Semi-rigid vs Flexible Cable for SMA: Insertion-loss Comparison
We compare insertion loss, phase stability, and flex life of semi-rigid and flexible cables terminated with SMA connectors at 6 GHz, 12 GHz, and 18 GHz.
Choosing between semi-rigid and flexible cable for an SMA assembly depends on whether the priority is electrical stability or mechanical convenience. The two cable types behave very differently under bending, temperature, and repeated flexure.
1. Construction differences
Semi-rigid cable uses a solid copper outer tube. It holds its shape after forming and offers excellent phase and amplitude stability. Flexible cable uses a braided outer shield, allowing repeated bending but introducing slightly higher loss and phase variation.
2. Measured insertion loss
Below are typical values for comparable-diameter cables with SMA connectors.
| Frequency | Semi-rigid (dB/m) | Flexible (dB/m) | Difference |
|---|---|---|---|
| 6 GHz | 0.45 | 0.65 | ~0.20 dB/m |
| 12 GHz | 0.70 | 1.05 | ~0.35 dB/m |
| 18 GHz | 0.95 | 1.45 | ~0.50 dB/m |
3. Phase stability
Semi-rigid cable shows phase change below ±2° per meter over temperature, while flexible cable can drift ±10°/m or more because the braid relaxes and the dielectric shifts. For vector-network-analyzer test cables and phase-critical systems, semi-rigid is usually preferred.
4. Flex life
Flexible cable is designed for hundreds or thousands of flex cycles. Semi-rigid cable should be bent once into shape; repeated bending will work-harden and eventually crack the copper outer conductor. If your assembly must move or be rerouted often, flexible cable is the only practical choice.
5. When to choose which
| Priority | Choice |
|---|---|
| Lowest loss, phase stability | Semi-rigid |
| Repeated bending, field deployment | Flexible |
| Precision test fixtures | Semi-rigid or hand-formable |
| Cost and quick installation | Flexible |
Bottom line: Use semi-rigid cable for phase-stable, low-loss, fixed layouts. Use flexible cable for dynamic routing and field service. Do not repeatedly bend semi-rigid cable once installed.
For precision-SMA performance at these frequencies, see SMA 18 GHz vs 26.5 GHz Frequency Margin Analysis.
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