Short Circuits (Phase Reversal)
The magnitude of the reflection coefficient (|Γ|) tells you how much power is reflected. The phase of the reflection tells you how the wave interacts at the physical boundary. Try clicking the Preset buttons above to observe this!
- Short Circuit (0Ω): If you short the end of the cable, the voltage at that exact physical point must be zero. To enforce this physical boundary condition, the reflected wave must invert its phase completely (180° shift) so it perfectly cancels out the incoming wave at the load. This creates a voltage node (zero point) exactly at the black LOAD line.
- Open Circuit (∞Ω): If the cable is left completely open, current cannot flow off the end, so the current must be zero. The voltage, however, reflects perfectly in-phase (0° shift) and stacks directly on top of the incoming wave. This creates a voltage antinode (peak) exactly at the load, where the voltage physically doubles!
- Matched Load (50Ω): A perfect match absorbs the wave entirely. |Γ| is 0, so there is no reflected wave and therefore no standing wave pattern.
Why does VSWR matter?
- Power Loss: Reflected energy is energy that isn't radiated by your antenna. A VSWR of 3.0 means 25% of your transmitter power is completely wasted.
- Transmitter Damage: That reflected energy travels straight back into your amplifier. High VSWR can cause voltage spikes that permanently fry output transistors.
- Signal Distortion: In digital communications, reflections cause "ghosting" and symbol interference, destroying your data throughput.