Calculator
Coaxial Cable Impedance Formula
Characteristic impedance is not the physical resistance of the cable, but the dynamic impedance encountered by a signal traveling through it. It depends on the cable's geometry and insulating material. The formula is:
| Variable | Meaning |
|---|---|
| \(Z_0\) | Characteristic impedance, in Ohms (\(\Omega\)) |
| \(D\) | Inner diameter of the outer conductor |
| \(d\) | Diameter of the inner conductor |
| \(\epsilon_r\) | Relative permittivity (dielectric constant) of the insulator |
Note: The formula is based on a logarithmic function where D and d must be in the same units.
How to Use the Calculator - A Step-by-Step Guide
For accurate results, you need to precisely measure the inner conductor diameter (d) and the inside diameter of the outer shield (D). Refer to the diagram below:
You also need to enter the relative permittivity (εᵣ) of the cable's insulating material. Here is a table of common values:
| Material | Relative Permittivity |
|---|---|
| Air | \(\epsilon_r = 1.0\) |
| Foamed Polyethylene | \(\epsilon_r = 1.6\) |
| PTFE | \(\epsilon_r = 2.1\) |
| Polyethylene | \(\epsilon_r = 2.3\) |
| PVC | \(\epsilon_r = 3.0\) |
Common Coaxial Cable Types & Impedances
Here is a table of common coaxial cables with their standard impedance and typical applications:
| Cable Type | Impedance | Common Application |
|---|---|---|
| RG-6 | \(75\ \Omega\) | Cable TV, Satellite TV |
| RG-58 | \(50\ \Omega\) | Ham Radio, WiFi, RF Test Equipment |
| RG-59 | \(75\ \Omega\) | CCTV Video Surveillance, Analog Video |
| RG-213 | \(50\ \Omega\) | High-Power RF Transmission, Broadcast Communications |
| LMR-400 | \(50\ \Omega\) | Wireless Communications, GPS, Cellular Networks |
Frequently Asked Questions (FAQ)
50 Ohms is considered the ideal impedance for high-power RF applications, as it provides a good balance between power handling capability and low loss. 75 Ohms is considered ideal for transmitting video signals (like CATV and satellite) because it minimizes signal attenuation.
Resistance is the opposition encountered by direct current (DC). Impedance, on the other hand, is the total opposition to alternating current (AC) signals, which includes both resistance and reactance (caused by capacitance and inductance). In a coaxial cable, impedance is a property of AC signals, while resistance is a DC property of the cable itself.
When the impedance of the source, cable, and load are matched, signal energy is transferred from the source to the load with maximum efficiency. If there is an impedance mismatch, part of the signal energy will be reflected back to the source, leading to signal loss (also known as return loss) and standing waves.
