Semi rigid coaxial cables have stable electrical properties and strong shielding performance. Unlike flexible coaxial cables that use braided outer conductors, semi rigid cables employ a solid metal outer conductor and a PTFE dielectric, which helps maintain consistent impedance and minimize signal leakage.
What Is a Semi Rigid Coaxial Cable
Semi rigid coaxial cable definition
A semi rigid coaxial cable is a type of coaxial transmission line built with a solid metal outer conductor instead of a braided shield. The outer conductor is typically formed from copper or silver plated copper tubing, while the dielectric is commonly PTFE. This construction maintains a fixed coaxial geometry that supports stable characteristic impedance, commonly 50 ohms, across a wide frequency range that can extend into microwave bands above several gigahertz.
Semi rigid coaxial cables are commonly manufactured in diameters such as 0.086 inch, 0.141 inch, and 0.250 inch. These cables are often used as internal interconnects in microwave assemblies, RF test systems, radar hardware, and satellite communication equipment where controlled impedance and strong shielding are required.
Structural differences from flexible coaxial cables
Flexible coaxial cables normally use braided outer conductors made from copper wire strands. The braided structure allows repeated bending and movement but introduces small variations in the cable geometry. Semi rigid coaxial cable replaces the braid with a seamless metal tube that fully surrounds the dielectric and center conductor.
The continuous metal shield provides higher shielding effectiveness and improved mechanical stability. Signal leakage is reduced and impedance variation along the cable length remains minimal. In microwave assemblies where cable routing remains fixed after installation, the rigid outer conductor helps maintain consistent electrical performance.
Internal Construction of Semi Rigid Coaxial Cable
Center conductor
The center conductor of a semi rigid coaxial cable is typically made from solid copper or silver plated copper. Solid conductors provide stable electrical characteristics and maintain a consistent transmission path for high frequency signals. Silver plating is often applied in microwave cables because it improves surface conductivity and reduces conductor loss at high frequencies where current mainly flows along the conductor surface.
Typical semi rigid cables designed for microwave applications operate with a characteristic impedance of 50 ohms. Maintaining precise conductor diameter and concentric alignment with the dielectric is important for preserving impedance stability across the transmission line.
PTFE dielectric insulation
Between the center conductor and the outer conductor lies a dielectric layer, commonly made from polytetrafluoroethylene, known as PTFE. This material is selected for its low dielectric constant, typically around 2.1, and its low loss tangent, which helps reduce signal attenuation in high frequency transmission.
PTFE also maintains stable electrical properties across a wide temperature range. In microwave and high frequency assemblies, this stability helps maintain predictable impedance and signal propagation characteristics throughout the cable length.
Continuous metal outer conductor
Semi rigid coaxial cable uses a seamless metal tube as the outer conductor instead of a braided shield. The outer conductor is usually made from copper tubing and sometimes plated with tin or silver depending on the application environment.
A continuous metal shield provides excellent shielding effectiveness by fully enclosing the electromagnetic field inside the cable structure. Shielding performance in semi rigid coaxial cables can exceed 90 dB in many microwave cable designs.
Advantages of Semi Rigid Coaxial Cable
Impedance stability from a fixed coaxial structure
Semi rigid coaxial cable maintains a precisely controlled coaxial geometry because the outer conductor is a solid metal tube rather than a braided shield. The center conductor, dielectric layer, and outer conductor remain in a fixed concentric alignment along the entire cable length. This stable coaxial geometry allows the cable to maintain a consistent characteristic impedance along the entire transmission line.
In high frequency signal paths, even small changes in conductor spacing can affect impedance and reflection performance. The rigid structure of semi rigid coaxial cable minimizes these variations and supports accurate signal transmission across microwave frequencies that may extend into tens of gigahertz depending on cable diameter.
High shielding effectiveness and low signal leakage
Semi rigid coaxial cable uses a continuous metal outer conductor that completely surrounds the dielectric and center conductor. This structure forms a closed electromagnetic shield around the signal path. Compared with braided shields used in many flexible coaxial cables, a seamless copper tube provides much higher shielding effectiveness.
In microwave assemblies and RF measurement systems where multiple transmission lines are placed close together, strong shielding helps limit electromagnetic coupling between adjacent cables. This improves signal isolation and keeps high frequency signal paths clean. Semi rigid coaxial cables commonly achieve shielding performance above 90 dB, which reduces signal leakage and limits external interference.
Suitability for high frequency and high power transmission
Semi rigid coaxial cable supports signal transmission across microwave and millimeter wave frequency ranges depending on cable size and design. Common microwave assemblies use semi rigid cables with diameters such as 0.086 inch or 0.141 inch for compact signal routing.The solid outer conductor also improves thermal and electrical stability when handling higher power levels compared with many small flexible coaxial cables.
Limitations of Semi Rigid Coaxial Cable
Limited flexibility
Semi rigid coaxial cable uses a solid metal outer conductor instead of a braided shield. This structure gives the cable high mechanical stability but also limits its flexibility. Once the cable is bent into shape during installation, it generally remains fixed in that position.
Because of this mechanical characteristic, semi rigid coaxial cables are typically used as internal interconnects inside microwave modules, RF assemblies, and test equipment where the cable routing does not change after installation.
Bending and installation require specialized tools
Forming a semi rigid coaxial cable into the required routing path normally requires bending tools designed for coaxial tubing. Improper bending may deform the outer conductor or change the spacing between the center conductor and dielectric, which can affect impedance and signal performance.
Precision bending tools help maintain the circular cross section of the outer conductor and protect the internal dielectric structure. In microwave assemblies where electrical parameters must remain stable, careful cable forming is an important step during installation.
Not suitable for systems with frequent movement
Semi rigid coaxial cable is designed for installations where the cable remains fixed after routing. Repeated bending or movement can damage the solid outer conductor or affect the internal coaxial structure.Applications that require continuous motion, frequent cable repositioning, or high mechanical flexibility usually use flexible coaxial cables instead. Flexible cable designs allow repeated bending without affecting the transmission line structure.
FAQ
Conclusion
Semi rigid coaxial cable is often selected in microwave and high frequency equipment when signal stability and shielding are important. The solid metal outer conductor and PTFE dielectric keep the internal coaxial structure consistent, which supports reliable signal transmission in many RF modules and test assemblies.If you have questions about semi rigid coax cables or need help choosing the right RF cable assembly, feel free to leave a comment or contact our engineers to discuss your application.