SWR (Standing Wave Ratio) Calculator
Forward + reflected power → SWR, reflection coefficient ρ, and return loss — the three standard antenna-matching metrics.
Result
- Reflection coefficient ρ0.2000
- SWR1.500 : 1
- Return loss13.98 dB
- Reflected power %4.00%
- Forward power delivered96.00%
- VerdictExcellent (≤ 1.5:1)
Step-by-step
- ρ = √(Pr / Pf) = √(4 / 100) = 0.2000.
- SWR = (1 + ρ) / (1 − ρ) = (1 + 0.2000) / (1 − 0.2000) = 1.500:1.
- Return loss = −20·log10(0.2000) = 13.98 dB.
How to use this calculator
- Pick whether you have forward/reflected power readings or impedance values.
- Enter the relevant numbers.
- Read SWR (1:1 ideal, ≤ 1.5:1 excellent, ≤ 2:1 good, > 3:1 problematic).
About this calculator
Standing wave ratio (SWR) is the ratio of voltage maxima to minima on a transmission line under standing-wave conditions. SWR = 1:1 means the load matches the line and all power flows to the load. SWR rises as the load deviates from the line impedance; the reflection coefficient ρ is the underlying wave-mechanics quantity and SWR = (1+ρ)/(1−ρ) is just a more intuitive readout of it. Return loss in dB is yet another presentation: RL = −20·log10(ρ), which is positive and large for a good match.
What this calculator does
This calculator computes SWR (standing wave ratio), reflection coefficient ρ, return loss in dB, and the percent of forward power delivered vs reflected. You can input either forward and reflected power (the readings from a typical RF wattmeter) or the transmission-line characteristic impedance Z₀ and the load impedance Zl. The two methods produce identical results and the calculator reports all three standard metrics so you can match against whichever scale the equipment uses.
How it works — the formula
From power readings: ρ = √(Pr / Pf)
From impedance: ρ = |Zl − Z₀| / (Zl + Z₀)
SWR = (1 + ρ) / (1 − ρ)
Return loss (dB) = −20·log10(ρ)Voltage reflection coefficient ρ is the fundamental microwave-engineering quantity. It is unitless and ranges 0 to 1. SWR is the maximum-to-minimum voltage ratio on a standing-wave line and is just a different presentation of ρ — easier to interpret intuitively (lower is better, 1:1 is perfect). Return loss inverts the sign convention so larger dB numbers are better, the convention preferred in microwave-engineering test sets.
Worked examples
- Inputs:
- mode=power, Pf=100, Pr=4
- Output:
- ρ = 0.2; SWR = 1.5:1; RL ≈ 13.98 dB; 96% power delivered
Borderline-excellent match — typical of a properly resonant antenna.
- Inputs:
- mode=z, Z₀=50, Zl=75
- Output:
- ρ ≈ 0.2; SWR = 1.5:1
Classic illustration of 50 Ω vs 75 Ω mismatch — same SWR as the power example.
- Inputs:
- mode=z, Z₀=50, Zl=10000
- Output:
- ρ ≈ 0.99; SWR ≈ 198:1; RL ≈ 0.087 dB
Open and short look the same from an SWR perspective — total reflection.
When to use this vs other tools
Use this for RF antenna-system matching. For lumped-element audio impedance the audio-impedance tool is calibrated for voltage-bridging analysis instead.
- Antenna Length
Use to cut an antenna near its resonant length — high SWR usually means the antenna is off-frequency.
- Transmission Line Impedance
Use to confirm the line Z₀ matches what your equipment expects (50 Ω ham/commercial, 75 Ω broadcast/CATV).
- Coax Loss
Use to estimate how much SWR is being masked by feedline loss between meter and antenna.
- Audio Impedance
Use for line-level signal-chain impedance — different math conventions apply (voltage bridging).
Authority note
The SWR/ρ/return-loss formulations are textbook microwave-engineering identities and appear identically in every professional RF reference. The amateur-radio convention (1.5:1 / 2:1 / 3:1 thresholds) is the standard practical interpretation.
Limitations
- Assumes a lossless transmission line for the strict SWR identity. Line loss masks SWR — measure near the antenna for accurate readings.
- Power-mode inputs assume a properly calibrated directional wattmeter. Cheap SWR bridges can mis-read at high power or off-frequency.
- Reactive load components (X) reduce to magnitude here. Full Smith-chart analysis is needed for matching-network design.
- High SWR plus high cable loss is dangerous — it can produce localized heating in the line and connectors.
Sustained transmit into high-SWR loads can damage transceivers. Always verify SWR with a calibrated meter before applying full power on a new antenna installation.