tools / coax loss calculator

Coax Loss Calculator

Estimate feedline loss for common coax and ladder line. Pick a cable, frequency, length, TX power, and antenna SWR — see the matched and total loss, the power that actually reaches the antenna, and how cable choice changes the picture.

cable

frequency

MHz

run

signal

W
:1

SWR at the antenna feedpoint, not at the radio. A long lossy cable will mask SWR as seen by the rig.

total loss

— —
power to antenna —
power lost in cable —
matched-line loss —
extra loss from SWR —
SWR at transmitter —
verdict —

power flow

cable comparison

Same frequency, length, power, and SWR — different cables.

formulas

Two formulas drive this calculator. The first gives the matched-line loss using a two-term model fitted to manufacturer data. The second adds the extra loss caused by reflected power bouncing back and forth on a mismatched line.

Matched-line loss

Loss per 100 ft (dB) k1 × √f + k2 × f
For length L (ft) α = (k1√f + k2f) × L ÷ 100

k1 captures conductor loss (skin effect, scales with √f). k2 captures dielectric loss (scales linearly with f). Frequency f is in MHz. Both constants are cable-specific.

Total loss with SWR

Reflection coefficient ρ = (SWR − 1) ÷ (SWR + 1)
Matched loss as ratio a = 10α÷10
Total loss (dB) 10 × log10[ (a² − ρ²) ÷ (a × (1 − ρ²)) ]

Standard ARRL form. When SWR = 1, ρ = 0 and the formula collapses back to the matched-line loss. Reflected power makes additional passes through the cable, so each pass picks up more attenuation. This is also why a long lossy run "improves" the SWR seen at the radio — the cable swallows the returning wave.

Power delivered

To antenna Pout = Pin × 10−αtotal÷10

Convert total loss in dB back to a power ratio, multiply by TX power. The rest is dissipated as heat in the cable.