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Additional loss from SWR calculator

The useful thing here is two numbers, not one

Mismatch does add loss, but far less than people assume. What is more interesting is this: cable loss attenuates both the forward and the reflected wave, so the SWR the rig sees is always better than the real one. A long bad cable simply hides the problem.

TRXZLSWR at the inputSWR at the load
A lossy line «improves» the SWR at the input — the meter in the shack reads better than things really are
Matched-line loss (dB)
SWR at the load
Transmitter power (W)
Loss
Total line loss
Extra loss caused by SWR
Reached the antenna
Lost as heat in the cable
SWR at both ends
At the load (true value)
At the line input (what the rig sees)

📐 The formulas​

a = 10^(A/10)                     A — matched-line loss, dB

Total loss:
    L_total[dB] = 10 · log10[ (a² − |Γ|²) / ( a · (1 − |Γ|²) ) ]

Additional:
    L_extra = L_total − A

SWR at the line input:
    |Γ_in| = |Γ_load| / a
    SWR_in = (1 + |Γ_in|) / (1 − |Γ_in|)

Worked examples:​

Line lossSWR at the loadExtraSWR at the input
1 dB3.00.50 dB2.32
3 dB3.00.97 dB1.67
1 dB2.00.21 dB1.72
Where this calculator stops:
  • Matched-line loss is entered, not calculated: take it from the coax loss calculator for your length and frequency
  • The formula is for a uniform line: connectors and adapters add their own
  • It does not account for loss rising with frequency — work it out at the frequency you use
  • It says nothing about the transmitter: a modern rig reduces power by itself above an SWR of 2

🎯 In practice​

  • Deciding whether the feedline is worth replacing. It often turns out not to be.
  • Explaining why the SWR meter in the shack reads better than the analyser at the antenna.
  • Choosing between a tuner and reworking the antenna: the calculator shows what the question really costs.
Practical advice:
  1. An SWR of 2 on good cable costs about 0.2 dB. That is a third of an S-point — nothing.
  2. A perfect SWR on a long cable is a reason to check the cable, not a reason to celebrate.
  3. A tuner in the shack does not change the SWR in the feedline: it only makes the rig see 50 Ω.
  4. On VHF the picture is different: cable losses are large there, and SWR really does cost you.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Additional loss from SWR calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/lines/swr_loss (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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