Free RF SWR & Return Loss Calculator
Instant bidirectional conversion between VSWR, Return Loss (dB), Reflection Coefficient (Γ), mismatch loss, and transmitted power.
📡 RF Measurement Input
📊 Power Flow & Reflection Metrics
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The Mathematics of RF Standing Waves
In radio frequency engineering, when an antenna or transmission line load does not exactly match the characteristic impedance ($Z_0$) of the feedline, a portion of the forward incident wave is reflected back toward the transmitter. The forward and reflected waves interfere to create a stationary interference pattern known as a standing wave.
1. Master Conversion Formulas
- Reflection Coefficient ($Gamma$ or $
ho$):
Γ = (VSWR - 1) / (VSWR + 1) - Return Loss (dB):
RL (dB) = -20 × log10(Γ) - VSWR from Return Loss:
Γ = 10^(-RL / 20) → VSWR = (1 + Γ) / (1 - Γ) - Reflected and Delivered Power Fractions:
P_reflected_% = Γ² × 100%
P_transmitted_% = (1 - Γ²) × 100% - Mismatch Loss (dB):
ML (dB) = -10 × log10(1 - Γ²)
2. Key Benchmark Reference Table
Frequently Asked Questions
Why do modern transceivers reduce power when SWR exceeds 2:1?
Solid-state bipolar or LDMOS power amplifier transistors are sensitive to high reflected voltage and current peaks. When SWR climbs above 2:1 or 3:1, the automatic ALC (Automatic Level Control) circuit folds back output power (e.g. from 100W down to 25W) to prevent transistor thermal destruction.
Can an antenna tuner at the transmitter eliminate high SWR on the feedline?
No! An antenna tuner located in the radio shack only provides a 50-ohm conjugate match for the transceiver itself. The high standing waves and dielectric losses still exist entirely along the coaxial cable between the tuner and the antenna feedpoint.
What is the relationship between SWR and Return Loss?
They measure the exact same physical phenomenon from different perspectives. VSWR (1.0 to infinity) is traditionally used by radio operators and antenna installers. Return Loss (dB, from 0 to 60 dB) is preferred by microwave engineers and network analyzers because decibels add linearly with cable attenuation.
How does feedline loss deceive an SWR meter?
A lossy coaxial cable attenuates both the forward wave traveling up and the reflected wave traveling back down. Because the reflected wave is attenuated twice, an SWR meter at the shack end will read a falsely low SWR (e.g. reading 1.3:1 when the antenna at the roof is actually 2.5:1).