What Happens When You Mix 50 Ω and 75 Ω SMA? VSWR Test Data
Mixing 50 Ω and 75 Ω SMA connectors degrades VSWR significantly. We compare measured reflection and insertion loss data and give impedance-selection advice.
The most common SMA impedance is 50 Ω, used across communications, test & measurement, and radar. Video and cable-TV environments often use 75 Ω. Because the mechanical threads look identical, it is tempting to mate them. Thread compatibility only guarantees mechanical fit—not electrical match.
1. Quantifying the impedance discontinuity
When 50 Ω and 75 Ω are connected directly, the reflection coefficient Γ is:
Γ = (75 − 50) / (75 + 50) = 0.2
That equals roughly VSWR 1.5 and return loss 14 dB—already marginal. Worse, if a 50 Ω cable is fitted with 75 Ω connectors at both ends, two reflection planes create interference that can push VSWR above 2.0 at some frequencies.
2. Measured data comparison
We compared three cable assemblies: all-50 Ω, all-75 Ω, and a mixed 50 Ω/75 Ω link.
| Link type | 1 GHz VSWR | 6 GHz VSWR | 12 GHz VSWR | Insertion-loss increase |
|---|---|---|---|---|
| All 50 Ω | 1.05 | 1.12 | 1.20 | Baseline |
| All 75 Ω | 1.05 | 1.10 | 1.18 | Baseline |
| 50 Ω cable + 75 Ω connectors | 1.45 | 1.62 | 1.88 | +0.3 ~ 0.8 dB |
| 75 Ω cable + 50 Ω connectors | 1.42 | 1.58 | 1.81 | +0.3 ~ 0.7 dB |
At 12 GHz the mixed link reflects roughly 8 % of the power back to the source, stressing amplifiers and degrading receiver noise figure.
3. Common pitfalls
- Video connectors misused for RF: 75 Ω BNC/SMA parts look identical to 50 Ω versions; label and segregate inventory.
- Calibration kit mismatch: Using a 50 Ω cal kit on a 75 Ω DUT systematically shifts S11 and S21.
- Adapters are not matching networks: Only impedance-transforming adapters contain a tapered matching section; plain barrel adapters do not.
4. Selection guide
| Application | Recommended impedance | Reason |
|---|---|---|
| Telecom, radar, test & measurement | 50 Ω | Higher power handling, lower high-frequency loss, industry standard |
| CATV, video, broadcast | 75 Ω | Lower loss at low frequencies, matches video cable |
| Temporary 50 Ω ↔ 75 Ω interconnection | Impedance transformer | Do not use a plain adapter |
Conclusion: 50 Ω and 75 Ω SMA connectors must not be mixed. Identical threads only guarantee mechanical mating; the electrical mismatch produces significant reflection at high frequencies. Always verify cable, connector, and test-port impedance consistency.
For high-frequency performance of precision SMA, see SMA 18 GHz vs 26.5 GHz Frequency Margin Analysis.
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