Calculate characteristic impedance of coaxial, twisted pair, and microstrip cables. Determine cable properties for RF, signal integrity, and transmission line design.
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.
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.
Coaxial: Z₀ = (138 / √εr) × log₁₀(D/d), where D = outer conductor diameter, d = inner conductor diameter, εr = relative permittivity. Twisted Pair: Z₀ = (120 / √εr) × ln(2s/d), where s = center-to-center spacing, d = wire diameter. Microstrip: Z₀ = (87 / √(εr + 1.41)) × ln(5.98h / (0.8w + t)).
Result: 50.2 Ω
For a coaxial cable with 7.24 mm outer and 1.83 mm inner conductor diameters with PTFE dielectric (εr=2.3): Z₀ = (138/√2.3) × log₁₀(7.24/1.83) = 50.2Ω, matching the standard 50Ω specification.
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.
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.
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.
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.
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.
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.
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.
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.
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.