Coax loss calculator
How much of your transmitter power the coax turns into heat: attenuation per 100 m and per 100 ft at your frequency, the loss for your length of cable, the extra loss caused by SWR, and how many watts reach the antenna. Results update as you type.
Indicative calculation; verify before use.
Cable and frequency
Length, SWR and power
- Attenuation (matched)
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- Matched loss for this length
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- Extra loss from SWR
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- SWR seen at the transmitter
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- Power at the antenna
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- Heat in the cable
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- Method
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All cables at this frequency and length
Datasheet attenuation only. Click a cable to use it above.
* Outside the frequency range of the datasheet's attenuation table; the value is extended from the nearest datasheet points.
How it works
Coax loss grows with frequency: the √f part is the resistance of the conductors (skin effect) and the f part is the loss in the dielectric. Where the manufacturer publishes the two constants K1 and K2, the calculator uses them as printed ("datasheet formula"). Where the datasheet only lists the attenuation at a few frequencies, the calculator draws a straight line between neighbouring points on log–log axes, so it reproduces every datasheet value exactly ("fitted to datasheet points"). Outside the datasheet's frequency range the nearest segment is extended, and the result is marked.
The datasheet value is the matched loss: the loss when the cable is terminated in its own impedance (SWR 1:1). With a mismatch at the antenna, the reflected power travels the cable twice and the loss rises. The formula above, from the ARRL Antenna Book (transmission lines chapter), gives the total loss from the matched loss and the SWR at the antenna. The SWR at the transmitter is lower than at the antenna, because the cable attenuates the reflected wave on its way back; a long, lossy cable can show a good SWR at the radio over a poorly matched antenna.
Worked example: LMR-400 at 145 MHz, 30 m, SWR 2:1 at the antenna. A = 0.12229 × √145 + 0.00026 × 145 = 1.4726 + 0.0377 = 1.510 dB/100 ft = 4.955 dB/100 m. 30 m = 98.43 ft, so ML = 1.510 × 0.9843 = 1.486 dB. ρ = 1/3, a = 10^0.1486 = 1.408, TL = 10 × log10[(1.983 − 0.111) / (1.408 × 0.889)] = 1.75 dB, so the SWR adds 0.26 dB. With 100 W into the cable, 66.9 W reaches the antenna and 33.1 W is heat in the cable. The SWR at the transmitter is 1.62:1.
The SWR correction is an approximation. It treats the cable's characteristic impedance as purely resistive, and AC6LA shows that this makes it exact only at certain line lengths, and Watts Engineering discusses the same limits. For station planning the difference is usually small, but at low frequencies, on lossy cable and at high SWR it can matter.
Sources (checked 2026-11-03):
- Total loss with SWR: ARRL Antenna Book (transmission lines chapter). Only the formula is used; no text, figures or tables from the book.
- Accuracy of that formula, AC6LA, "Additional Loss Due to SWR": https://www.ac6la.com/swrloss.html
- Watts Engineering, SWR and loss: https://wattsengineeringlabs.com/swr-and-loss
- Cable attenuation, impedance and velocity factor: the manufacturer datasheet linked on each cable row above.
Indicative calculation; verify before use. Attenuation is the manufacturer's nominal matched-line value for new cable at the datasheet's reference temperature. Connectors, ageing, moisture and installation add loss. The SWR correction is an approximation. This page does not check whether a cable can safely carry a given power; see the manufacturer's datasheet.
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