- Mechanical Stress Management
A. Bending and Shaping Guidelines
Semi-rigid cables are designed for one-time bending. Improper shaping can crack the outer conductor, causing signal leakage or impedance mismatches.
Cable Diameter (mm) Minimum Bend Radius (Static) Maximum Bend Angle (per 10cm length)
2.2 8 mm 90°
3.0 12 mm 60°
5.0 20 mm 45°
Tools: Use mandrel benders (e.g., Times Microwave SMB-2200) to ensure uniform curvature.
B. Avoiding Fatigue Fractures
Vibration-Prone Environments: Secure cables with clamps every 15–20 cm (per MIL-STD-348).
Do NOT Re-bend: Once shaped, re-adjusting bends risks metal fatigue. Replace deformed sections.
- Thermal Considerations
A. Temperature Limits During Installation
Operating Range: -55°C to +165°C (PTFE dielectric).
Soldering/Assembly: Keep localized heat below 300°C to prevent dielectric melting. Use heat sinks on connectors.
B. Thermal Expansion Mismatch
Example: A 3.0mm aluminum-jacketed cable in a satellite payload failed due to CTE (Coefficient of Thermal Expansion) mismatch with a steel chassis.
Solution:
Use flexible transition sections (e.g., braided jumpers).
Match materials (e.g., copper cables with copper brackets).
- Connector Installation Best Practices
A. Soldering Techniques
Flux Selection: Non-corrosive rosin flux (e.g., Kester 1544) to avoid residue-induced corrosion.
Soldering Iron: 60W with a chisel tip (350–400°C) for rapid heat transfer.
Step-by-Step:
Clean the cable end with isopropyl alcohol.
Tin the inner conductor and connector pin.
Solder within 3 seconds to prevent dielectric damage.
B. Connector Alignment
Misalignment causes impedance spikes (>2.0 VSWR at 18 GHz).
Tool: Use alignment jigs (e.g., Southwest Precision SNAPA-LOK).
Verification: Test with a vector network analyzer (VNA) post-installation.
- Grounding and Shielding Integrity
A. Grounding the Outer Conductor
Direct Grounding: Solder the cable’s outer conductor to a low-impedance ground plane.
Avoid Paint/Coating: Scrape off anodized layers on mounting surfaces for metal-to-metal contact.
B. Shield Continuity Testing
Method: Use a milliohm meter to measure resistance between cable shield and chassis (<5 mΩ).
Failure Case: A radar system EMI issue traced to a 120 mΩ ground discontinuity in a 5.0mm cable.
- Environmental Protection
A. Moisture and Corrosion
Sealing: Apply silicone sealant (e.g., Dow Corning 732) at connector interfaces in humid environments.
Material Choice: Opt for silver-plated copper in salt-spray conditions (per ASTM B117).
B. Abrasion Resistance
Sleeving: Protect cables in high-friction zones with PTFE sleeving (e.g., Gore Tex GORE™ Protective Sleeve).
- Common Mistakes and Fixes
Mistake Consequence Solution
Over-tightening clamps Outer conductor crushing Torque to 0.6–0.8 Nm
Using non-mandrel tools Oval deformation Replace damaged section
Skipping post-installation VNA test Undetected impedance mismatch Re-solder or replace connector
- Case Study: Aerospace Antenna Feed Failure
Issue: A bent 2.2mm semi-rigid cable in a UAV’s phased array antenna caused 6 dB loss at 15 GHz.
Root Cause: Hand-bent beyond 8mm radius, cracking the outer conductor.
Fix: Replaced with a pre-shaped section and added vibration-damping mounts.
- Industry Standards Summary
Standard Focus Area Key Requirement
MIL-STD-348 RF connector interfaces VSWR <1.3 up to 18 GHz
IEC 60169 Soldering and grounding Shield continuity <5 mΩ
NASA-STD-8739.4 Spacecraft cable assembly No sharp bends, 2x thermal cycle testing
