Enter Coaxial Cable Details
Use the conductor outside diameter and the inside diameter of the shield or outer tube.
Example Data Table
| Center Diameter | Shield Inner Diameter | Dielectric Constant | D/d Ratio | Estimated Impedance |
|---|---|---|---|---|
| 0.90 mm | 2.30 mm | 2.25 | 2.5556 | 37.5 Ω |
| 0.92 mm | 3.35 mm | 2.25 | 3.6413 | 51.7 Ω |
| 0.91 mm | 4.60 mm | 2.25 | 5.0549 | 64.8 Ω |
| 0.75 mm | 5.30 mm | 2.25 | 7.0667 | 78.2 Ω |
Formula Used
The calculator uses the standard lossless coaxial line impedance equation:
Z0 = 60 / √εr × ln(D / d)
Here, Z0 is characteristic impedance in ohms. εr is the relative dielectric constant. D is the inside diameter of the shield. d is the outside diameter of the center conductor.
The same result can be checked with:
Z0 = 138.155 / √εr × log10(D / d)
Capacitance per meter is calculated as C = 2π ε0 εr / ln(D / d). Inductance per meter is calculated as L = μ0 / 2π × ln(D / d). Velocity factor is 1 / √εr.
How to Use This Calculator
- Measure the outside diameter of the center conductor.
- Measure the inside diameter of the outer shield or tube.
- Select the correct diameter unit.
- Enter the dielectric constant for the cable material.
- Add target impedance, cable length, frequency, and load values.
- Press the calculate button.
- Review impedance, ratio, capacitance, inductance, and matching values.
- Download the CSV or PDF file for records.
Coaxial Cable Impedance Guide
Why Coaxial Impedance Matters
Coaxial cable keeps radio energy inside a controlled path. Its impedance depends mainly on geometry and dielectric material. A stable impedance reduces reflections. It also protects power transfer. Designers use it for antennas, instruments, video systems, and fast digital links.
Geometry Controls the Result
The calculator compares the shield inner diameter with the center conductor diameter. A wider spacing raises impedance. A larger center conductor lowers it. The dielectric constant also matters. Higher dielectric values reduce impedance and slow wave travel. Clean measurements are important. Small errors can shift the result.
Useful Advanced Outputs
Use the planner before ordering parts. Test several ratios. Choose dimensions that stay practical for machining, stripping, shielding, connector assembly, repairs, and future inspection during routine maintenance work in harsh sites.
The tool reports characteristic impedance, capacitance per meter, inductance per meter, velocity factor, and propagation speed. These values help users compare cable choices. They also support matching networks and test setups. When frequency and length are entered, the calculator estimates wavelength and electrical length. That helps with stubs, feed lines, and phased designs.
Matching and Practical Checks
The target impedance field gives a quick design comparison. Common systems use 50 ohms for radio work and 75 ohms for video or receive systems. The required diameter ratio shows what geometry would meet the selected target. Reflection coefficient and VSWR estimates help when a load value is supplied. Lower VSWR usually means better matching.
Measurement Tips
Measure the conductor diameter after removing insulation carefully. Measure the inside diameter of the outer shield or tube. Use the same unit for both diameter fields. Select a dielectric constant that matches the material. Air is near 1.0. Solid polyethylene is commonly near 2.25. Foam dielectric values are often lower. Real cables may vary because of braid shape, plating, moisture, and manufacturing tolerance. Treat the result as an engineering estimate.
Exporting the Results
The CSV export helps record repeated trials. The PDF export is useful for reports and shop notes. Keep each exported file with the cable drawing, material choice, and frequency assumptions. This makes later troubleshooting easier. Recalculate whenever dimensions, dielectric material, or target impedance changes.
FAQs
What is coaxial cable impedance?
It is the characteristic impedance formed by the cable geometry and dielectric material. It describes how the cable carries RF energy along its length.
Which diameters should I enter?
Enter the outside diameter of the center conductor and the inside diameter of the outer conductor, shield, or tube.
What dielectric constant should I use?
Use the relative dielectric constant for the insulation. Air is near 1.0, while solid polyethylene is commonly near 2.25.
Why must the shield diameter be larger?
The shield surrounds the center conductor. The formula needs a valid diameter ratio, so the shield inner diameter must be greater.
Can this calculate 50 ohm cable dimensions?
Yes. Enter 50 as the target impedance. The calculator shows the required diameter ratio and shield diameter estimate.
Does this include cable losses?
No. It estimates lossless characteristic impedance and related line values. Real cables may also have conductor and dielectric losses.
Why is velocity factor included?
Velocity factor shows how fast the signal travels compared with light in free space. It affects wavelength and electrical length.
Are CSV and PDF files useful?
Yes. CSV files help with spreadsheets. PDF files are useful for reports, design notes, client records, and lab documentation.