📡 VSWR Calculator
Calculate Voltage Standing Wave Ratio
Magnitude of reflection coefficient (0 ≤ |Γ| < 1)
How to Use This Calculator
Select Input Method
Choose to calculate VSWR from: (1) Reflection coefficient magnitude, (2) Forward and reflected power, or (3) Load and characteristic impedance.
Enter Values
Input the required values based on your selected method. For reflection coefficient, enter a value between 0 and 1. For power method, ensure reflected power ≤ forward power.
Calculate
Click "Calculate VSWR" to get the Voltage Standing Wave Ratio. VSWR is displayed as a ratio (e.g., 1.5:1). Lower VSWR indicates better impedance matching (ideal is 1:1).
Formula
VSWR = (1 + |Γ|) / (1 - |Γ|)
From Reflection Coefficient
|Γ| = √(Pr / Pf)
From Power
|Γ| = |(ZL - Z0) / (ZL + Z0)|
From Impedance
Where:
- VSWR = Voltage Standing Wave Ratio (dimensionless)
- Γ = Reflection Coefficient (dimensionless, 0 ≤ |Γ| < 1)
- Pf = Forward Power (Watts, W)
- Pr = Reflected Power (Watts, W)
- ZL = Load Impedance (Ohms, Ω)
- Z0 = Characteristic Impedance (Ohms, Ω)
Example Calculation 1:
For |Γ| = 0.2:
VSWR = (1 + 0.2) / (1 - 0.2) = 1.2 / 0.8 = 1.5 : 1
A VSWR of 1.5:1 indicates good impedance matching. The maximum voltage on the line is 1.5× the minimum voltage.
Example Calculation 2:
For Pf = 100 W, Pr = 4 W:
|Γ| = √(4/100) = √0.04 = 0.2
VSWR = (1 + 0.2) / (1 - 0.2) = 1.5 : 1
4% power is reflected, resulting in 96% power delivered to the load.
Example Calculation 3:
For ZL = 75 Ω, Z0 = 50 Ω:
|Γ| = |(75 - 50) / (75 + 50)| = 25/125 = 0.2
VSWR = (1 + 0.2) / (1 - 0.2) = 1.5 : 1
A 75Ω load on a 50Ω line causes a VSWR of 1.5:1.
About VSWR Calculator
The VSWR (Voltage Standing Wave Ratio) Calculator determines the impedance mismatch between a transmission line and its load. VSWR measures the ratio of maximum to minimum voltage on a transmission line and indicates how well the load is matched to the characteristic impedance. Perfect matching gives VSWR = 1:1 (no reflection). Higher VSWR indicates more reflection and poorer matching.
When to Use This Calculator
- RF Engineering: Analyze impedance matching in RF circuits
- Antenna Design: Measure antenna matching to transmission lines
- Amateur Radio: Optimize transmitter-to-antenna matching
- Microwave Systems: Evaluate impedance matching in microwave circuits
- Transmission Lines: Assess signal reflection and power transfer
Why Use Our Calculator?
- ✅ Multiple Methods: Calculate from reflection coefficient, power, or impedances
- ✅ Quick Analysis: Instantly determine VSWR and matching quality
- ✅ RF Applications: Essential for radio frequency engineering
- ✅ Free Tool: No registration or payment required
- ✅ Educational: Learn about VSWR and impedance matching
Common Applications
Antenna Matching: In radio and antenna systems, VSWR indicates how well an antenna is matched to the transmission line. Low VSWR (close to 1:1) means most power is delivered to the antenna. High VSWR causes power reflection back to the transmitter, potentially damaging equipment. Most systems aim for VSWR < 2:1, with 1.5:1 considered excellent.
RF Circuit Design: In RF amplifiers, filters, and matching networks, VSWR helps evaluate impedance matching. Poor matching causes signal reflection, loss, and distortion. Designers use VSWR to optimize matching networks (e.g., L-networks, pi-networks) to achieve low VSWR across the operating frequency band.
Transmitter Protection: High VSWR reflects power back to transmitters, potentially causing overheating and damage. Many modern transmitters include VSWR protection circuits that reduce output power or shut down when VSWR exceeds safe limits (typically >3:1). Monitoring VSWR protects expensive RF equipment.
Tips for Best Results
- Ideal VSWR = 1:1 (perfect match, no reflection)
- VSWR < 2:1 is generally acceptable for most applications
- VSWR > 3:1 indicates poor matching and significant power loss
- VSWR varies with frequency - check across operating band
- Use matching networks (tuners) to improve VSWR
Frequently Asked Questions
What is VSWR?
VSWR (Voltage Standing Wave Ratio) is the ratio of maximum to minimum voltage on a transmission line. It measures impedance mismatch between the line and load. VSWR = 1:1 means perfect match (no reflection). Higher VSWR (e.g., 2:1, 3:1) indicates more reflection and poorer matching.
What is a good VSWR value?
VSWR < 1.5:1 is excellent, < 2:1 is good, < 3:1 is acceptable for many applications. VSWR > 3:1 indicates poor matching. Perfect match is 1:1 (no reflection). Most RF systems aim for VSWR < 2:1 across the operating frequency band.
How is VSWR related to reflected power?
Reflected power percentage = |Γ|² × 100%, where |Γ| = (VSWR - 1) / (VSWR + 1). For VSWR = 2:1: |Γ| = 1/3 ≈ 0.333, reflected power = 11.1%. For VSWR = 3:1: |Γ| = 0.5, reflected power = 25%. Higher VSWR means more power is reflected instead of delivered to the load.
What causes high VSWR?
High VSWR is caused by impedance mismatch between load and transmission line. Common causes: wrong antenna impedance, damaged cables, poor connectors, incorrect cable length, frequency-dependent impedance changes, or disconnected load (open circuit VSWR = ∞).
How do I reduce VSWR?
Use impedance matching networks (tuners, matching circuits), select appropriate antenna/load impedance, ensure good cable connections, use correct cable length, or add matching components (capacitors, inductors, transformers). Antenna tuners can match various impedances to 50Ω or 75Ω lines.
Is VSWR the same as return loss?
No, but they're related. Return Loss (dB) = -20×log₁₀(|Γ|) = 20×log₁₀((VSWR+1)/(VSWR-1)). Higher return loss (in dB) means lower VSWR and better matching. VSWR = 1.5:1 corresponds to RL ≈ 14 dB. VSWR = 2:1 corresponds to RL ≈ 9.5 dB.