SMA Connector
Understanding SMA Connectors
The SMA connector was developed in the 1960s by Bendix Scintilla (now Amphenol) and quickly became the standard small-form-factor RF connector. Its threaded coupling provides a reliable, repeatable connection that maintains consistent VSWR across the operating band. SMA connectors are available in bulkhead, edge-launch, end-launch, and cable-mount configurations.
Key Specifications
- Impedance: 50 ohms
- Frequency range: DC to 18 GHz (standard), DC to 26.5 GHz (precision)
- VSWR: 1.15:1 typical at 12.4 GHz; 1.25:1 at 18 GHz
- Mating cycles: 500 minimum per MIL-STD-348
- Coupling torque: 5 in-lbs (0.56 N-m)
- Thread: 1/4-36 UNS-2
- Dielectric: PTFE (Teflon)
- Power handling: 500W at 1 GHz; derated at higher frequencies
SMA Connector Family
The SMA interface is compatible with several extended-frequency connectors that share the same outer conductor thread:
- 3.5mm: Air-dielectric interface, usable to 34 GHz. Mode-free. Mates with SMA.
- 2.92mm (K connector): Usable to 40 GHz. Mates with SMA and 3.5mm.
- SSMA: Smaller version (not compatible) for higher density applications to 40 GHz.
Common Applications
- Test and measurement: VNA port cables, signal generator outputs, spectrum analyzer inputs
- PCB launches: Edge-launch and end-launch connectors for microstrip and stripline transitions
- Cable assemblies: Semi-rigid (RG-405, RG-402) and flexible coaxial cables
- Military/aerospace: MIL-spec qualified connectors for harsh environment systems
Best Practices
Always use a calibrated torque wrench set to 5 in-lbs. Inspect the center pin for bent or recessed contacts before mating. Never spin the connector body; only rotate the coupling nut. For precision measurements, clean mating surfaces with isopropyl alcohol and lint-free wipes. Replace connectors showing visible wear on the center pin or thread damage.
Key Equations
The SMA (SubMiniature version A) connector is a threaded coaxial RF connector with a 50-ohm impedance. Operating from DC to 18 GHz in standard configurations...
Key specifications:
18 GHz | 26.5 GHz | 50 ohm | 12.4 GHz
S-params: IL=−20log|S21|, RL=−20log|S11|
Comparison
| Connector | Freq Max | Impedance | Power | Interface |
|---|---|---|---|---|
| SMA | 18 GHz | 50 Ω | 0.5 W | Threaded |
| N-Type | 11 GHz | 50 Ω | 5 W | Threaded |
| 2.92mm (K) | 40 GHz | 50 Ω | 0.3 W | Threaded |
| 1.85mm (V) | 67 GHz | 50 Ω | 0.2 W | Threaded |
| 1.0mm (W) | 110 GHz | 50 Ω | 0.1 W | Threaded |
Frequently Asked Questions
What is an SMA connector?
An SMA (SubMiniature version A) connector is a threaded coaxial RF connector with 50-ohm impedance. It operates from DC to 18 GHz in standard configurations, or up to 26.5 GHz in precision-grade versions. The SMA is one of the most common RF connectors, used on test cables, PCB launches, adapters, and cable assemblies.
What is the difference between SMA and 3.5mm connectors?
SMA and 3.5mm connectors share the same outer conductor diameter and thread, so they can mate with each other. However, 3.5mm connectors use an air dielectric interface (no plastic support bead) and tighter tolerances, extending usable frequency to 34 GHz. Use 3.5mm when you need performance above 18 GHz while maintaining SMA compatibility.
What torque should I use on SMA connectors?
Standard SMA connectors should be tightened to 5 inch-pounds (0.56 N-m) using a torque wrench. Over-tightening damages the center pin and dielectric; under-tightening causes poor contact and intermittent connections. Always use a torque wrench for test and measurement applications.