Cable Impedance Calculator

Characteristic impedance is the inherent impedance of a transmission line, meaning the ratio of voltage to current for a wave traveling along the cable. When cable impedance is mismatched with connected devices, signal reflections occur, causing signal loss, interference, and in RF systems, reduced power transfer.

Why Impedance Matching Matters

In RF and high-frequency systems, impedance mismatch causes reflected power. The VSWR (Voltage Standing Wave Ratio) measures this mismatch:

• VSWR of 1:1 = perfect match, all power transferred
• VSWR of 2:1 = ~11% power loss from reflection
• VSWR of 3:1 = ~25% power loss

In audio systems, impedance mismatch affects frequency response and signal level but rarely causes reflections due to the low frequencies involved.

Coaxial Cable Impedance Formula

Z = (138 / √εr) × log₁₀(D / d)

Where:

• Z = characteristic impedance (ohms)
• εr = relative permittivity (dielectric constant) of insulation
• D = inner diameter of the outer conductor (shield)
• d = outer diameter of the inner conductor (center wire)

Common Dielectric Constants (εr)

Foam dielectrics are worth a note. Blowing gas into the polyethylene drops the effective εr toward 1.4 or 1.5 depending on how much air is in there, which raises both the impedance and the velocity factor for the same physical dimensions. That is why low-loss cables like LMR-400 quote a velocity factor around 0.85 while solid-PE RG-58 sits at 0.66.

The dropdown carries two foams for exactly this reason. The 1.6 entry is a mid-density foam with a velocity factor near 0.79. The 1.4 entry is the heavily gassed sort used in low-loss coax and comes out at 0.85, which is the figure on the LMR-400 datasheet. Two cables described in a catalogue as “foam dielectric” can therefore sit some way apart, so read the actual number rather than the word.

Foaming only ever lowers εr, because the gas it adds is close to 1.0. A foam can never come out above solid polyethylene’s 2.3, so a figure above that belongs to a different plastic. The 2.56 in the table is polystyrene.

Standard Cable Impedances

• 50 Ω: RF, antenna, instrumentation (RG-58, RG-213, LMR-400, SMA connectors)
• 75 Ω: Video, cable TV, satellite (RG-6, RG-59, F connectors)
• 93 Ω: Older computer and instrumentation coax (RG-62, ArcNet)
• 300 Ω: Flat twin-lead, the ribbon that used to run to a TV aerial
• 450 Ω: Open-wire ladder line, still popular for amateur antenna feeds

The 600 Ω figure you sometimes see quoted for audio is a different thing and does not belong on this list. It came from telephone and broadcast practice, where equipment was built to transfer maximum power into a matched 600 Ω load. Modern balanced audio gear does not work that way: a microphone or line output has a low source impedance, typically under 200 Ω, feeding a bridging input of several kilohms. Nothing is matched, and at audio frequencies a cable is far shorter than a wavelength, so its characteristic impedance never comes into play. This calculator is for transmission lines, which in practice means RF and fast digital signals.

Parallel Wire Formula

For two parallel conductors of diameter d, separated by center-to-center distance D:

Z = (276 / √εr) × log₁₀(2D / d)

Parallel line is almost always run in air, so leave the dielectric on 1.0 unless the pair is embedded in something. Spacing matters much more than it does for coax: two 1 mm wires 10 mm apart give about 360 Ω, and moving them to 20 mm apart only raises that to about 442 Ω, because the relationship is logarithmic. Doubling the spacing is nowhere near doubling the impedance.