VSWR & Return Loss Calculator
Enter a load impedance, VSWR, return loss or |Γ| and see the mismatch, reflected power and delivered power.
Step-by-step
- Enter valid values to begin.
Assumes a lossless line of characteristic impedance Z0 (real) and the forward power measured at the load plane. Line loss makes the VSWR seen at the transmitter look better than the VSWR at the load. The status bands (VSWR up to 1.5 good, up to 2 acceptable, up to 3 poor, above 3 bad) are rules of thumb, not standards; the allowable VSWR depends on your transmitter, receiver and application. Many transmitters reduce power or shut down for a high VSWR (a trip near 3:1 is typical): check the equipment datasheet for its protection threshold.
VSWR and Return Loss: Counting What Bounces Back
Core Engineering Principles
When a line of impedance Z₀ meets a load ZL that does not match it, part of the wave reflects. The reflection coefficient Γ = (ZL − Z₀) / (ZL + Z₀) is a complex number, and its magnitude is all most people quote. VSWR, return loss and mismatch loss are the same fact told three ways: VSWR = (1 + |Γ|) / (1 − |Γ|), return loss is −20 log₁₀|Γ|, and mismatch loss is −10 log₁₀(1 − |Γ|²). A 2:1 VSWR sounds alarming, but it means |Γ| = 1/3, only 11% of the power reflected, and a loss of 0.51 dB.
The numbers worth remembering are 1.5:1 (14 dB return loss, 4% reflected), 2:1 (9.5 dB, 11%) and 3:1 (6 dB, 25%). Return loss of 20 dB is 1.22:1 and costs under 0.05 dB. What hurts is not usually the lost power. It is the standing wave. At a 3:1 mismatch the voltage peaks at 1.5 times the forward voltage, so connectors and amplifier stages see stress that the average power figure hides, and a reflected wave returning to a power amplifier may push it into protection or damage it. Cable loss also hides a bad antenna: a lossy feed line can show 1.3:1 at the transmitter while the antenna sits at 3:1.
RL = −20 log10|Γ| • ML = −10 log10(1 − |Γ|²) • Prefl = |Γ|² Pfwd
NEC & Standard References
IEEE Std 145 defines antenna terms, and IEEE Std 100 and IEC 60050 (IEV) define reflection coefficient, standing-wave ratio and return loss. IEC 61169 specifies the RF coaxial connector families, and MIL-STD-348 their interface dimensions, in the 50 Ω and 75 Ω practice that sets Z₀ for most equipment. Radio rules such as FCC Parts 15 and 97 and ETSI EN 300 limit radiated power, not VSWR, but a bad match lowers the power reaching your antenna. Check your equipment datasheet for its VSWR protection setting.1. Γ = (100 − 50) / (100 + 50) = 0.3333 at a phase of 0°.
2. VSWR = 1.3333 / 0.6667 = 2.000 : 1.
3. RL = −20 log₁₀(0.3333) = 9.54 dB.
4. Mismatch loss = −10 log₁₀(1 − 0.1111) = 0.512 dB.
5. Reflected power = 0.1111 × 100 = 11.1 W, so 88.9 W is delivered.
6. The standing wave peaks at 1.333 × the forward voltage. Verdict: acceptable by rule of thumb.
- Don’t use VSWR meters at the wrong frequency. Cheap meters read only near their design band and give false comfort.
- Measure at the antenna end where possible. Line loss improves apparent VSWR at the transmitter.
- A short or open is the worst case. |Γ| = 1 means no delivered power and large standing voltages and currents.
- Watch for hot connectors. Reflected power can overheat poor connections and arc in high-power systems.
- Reduce power before testing a suspect antenna. A sweep at low power finds the fault without risking the final amplifier.