Published on 10/13/2026 • Updated on 10/13/2026
A lab chain has four adapters stacked between a signal generator and a device under test: SMA female to female, SMA to N, N to BNC, BNC to SMA. Each one is a small part. Together they can add several tenths of a dB and push VSWR past what the amplifier likes.
Short answer: a good straight SMA adapter adds about 0.05 to 0.1 dB of insertion loss at 6 GHz and 0.1 to 0.3 dB at 18 GHz, with VSWR typically between 1.1:1 and 1.25:1. Cheap or mixed-series adapters can be two to three times worse. Losses stack, so count every one.
The numbers below are typical figures from datasheets. Use them for planning, then measure the real chain on a VNA.
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| Adapter type | Frequency | Insertion loss (typical) | VSWR (typical) |
|---|---|---|---|
| SMA to SMA straight (F-F or M-F) | 6 GHz | 0.05 to 0.1 dB | 1.10 to 1.15 |
| SMA to SMA straight | 18 GHz | 0.1 to 0.3 dB | 1.15 to 1.25 |
| SMA to N | 6 GHz | 0.1 to 0.2 dB | 1.15 to 1.25 |
| SMA to BNC (50 ohm) | 2 GHz | 0.1 to 0.2 dB | 1.15 to 1.3 |
| 3.5 mm to SMA (precision) | 26.5 GHz | 0.1 to 0.2 dB | 1.1 to 1.2 |
| Right angle SMA | 6 GHz | 0.1 to 0.2 dB | 1.2 to 1.3 |
A rough estimate for a good SMA adapter is loss in dB = 0.03 x sqrt(f in GHz) plus a small constant. Precision parts beat the figures in the table, and cheap parts exceed them.
Key idea: Do not judge an adapter by its connectors. Judge it by its measured VSWR and insertion loss at your highest frequency.
Insertion loss adds in dB. Mismatches also add, but not simply, because reflections interact. For planning, add the loss figures and a margin for VSWR.
Each adapter also adds length and a place for a bad torque or loose nut to degrade the connection.
Skip the adapter chain
A single custom assembly with the right connectors at each end removes three or four junctions. We sweep every assembly for VSWR and insertion loss before shipment.
Talk to an Engineer| Effect | Result |
|---|---|
| Impedance step | 50 ohm vs 75 ohm adapters add mismatch |
| Frequency cap | BNC adapters limit a chain to about 4 GHz |
| Mechanical stress | Heavy adapters strain bench connectors |
| Phase shift | Adds a few picoseconds of delay; matters for phase-matched pairs |
| Intermodulation | Plated and loose adapters can add PIM in high-power systems |
Good practice
Avoid
Do RF adapters cause signal loss?
Yes. A good SMA adapter adds about 0.05 to 0.1 dB at 6 GHz and more at higher frequency, plus a small mismatch.
How much insertion loss does an SMA adapter add?
Typically 0.05 to 0.1 dB at 6 GHz and 0.1 to 0.3 dB at 18 GHz.
Do multiple adapters add up?
Yes. Losses in dB add, and mismatches can add further, so a chain of four adapters can cost roughly 0.4 to 1 dB at 6 GHz.
Is an adapter worse than a cable?
Per junction, often yes. One good adapter adds about as much loss as 3 to 4 cm of RG316 at 6 GHz, and it also adds a reflection point.
Can I measure adapter loss?
Yes. Use a VNA with a through calibration and measure S21 and S11 with and without the adapter.
What is the best way to avoid adapters?
Order a cable assembly with the exact connectors you need at each end.
Adapters cost less than a tenth of a dB each when they are good, and several tenths when they are not. They add up. When a chain has three or more, replace it with one assembly.
SigmaRF builds custom assemblies in India (Vadodara) and the USA (Cleveland area), VNA-swept before shipment, with lead times as short as 2 days.
Remove the adapters from your test setup
Tell us the connectors at each end.
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