What Is Semi-Rigid Coaxial Cable?

Semi-rigid coaxial cable is widely regarded as the closest practical implementation of an ideal coaxial transmission line. For high-frequency RF and microwave systems, it remains the benchmark against which other coaxial cable constructions are measured.

Tek-Stock LLC supplies a broad range of semi-rigid coaxial cable options — including multiple diameters, materials, impedance values, temperature ratings, and performance grades — available for immediate shipment to aerospace, defense, telecom, medical, and RF test customers.

This guide explains what defines semi-rigid coax, how it is constructed, and why it continues to be specified in precision microwave systems operating from low frequencies through millimeter-wave ranges.

What Qualifies as Semi-Rigid Coaxial Cable?

To be classified as semi-rigid coaxial cable, the construction must use a solid metallic outer conductor tube — not braided shielding.

This solid outer conductor provides:

• Extremely high shielding effectiveness
• Precise dimensional control
• Superior impedance stability
• Environmental sealing without an external jacket

Most semi-rigid cables fall below 0.250″ diameter, although certain constructions extend up to approximately 0.500″ for higher-power or specialized applications.

Core Construction Elements

Outer Conductor Materials

Semi-rigid cable outer conductors may be manufactured from:

• Copper
• Tin-plated aluminum (TP)
• Stainless steel (SS)

Material selection affects weight, bend characteristics, thermal conductivity, and corrosion resistance.

Dielectric Options

Most semi-rigid cables use solid PTFE dielectric for stable electrical properties and wide temperature capability.

Certain low-loss (LL) constructions use modified or low-density PTFE formulations to improve:

• Attenuation
• Phase stability
• Thermal performance

Typical maximum operating temperatures range from 125°C up to 250°C, depending on construction.

Center Conductor Materials

Semi-rigid coaxial cables are available with multiple center conductor materials, including:

• Silver-Plated Copper (SPC)
• Silver-Plated Copper Weld (SPCW)
• Stainless Steel
• Silver-Plated Beryllium Copper

Each option balances conductivity, mechanical strength, and environmental suitability.

• SPCW improves tensile strength and vibration resistance.
• Stainless steel reduces thermal conductivity for cryogenic systems.
• Silver-plated beryllium copper offers fatigue resistance with excellent electrical performance.

Impedance Range

Although 50 Ω is the most commonly specified impedance in RF and microwave systems, semi-rigid coaxial cable is available in impedance values ranging from 5 Ω to 100 Ω.

This allows use in:

• 75 Ω video and instrumentation systems
• Specialized RF architectures
• Legacy platforms
• Custom impedance matching applications

Frequency Capability

Semi-rigid coaxial cable is suitable for applications from very low frequencies through millimeter-wave ranges — with some constructions supporting operation up to 110 GHz.

In systems operating above 500 MHz where shielding integrity and stable performance are critical, semi-rigid coax is often the preferred transmission medium.

Performance Advantages

Semi-rigid coax is widely specified because it offers:

• RF shielding exceeding –130 dB
• Extremely stable impedance control
• Excellent VSWR performance
• Low attenuation for its geometry
• Small overall diameter options
• Tight bend radii for compact packaging
• Environmental sealing without risk of jacket damage

Once formed, semi-rigid cable maintains its shape, allowing maximum packaging efficiency without wasted space inside assemblies.

Available Constructions

Tek-Stock LLC supports a broad range of semi-rigid constructions, including:

MIL-DTL-17 Qualified Cables

For aerospace and defense applications requiring additional qualification testing and compliance verification.

Standard Copper 50 Ω Cables

Diameters ranging from approximately 0.013″ up to 0.390″, with extended lengths available in select constructions.

Dimensionally Stable (“DS”) Cables

Utilizing thermally enhanced PTFE formulations that reduce dielectric movement during soldering and improve dimensional consistency under temperature cycling.

Low-Loss & Ultra-Low-Loss Cables

Engineered to reduce attenuation and extend operating temperature range, often specified in satellite, radar, and high-frequency microwave systems.

Aluminum-Jacketed Cables

Offering weight reduction and improved bendability, useful in aerospace applications where mass is critical.

Stainless Steel-Jacketed Cables

Used in cryogenic systems, medical electronics, and environments requiring reduced thermal conductivity or corrosion resistance.

Non-50 Ω Cables

Available in impedance values from 5 Ω to 100 Ω in multiple diameters.

Typical Applications

• Defense electronics
• Radar systems
• Spaceflight hardware
• Test and measurement instrumentation
• Medical electronics
• Telecommunications systems
• Microwave amplifiers and oscillators
• PCB interconnects
• Delay lines
• Capacitive and filter sections

Because semi-rigid cable can be precisely formed, it is ideal for tightly packaged microwave assemblies and high-density electronics.

Why Inventory Depth Matters

RF and microwave programs often require very specific combinations of:

• Diameter
• Impedance
• Jacket material
• Center conductor type
• Qualification level
• Temperature rating

Final Thoughts

Semi-rigid coaxial cable remains the benchmark microwave transmission medium because it provides unmatched shielding performance, stable impedance control, and precise mechanical formability.

With impedance options from 5 Ω to 100 Ω, frequency capability through millimeter-wave ranges, multiple conductor materials, and performance grades ranging from standard to ultra-low-loss, semi-rigid coax continues to serve as the foundation of modern RF and microwave systems.

Tek-Stock LLC maintains a broad semi-rigid inventory to support both prototype development and full production programs. Immediate availability reduces delays in qualification testing, aerospace builds, and high-frequency system integration.

For guidance on diameter selection, impedance requirements, or available constructions, Tek-Stock LLC provides knowledgeable support and ready-to-ship inventory across a wide range of semi-rigid configurations. For more information please email sales@tek-stock.com