SWR Reflected Power

Formula

Description

Standing Wave Ratio (SWR) measures the impedance mismatch between a transmission line and its load (antenna). When the antenna impedance does not match the feedline characteristic impedance, a portion of the forward power is reflected back toward the transmitter. The reflection coefficient Γ = (SWR-1)/(SWR+1) determines the fraction of voltage reflected, and the reflected power is proportional to Γ². An SWR of 1:1 means a perfect match with zero reflection, while higher SWR values indicate worse mismatch and more wasted power.

Variables

• P_reflected — Power reflected back toward the transmitter (W)
• P_forward — Power traveling toward the antenna (W)
• SWR — Standing wave ratio (dimensionless, ≥ 1)

Practical Notes

SWR 1.5:1 reflects only 4% of power, SWR 2:1 reflects 11%, and SWR 3:1 reflects 25%. Most amateur transmitters can tolerate SWR up to 2:1 or 3:1 before automatically reducing power. The real concern with high SWR is increased feedline loss (lossy cable dissipates more power at high SWR) and voltage/current hot spots that can damage cable connectors. An antenna tuner at the transmitter improves the match seen by the transmitter but does not reduce feedline losses.