If you're building, selecting, or troubleshooting RF and high-speed signal paths, characteristic impedance matters. Use this coaxial cable impedance calculator to estimate Z0 from cable geometry and dielectric properties, then compare your result with common design targets like 50 Ω and 75 Ω.
Coaxial Cable Impedance Calculator
Enter dimensions in the same unit (mm, inches, etc.). The ratio is what matters.
How coaxial cable impedance is calculated
For a coaxial transmission line with uniform dielectric, characteristic impedance depends on geometry and material properties. The standard expression is:
Where:
- Z0 = characteristic impedance in ohms
- D = inner diameter of the outer conductor
- d = outer diameter of the inner conductor
- εr = relative permittivity of the dielectric
- μr = relative permeability (typically very close to 1 for non-magnetic dielectrics)
Why impedance matters
When source, transmission line, and load impedances are matched, reflections are minimized. In practical systems, that means better power transfer, cleaner waveforms, lower VSWR, and fewer measurement artifacts. Mismatch can produce standing waves, ripple, and amplitude errors—especially at higher frequencies and longer cable runs.
Typical coax impedance standards
- 50 Ω: common in RF transmit/receive systems, test equipment, and antennas
- 75 Ω: common in video distribution, broadcast, and cable TV
- 93 Ω: used in some instrumentation and specialty applications
Input guidance for better results
1) Use consistent dimensions
The formula uses the ratio D/d, so both dimensions can be in mm, inches, or mils—as long as you use the same unit for both.
2) Choose the right dielectric constant
Different dielectric materials shift impedance significantly. Some rough references:
- Air: εr ≈ 1.0006
- Foamed PE: often around 1.4 to 1.7
- Solid PE: about 2.25
- PTFE: about 2.1
Manufacturing structure (solid, foam, spacers) can change the effective dielectric constant, so datasheet values are best.
3) Keep manufacturing tolerances in mind
Small shifts in conductor diameter or dielectric placement can nudge impedance away from target. High-frequency systems often require tight concentricity and dimensional control.
Worked example
Suppose a cable has:
- d = 0.9 mm
- D = 3.0 mm
- εr = 2.25
- μr = 1.0
The calculator will estimate impedance near the 48–50 Ω range (depending on rounding), which is close to a classic 50 Ω design target.
Common design and troubleshooting mistakes
- Mixing diameter and radius values accidentally
- Using inconsistent units for D and d
- Ignoring dielectric variation with temperature/frequency
- Assuming connectors/adapters are perfectly matched
- Confusing DC resistance with characteristic impedance
Quick FAQ
Is this only valid at RF?
Characteristic impedance is a transmission line concept and applies broadly, but it becomes critical as electrical length grows relative to signal rise time or wavelength.
Why does the calculator also show velocity factor and capacitance?
Those are practical line properties derived from the same geometry/material assumptions. They help estimate propagation delay and loading.
Can I use this for lossy cables?
This is an idealized geometric model for nominal impedance. Real cables include losses, surface roughness, braid effects, and frequency dependence.
Use this tool for quick design checks, sanity checks against datasheets, and educational understanding of how cable geometry controls impedance.