Feedline and SWR

SWR and mismatch

Enter any one of SWR, |Γ|, return loss, mismatch loss, reflected percentage, forward and reflected power, or a load impedance, and get the rest:

|Γ| = (SWR − 1)/(SWR + 1)
return loss = −20·log₁₀|Γ|
mismatch loss = −10·log₁₀(1 − |Γ|²)

A 2:1 SWR is |Γ| = 1/3, 9.54 dB return loss and 0.51 dB mismatch loss.

Cable loss

Matched loss per 100 ft comes from the manufacturers' data:

LMR-240, LMR-400, LMR-600: Times Microwave’s equation k1·√F + k2·F (F in MHz) from its LMR Coax Selection Guide.
RG-58, RG-8X, RG-213 and RG-6: W4RP’s coax attenuation table.
Ecoflex 10 and Aircell 7: SSB Electronic’s data sheets (dB/100 m at 20 °C).

Values between published points are interpolated on a log–log scale; outside the published range the result is marked as extrapolated. The total loss with a mismatched load uses the ARRL formula 10·log₁₀[(a² − |Γ|²)/(a(1 − |Γ|²))], where a is the matched loss as a power ratio and Γ is at the load. Because the reflected wave is attenuated twice, the SWR at the transmitter is lower than at the antenna — a long lossy cable can make a bad antenna look good.

Brands and batches vary; use your cable’s data sheet for exact work.

Electrical length

The wavelength in a line is (c/f) × velocity factor. The tool converts physical length to degrees, wavelengths and delay, and gives the length for any electrical length — quarter-wave stubs and transformers, half-wave repeaters and phasing lines.

Related: antennas, matching networks, Smith chart, decibels; for repeaters and satellite work, repeater offsets and tones and satellite passes (coax loss matters most at 435 MHz).