Calculate SWR, positive return loss, reflection magnitude, mismatch loss and net forward power for passive loads. Convert any of the three mismatch metrics, use same-plane forward/reflected powers, or enter a load impedance with a positive real reference impedance. Resistive limits are also available.

Passive loads and a positive real reference impedance. Compare powers at the same reference plane; mismatch is not antenna radiation efficiency.

Standing Wave Ratio Formula

SWR = (1 + √(Pᵣ / Pf)) / (1 - √(Pᵣ / Pf))

Variables:

  • SWR is the standing wave ratio (dimensionless, typically written as “:1”).
  • Pr is the reflected power
  • Pf is the forward power
  • Pr and Pf must be measured at the same point on the line (in the same power units), with 0 ≤ Pr ≤ Pf; this relationship assumes a low-loss (approximately lossless) line between the measurement point and the load.

How to Calculate Standing Wave Ratio?

The following steps outline how to calculate the Standing Wave Ratio.

  1. First, determine the reflected power. 
  2. Next, determine the forward power. 
  3. Next, gather the formula from above = SWR = (1+√(Pr/Pf)) / (1-√(Pr/Pf)).
  4. Finally, calculate the Standing Wave Ratio.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

reflected power (Pr) = 5 W

forward power (Pf) = 25 W, so SWR = (1+√(5/25)) / (1-√(5/25)) ≈ 2.618:1

FAQs

What is the significance of Standing Wave Ratio (SWR) in telecommunication?

SWR measures impedance mismatch and reflection at a reference plane. Lower SWR means less reflected power there; it does not measure antenna radiation efficiency. Feed-line loss and loss in the antenna or matching network can reduce useful radiated power even with low SWR.

How does reflected power affect the SWR?

Reflected power directly impacts the SWR value. Higher reflected power indicates more power is being reflected back towards the source, resulting in a higher SWR. This signifies inefficient power transfer and potential damage to the transmitter over time.

Can SWR be reduced and how?

Yes, SWR can be reduced by improving the match between the transmission line and the antenna. This can be achieved by using matching devices like tuners, stubs, or baluns to minimize reflections and thus lower the SWR.

Is it possible to have an SWR of 1:1, and what does it mean?

SWR of 1:1 means zero reflection at the measurement plane in the ideal model. Return loss is then infinite and mismatch loss is zero. It does not establish the efficiency of the antenna or the complete transmitting system.