All You Need to Know About Semi-Flexible Cable Assembly
Semi-flexible RF coaxial cable assemblies bridge the gap between fully flexible braided coax and semi-rigid coax, offering stable microwave performance with hand-formable routing—ideal for compact RF modules and repeatable test setups.
Why semi-flex?
- Shape retention after forming
- Better electrical stability than soft coax
- High shielding in dense RF packaging
Where it’s used
- RF modules (radar, avionics, telecom)
- ATE & lab test fixtures
- Interconnects: amps, filters, couplers
What you specify
- Cable size (0.086 / 0.141 / 0.250)
- Connector series & part numbers
- Length definition + RF test limits
Why You Need Semi-Flexible Cable Assemblies
Semi-flex coaxial RF cable assemblies are selected when you need stable RF performance and controlled routing in compact enclosures. Compared with common braided flexible coax, semi-flex cables tend to hold their formed shape and are less sensitive to movement—supporting more repeatable return loss and insertion loss in real-world packaging.
Types of Semi-Flexible Coaxial Cable
This is the most common meaning of “semi-flex”: a hand-formable coax that can be bent into shape and will stay there. Typical construction includes a solid (or plated) center conductor, PTFE dielectric, and a form-stable outer conductor (often braid/composite).
Typical procurement specs to compare
- Impedance: typically 50 Ω
- Frequency capability: commonly 18–33+ GHz (depends on size/design)
- Velocity of propagation (VoP): often ~0.70–0.71 for PTFE designs
- Shielding: very high; depends on construction
- Temperature range: depends on dielectric/jacket; PTFE supports high temperatures
- Bend/handling: can be formed; repeated re-bending can degrade performance
Common size families
Semi-rigid coax uses a solid metal tube outer conductor. It provides excellent shielding and electrical stability, but routing typically requires proper forming tools and it is not intended for repeated bending. Many engineers compare semi-flex against semi-rigid when evaluating phase stability and shielding.
Corrugated coax is commonly used in RF infrastructure to balance loss, power handling, and mechanical durability. It’s generally larger and used in different contexts than module-level conformable cables, but may be considered a “semi-flex” option in some procurement catalogs due to its controlled flexibility.
Premium flexible microwave cables can offer low loss and improved phase stability compared to common braided cables, while remaining more flexible than conformable or semi-rigid options. They typically don’t “hold shape” like conformable cable but are useful where movement tolerance is required.
RF Coaxial Connectors Used with Semi-Flexible Cable
Connector selection depends on frequency, size constraints, mating style, and reliability requirements. Semi-flex assemblies commonly use precision microwave connector series for module-level RF.
| Connector family | Why it’s used | Typical use cases |
|---|---|---|
| SMA | Compact, widely available, common for microwave interconnects | General RF modules, lab fixtures |
| 2.92 mm (K), 2.4 mm, 1.85 mm, 1.0 mm | Higher-frequency precision ecosystems | Millimeter-wave modules, calibration-grade setups |
| SMP / SMPM / SMPS | Push-on / blind-mate for dense packaging | Board-to-board / module interconnects |
| BNC / TNC / N-type | More rugged / larger, often system-level | Benchtop, rack systems, ruggedized I/O |
| MCX / MMCX / SMB | Small form factor (frequency depends on series and design) | Compact RF subsystems |
Selecting a Supplier: What to Share and What to Request
What to provide the supplier
- Cable type & size family (0.086 / 0.141 / 0.250)
- Connector part numbers for both ends + straight/right-angle
- Finished length definition and tolerance
- Frequency range + max VSWR/return loss + insertion loss limits
- Power handling (CW/peak) if applicable
- Environment (temperature, vibration, corrosion), packaging constraints
- Marking, caps, and cleanliness/handling expectations
Quality & documentation to request
- Assembly drawing: recommended for clarity (length reference planes, clocking, keep-outs)
- Certificate of Conformance (CoC): materials/parts traceability as needed
- RF test data: VSWR/return loss and insertion loss across your band
- Test method notes: VNA calibration approach & reference planes
- Workmanship standard: IPC/WHMA-A-620 (Class 2 or 3 depending on reliability needs)
- Connector/cable standards: if required, specify MIL connector/cable ecosystems
Handling During Interconnection (Amps, Filters, Waveguides, etc.)
Installation best practices
- Respect minimum bend radius; avoid sharp bends behind connectors.
- Use correct mating torque and proper wrenches; avoid over-tightening.
- Keep connector interfaces clean; cap ends when not in use.
- Add strain relief so component ports aren’t used as mechanical supports.
Integration tips
- Avoid twisting the cable while tightening; use a backup wrench where applicable.
- Route away from hot zones (e.g., power amps) to reduce thermal cycling stress.
- When transitioning to waveguide chains, use coax-to-waveguide adapters and avoid side loading.
- For dense modules, consider blind-mate families (SMP/SMPM) to reduce rework risk.