Cable Impedance Calculator

About the Cable Impedance Calculator

The Cable Impedance Calculator determines the characteristic impedance of transmission lines including coaxial cables, twisted pair cables, and microstrip traces. Characteristic impedance is a fundamental parameter in RF engineering, telecommunications, and high-speed digital design that determines how electromagnetic waves propagate through a cable.

Impedance mismatches cause signal reflections, standing waves, and power loss — problems that become critical at high frequencies. Whether you're designing an antenna feed line, laying Ethernet cable, or routing high-speed PCB traces, matching impedances at 50Ω, 75Ω, or other target values is essential for reliable signal transmission.

This calculator supports all major cable geometries and dielectric materials. Enter the physical dimensions and material properties to compute impedance, velocity factor, capacitance per unit length, inductance per unit length, and propagation delay. Use it for cable selection, custom cable design, or verifying specifications of existing installations. That is especially helpful when a standard impedance needs to be confirmed against a custom geometry.

When This Page Helps

Use this calculator when you need the characteristic impedance of a cable or trace before routing or ordering parts. It is useful for RF, Ethernet, and PCB work where matching the line to the source and load matters more than the cable length itself. That keeps the design focused on signal integrity instead of guesswork.

How to Use the Inputs

1. Select the cable type: coaxial, twisted pair, or microstrip
2. Enter the conductor dimensions (inner/outer diameter, spacing, trace width)
3. Enter or select the dielectric constant (εr) of the insulating material
4. Review the calculated characteristic impedance in ohms
5. Check additional parameters: velocity factor, capacitance, inductance, and delay
6. Use the common cable presets to quickly look up standard cable types
7. Compare results with the reference table of standard impedances

Example Calculation

Tips & Best Practices

• Always match cable impedance to source and load impedance to minimize reflections
• Use 50Ω cables for transmitting applications (radio, test equipment) and 75Ω for video/receive
• The dielectric constant of the insulation is crucial — even small changes significantly affect impedance
• For PCB traces, impedance depends on trace width, substrate thickness, and dielectric constant
• Cable impedance is independent of cable length, but attenuation increases with length
• Measure actual impedance with a TDR (Time Domain Reflectometer) to verify calculations

Transmission Line Theory Basics

Transmission line theory describes how electromagnetic waves propagate along conductors. Unlike simple circuit theory where wires are assumed to have zero impedance, transmission line theory accounts for distributed inductance and capacitance along the cable's length. When the cable length approaches a significant fraction of the signal's wavelength, these distributed effects become critical.

The characteristic impedance Z₀ = √(L/C), where L is inductance per unit length and C is capacitance per unit length. This relationship shows that impedance depends only on the cable's geometry and materials, not its length. Understanding this principle is fundamental to all RF and high-speed digital design.

Common Cable Types and Applications

Coaxial cables are the most common high-frequency transmission lines. RG-58 (50Ω) is standard for radio and test equipment, while RG-6 and RG-59 (75Ω) serve TV and video distribution. Higher-performance cables like LMR-400 offer lower loss for long runs. The coaxial geometry provides excellent shielding, making these cables resistant to electromagnetic interference.

Twisted pair cables are used extensively in telecommunications and networking. Cat5e/Cat6 Ethernet cables have a characteristic impedance of 100Ω (differential). The twisting reduces crosstalk and EMI pickup. For telephone systems, 120Ω or 150Ω balanced pairs are standard in various regions.

PCB Microstrip Design Considerations

For printed circuit board traces, microstrip and stripline geometries are used to create controlled-impedance transmission lines. Microstrip traces run on the outer layer above a ground plane, while striplines are embedded between ground planes. Modern high-speed designs routinely require 50Ω single-ended and 100Ω differential impedance control with ±10% tolerance.

Frequently Asked Questions

• What is characteristic impedance?

  Characteristic impedance (Z₀) is the ratio of voltage to current of a wave traveling along a transmission line. It depends on the cable's physical geometry and dielectric material, not its length.

• Why are 50Ω and 75Ω the standard impedances?

  50Ω is a compromise between minimum attenuation (77Ω) and maximum power handling (30Ω) for air-dielectric coax, making it ideal for transmitting applications. 75Ω provides minimum attenuation and is standard for receive-only applications like TV/video.

• What happens with impedance mismatch?

  Impedance mismatch causes partial reflection of the signal. The reflection coefficient Γ = (ZL - Z0) / (ZL + Z0). This creates standing waves measured by VSWR, causes signal distortion, and reduces power transfer efficiency.

• What is velocity factor?

  Velocity factor is the ratio of signal speed in the cable to the speed of light in vacuum: VF = 1/√εr. For solid PTFE (εr=2.1), VF ≈ 0.69, meaning signals travel at 69% the speed of light.

• How does frequency affect impedance?

  Characteristic impedance is largely frequency-independent for low-loss cables. However, at very low frequencies, conductor resistance becomes significant and impedance increases. At very high frequencies, skin effect and dielectric losses matter.

• Can I use this for PCB trace design?

  Yes, the microstrip mode calculates impedance for PCB traces above a ground plane. For differential pairs or striplines, you'll need additional parameters not covered by basic microstrip equations.