Q8Microwave Theory And Techniques
Question
Q.8. What is VSWR?
Answer
VSWR (Voltage Standing Wave Ratio) is the ratio of maximum to minimum voltage magnitude of the standing-wave pattern on a mismatched transmission line, VSWR = Vmax/Vmin = (1+|Γ|)/(1−|Γ|), a dimensionless measure of impedance match quality ranging from 1 (perfect match) to ∞ (total mismatch).
VSWR quantifies how well a load (or any discontinuity) is impedance-matched to the characteristic impedance of the line or waveguide feeding it. When a wave reflects at a mismatch, the superposition of the forward and reflected waves produces a standing-wave pattern with alternating maxima and minima; VSWR is defined as the ratio of the maximum to minimum voltage magnitude of this pattern:
where Γ is the voltage reflection coefficient. VSWR=1 (|Γ|=0) represents a perfectly matched load with no reflected wave; VSWR→∞ (|Γ|→1) represents total reflection, as at an open or short circuit. In microwave engineering VSWR is typically measured directly using a slotted line and movable probe (or, in modern practice, a VNA), and is one of the most commonly specified figures of merit for antenna feeds, filters, and any microwave component interface, with well-designed systems typically targeting VSWR below about 1.5 (equivalent to under 4% reflected power).