Cable Characteristic Impedance Calculator
Compute characteristic impedance Z₀ of cables: coaxial, twisted pair, microstrip. Z₀ = √(L/C). For impedance matching.
Characteristic impedance of a coaxial cable: Z0 = (138/√εr) · log10(D/d)
The characteristic impedance Z0 of a coaxial cable depends only on geometry and dielectric: Z0 = (138/√εr) · log10(D/d), where D is the shield inner diameter, d the inner conductor diameter, and εr the dielectric constant (≈2.3 for polyethylene). For transmission lines in general, Z0 = √(L/C). Standard values: 50 Ω for RF, lab instrumentation and scientific networks (RG-58, RG-213, LMR-400); 75 Ω for cable TV and video distribution (RG-6, RG-59); 93 Ω in legacy computer links. Twisted pair Cat6 carries 100 Ω differential. Example: D = 6 mm, d = 2 mm, εr = 2.3 → Z0 = (138/1.517) · log10(3) ≈ 43.4 Ω. Matching: reflection vanishes when the load equals Z0; VSWR measures mismatch and the closer to 1:1, the better.
Applications: cable TV, Wi-Fi antennas, Ethernet, ham radio
Characteristic impedance is the basis for sizing cable TV networks (NET, Claro), Wi-Fi antenna feeds, broadcast and ham-radio RF chains, Ethernet twisted-pair (100 Ω), laboratory instrumentation (oscilloscopes, network analyzers) and any link that must avoid reflections.
FAQ
Why 50 Ω in RF and 75 Ω in TV? 50 Ω is the compromise between minimum loss (≈77 Ω) and maximum power-handling (≈30 Ω); 75 Ω minimises loss for video signals.
Does Z0 depend on cable length? No — it depends only on cross-sectional geometry and the dielectric, not length.
What is VSWR? Voltage Standing Wave Ratio: 1:1 means perfect match, 2:1 already indicates appreciable reflection.
Can I connect 50 Ω to 75 Ω? Only with an impedance transformer (matching network); direct connection causes reflections and power loss.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.