Coax power handling calculator
A cable can fail in two ways: the voltage punches through the dielectric, or it overheats on its own losses. On HF the first limit usually bites, on VHF the second, and the remedies are different. The calculator works out both and tells you which one is closer.
| Cable type | |
| Centre conductor diameter (mm) | |
| Diameter over insulation (mm) | |
| Characteristic impedance (Ω) | |
| Loss at 14 MHz (dB/100 m) | |
| Insulation | |
| Voltage safety factor | |
| SWR in the line | |
| Ambient temperature (°C) | |
| Limits | |
| Peak voltage across the insulation | |
| Limit set by breakdown | |
| Limit set by heating | |
| What limits it | |
| Permissible power | |
| In a matched line | |
| Taking SWR into account | |
📐 The formulas
The dielectric breakdown limit:
U_peak = E_break × margin × a × ln(b/a)
P_max = U_peak² / (2 × Z₀)
a, b are the radii of the centre conductor and of the dielectric. The field is strongest at the centre conductor, and that is where breakdown happens.
The heating limit scales inversely with loss per unit length, from the practical rule that “a cable with 1 dB/100 m takes about a kilowatt on HF”, corrected for ambient temperature.
Correction for SWR:
P_allowed = P_max / SWR
At an antinode of the standing wave the voltage rises, so be conservative with the margin.
Dielectric strength of the insulation:
| Material | kV/mm |
|---|---|
| Solid polyethylene | 20 |
| Foamed polyethylene | 10 |
| PTFE | 25 |
| Air | 3 |
- Both limits are estimates. For an exact figure use the maker's curve for your particular cable
- The thermal limit is pinned to a practical rule of thumb, not to a heat calculation
- Installation is not accounted for: cable in a conduit or in a bundle runs far hotter than cable hanging in air
- Altitude is not accounted for: the voltage limit falls about 3 % per 300 m
🎯 In practice
- Choosing cable for an amplifier. Will RG-58 take a kilowatt — the short answer is no, and here you can see why.
- Judging the risk at high SWR, when the antenna has drifted.
- Understanding what actually limits you at your frequency — voltage or heat.
- The commonest real cause of a burnt feedline is not power but water and an old connector.
- The connector is always the weaker link: a PL-259 does not exist at 10 GHz, and at a kilowatt it is already getting warm.
- Cable in a black conduit in the sun runs at +60 °C, not +25 — the temperature correction is not a formality.
- At an SWR of 3, take a third of the power the matched-line calculation gives.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
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UR3PKI. «Coax power handling calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/lines/coax_power (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.