Directional coupler calculator
Why SWR meters disagree with each other
Directivity is how well a coupler tells the forward wave from the reflected one. At 20 dB, on an antenna with an SWR of 1.2 a meter will read anything from 1.0 to 1.5 — and two different meters will honestly show different numbers. Here you can see where that spread comes from.
| Coupling C (dB) | |
| Directivity D (dB) | |
| True SWR at the load | |
| Coupler | |
| Insertion loss | |
| Fraction of power into the coupled line | |
| Error in |Γ| | |
| The meter will read an SWR of | |
📐 The formulas
Coupling: C[dB] = 10 · log10(P_in / P_coupled)
Insertion loss: IL[dB] = −10 · log10(1 − 10^(−C/10))
Directivity: D[dB] = C(reverse) − C(forward)
Measurement error:
|Γ_measured| = |Γ_true| ± 10^(−D/20)
Insertion loss:
| Coupling | Insertion loss |
|---|---|
| 20 dB | 0.044 dB |
| 10 dB | 0.458 dB |
| 6 dB | 1.256 dB |
| 3 dB | 3.0 dB (an equal split) |
Where this calculator stops:
- The error is the worst case — when the unwanted signal adds to the wanted one in phase
- Directivity depends on frequency: the quoted figure is usually for mid-band
- The coupler's own mismatch is not accounted for, and it adds an error of its own
- The model says nothing about the direction of the error: the real reading may be above or below the true value
🎯 In practice
- Judging whether to believe an SWR meter on a well-matched antenna.
- Choosing the coupling when designing one: light for measurement, tight for tapping off power.
- Comparing two instruments that read differently.
Practical advice:
- 20 dB of directivity is typical of an amateur SWR meter; 35–40 dB of a laboratory one.
- Telling an SWR of 1.05 from one of 1.2 is impossible with 20 dB of directivity.
- A tandem match gives better directivity than the classic Bruene circuit.
- Calibrating on a known-good load shows the real directivity of your instrument.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
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UR3PKI. «Directional coupler calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/coupler (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.