Coaxial Cable Impedance & Attenuation Calculator
Explore how conductor diameters, dielectric properties, frequency, and cable length affect impedance and signal loss. Calculate an existing geometry or solve the shield inner diameter for a target impedance.
This tool models a smooth, concentric coax with solid nonmagnetic conductors and a uniform dielectric. Compare estimates with the actual cable or assembly’s specified performance.
Calculate your coaxial line
d is the center conductor diameter. D is the inner diameter of the shield—not the jacket outside diameter.
Calculated estimates
Enter your geometry and calculate.
- Ideal velocity factor
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- Ideal one-way delay
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- Capacitance per meter
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- External inductance per meter
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The impedance, velocity, and delay above are ideal TEM references—not a complex, frequency-dependent impedance or dispersive group-delay calculation.
Only the ideal geometry references remain. Use measured cable data or a more complete model for this operating point.
Separate the loss mechanisms
| Loss component | dB/m | dB/100 m | dB/100 ft |
|---|
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Frequency comparison
The same geometry, length, εr, tanδ, and conductivities are held fixed. This is not a manufacturer’s measured attenuation curve. Unavailable rows identify the limiting assumption.
| Frequency | Conductor dB/m | Dielectric dB/m | Cable loss dB |
|---|
Check the actual cable assembly
Equal impedance does not mean equal attenuation. Braid construction, plating, foam, surface condition, connectors, bends, and temperature can change real performance. Use the selected part’s data and assembly test requirements before purchase.
Which diameters should you enter?
Three inputs to verify first
- Center conductor d: the metal diameter. Stranded conductors need a construction-specific treatment.
- Shield inner diameter D: the inner boundary of the outer conductor. In the ideal filled model, this also bounds the dielectric.
- Dielectric εr: the relevant material value, not a generic assumption based only on a cable family name.
The radial dielectric gap is (D − d) / 2. The solid shield’s outside diameter is D + 2t; neither is a substitute for D.
For a nonmagnetic homogeneous TEM line, velocity factor ≈ 1/√εr. A published velocity factor can help check the dielectric assumption, but it does not provide the loss tangent or predict attenuation by itself.
Equations & validity checks
The calculation separates ideal TEM geometry from low-loss RF attenuation. It assumes μr = 1 throughout and uses meters, hertz, and S/m internally.
Impedance & propagation
K = μ0c / (2π) ≈ 59.9585 Ω
Z0 = K × ln(D/d) / √εr
D = d × exp(Ztarget × √εr / K)
C′ = 2πε0εr / ln(D/d)
L′ = μ0 × ln(D/d) / (2π)
v = c / √εr; delay = ℓ / vln is the natural logarithm. L′ is the external inductance approximation; conductor internal inductance is excluded. Source: Ellingson, Coaxial Line.
Conductor & dielectric loss
δi = 1 / √(πfμ0σi); δo = 1 / √(πfμ0σo)
Rsi = √(πfμ0/σi); Rso = √(πfμ0/σo)
R′ = Rsi / (πd) + Rso / (πD)
αc = R′ / (2Z0)
αd = πf√εr × tanδ / c
A = (20/ln 10) × (αc + αd) × ℓα is in Np/m; A is positive cable attenuation in dB. The model uses separate inner and outer conductivities. Conductor-loss derivation · Loss-tangent model.
When RF loss is withheld
This tool requires δi ≤ a/10, δo ≤ b/10, δo ≤ t/5, R′/(ωL′) ≤ 0.1, and tanδ ≤ 0.05, where a = d/2 and b = D/2. These software guardrails screen the thin-skin and low-loss assumptions; they are not an accuracy guarantee.
Frequency must also be ≤ 0.5 × the approximate TE11 cutoff, fc ≈ 2c / [π(D + d)√εr]. This conservative screening margin is not a cable or connector frequency rating. The TEM mode itself has no lower cutoff. About higher-order modes.
Input window & exclusions
d: 0.05–20 mm; D: 0.06–100 mm; D/d: 1.2–20; εr: 1–10; frequency: 0.1 MHz–100 GHz; length: 0.001–10,000 m; shield wall: 1–5,000 µm; conductivities: 1–65 MS/m.
tanδ accepts 0–0.2, but values above 0.05 return geometry references only. Diameter solving accepts targets of 10–200 Ω, subject to the geometry limits.
No braid, foil seams, plating layers, roughness correction, connectors, bends, leakage, separate DC dielectric conduction, thermal power rating, or frequency dispersion is modeled. The sweep assumes material values stay constant.
Choose the cable, not just the impedance
Specify the operating band
Give the target impedance, frequency range, installed length, and cable-only attenuation allowance. Check the manufacturer’s published loss at your frequency and temperature, not just the nominal 50 Ω or 75 Ω label.
Match the assembly
Confirm connector series, gender, interface dimensions, cable construction, minimum bend radius, jacket requirements, and installation environment. A connector transition can limit performance even when the bulk cable is suitable.
Agree the evidence
For a finished assembly, request the relevant datasheet and agree any insertion-loss, return-loss, continuity, or dimensional records needed for acceptance. Keep measured assembly performance separate from this idealized estimate.
Coaxial cable calculation questions
Does making a cable longer change its impedance?
Not the characteristic impedance of a uniform line. In this model, Z0 depends on D/d and εr. Length increases delay and cable attenuation. The input impedance of a terminated cable is a different quantity and can depend on length, frequency, and load.
Why do two 50 Ω cables have different loss?
The diameter ratio sets ideal impedance, but conductor size, surface conductivity, dielectric loss, and construction determine attenuation. With fixed D/d and material properties, scaling both diameters up lowers the modeled conductor loss; the dielectric-loss term stays the same. The larger structure also has a lower approximate higher-mode cutoff.
Can I enter my cable’s jacket diameter as D?
No. D is the inside diameter of the metal shield. A jacket diameter includes the shield and protective layers. Ask for a construction drawing or use the manufacturer’s impedance and attenuation data if the required dimensions are unavailable.
Can this predict RG58, RG6, or LMR cable loss?
Not reliably from the family name alone. Real cables may use stranded or plated centers, foam dielectrics, foil, and braid. The examples here are generic solid-conductor geometries, not presets or specifications for a commercial cable. Use the exact manufacturer and part number for selection.
Are connector loss and impedance mismatch included?
No. The attenuation estimate assumes a uniform, matched line and includes only the modeled conductor and dielectric loss. Connector transitions, return loss, adapter chains, and installation effects require separate data or a measured assembly result.
Why does the tool show impedance but no attenuation?
The dimensions still define an ideal TEM geometry reference, but the chosen operating point may fail the thin-skin, low-loss, or higher-mode checks. The result lists the reason. Do not treat the remaining ideal values as a complete RF model at that frequency.
Related engineering tools
Need RF parts for your build?
Share the approved part numbers, quantities, connector requirements, and required date. YURUNOX can review sourcing options for RF connectors, interconnect components, and supporting electronics.
Technical references
- Steven W. Ellingson, Virginia Tech — Electromagnetics I, Coaxial Line.
- Steven W. Ellingson, Virginia Tech — Electromagnetics II, Attenuation in Coaxial Cable.
- Microwaves101 — Coax Loss Calculations.
- Microwaves101 — Coax Cutoff Frequency.
- Michael Steer — Microwave and RF Design II, Transmission Line Theory.