SWR Standing Wave Ratio Calculator

This small tool computes the Standing Wave Ratio and estimates power lost on a transmission line. You can supply data in three ways: by forward and reflected power, by characteristic and load impedance, or by peak and trough voltages.

Input modes

• By power — enter forward and reflected power values.
• By impedance — enter line characteristic impedance and load impedance.
• By voltage — enter maximum and minimum voltages along the line.

How to use

1. Choose an input mode from the dropdown.
2. Fill the relevant fields with measured or assumed values.
3. Click Calculate to get SWR and estimated line loss.

Formulas

1. From powers

Calculate the square root of the reflected to forward power ratio, then compute SWR:

$$
\text{SWR} = \frac{1 + \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}}}{1 – \sqrt{\frac{P_{\text{ref}}}{P_{\text{fwd}}}}}
$$

2. From impedances

First compute the reflection coefficient magnitude, then SWR:

$$
\Gamma = \left|\frac{Z_L – Z_0}{Z_L + Z_0}\right|
$$

$$
\text{SWR} = \frac{1 + \Gamma}{1 – \Gamma}
$$

3. From voltages

SWR equals the ratio of maximum to minimum voltage along the line:

$$
\text{SWR} = \frac{V_{\max}}{V_{\min}}
$$

Line loss, percent

Approximate percent of power reflected back and thus not delivered to the load:

$$
\text{Loss \%} = \left(\frac{\text{SWR} – 1}{\text{SWR} + 1}\right)^{2} \times 100\%
$$

Worked examples

Quick facts

• SWR is always at least 1. Value 1 means perfect match between load and line.
• High SWR increases reflected power and can stress or damage transmitters.
• Use an SWR meter or directional wattmeter for accurate on-air measurements.
• This calculator is handy for hobbyists, RF engineers and communications technicians.

For practical guides on building simple directional wattmeters and SWR meters see the digital and analogue projects referenced below.

Further reading

1. Constantine A. Balanis — Antenna Theory: Analysis and Design. A solid theoretical reference for antenna behaviour and transmission line interaction.
2. David M. Pozar — Microwave Engineering. Covers transmission line theory and practical microwave techniques.
3. ARRL — The ARRL Antenna Book. Practical projects, measurement techniques and antenna tuning tips useful for amateurs.
4. G. Gonzalez — Microwave Transistor Amplifiers. Good coverage of reflections, matching and stability considerations.

David Parry — Senior Engineering Analyst

Specializing in electronics and physics-based simulations with 20+ years of engineering experience. David ensures the mathematical and physical accuracy of the tools at ProCalcLab.