Coax loss · SWR · Return loss
Feedline loss & SWR
How much of your power actually reaches the antenna? Loss figures come straight from the cable makers' datasheets, and the extra loss from a mismatched antenna is included.
Result
Every cable at
Total loss includes the SWR you entered. * = below that cable's lowest datasheet frequency, so it's an estimate. Tap a row to select that cable.
SWR · return loss · reflected power
Type any one value; the rest are calculated.
How it works
Matched loss comes from each manufacturer's published attenuation table (dB per 100 ft at a perfect match), using log-log interpolation between the published frequencies, so every datasheet value is reproduced exactly. A little below the lowest published frequency, the manufacturer's own k1·√f + k2·f formula is used where one exists (Times LMR), otherwise a curve fitted to the datasheet; those results are flagged. Nothing is extrapolated above the highest published frequency.
Extra loss from SWR. Reflected power travels the line twice, losing more on each pass. With matched loss ML (as a ratio a = 10ML/10) and ρ at the load, total loss = 10·log10[(a² − ρ²) / (a(1 − ρ²))], and the SWR the transmitter sees is lower: ρin = ρ / a. This is the relationship behind the ARRL Antenna Book's “additional loss due to SWR” chart. It assumes a lossless (real) Z0, which is accurate for normal amateur use; very lossy thin coax at low frequency departs from it slightly.
Ladder line and twin lead aren't included: no manufacturer publishes loss data for them that I could verify.