About the Coax Cable Loss Calculator
This coax cable loss calculator estimates how much signal a coaxial feed line throws away between your radio and your antenna (or between an antenna and a receiver). Choose a cable, enter the frequency and run length, and it returns the total attenuation in decibels, the loss per 100 feet and per 100 metres, and how many watts of your transmitter power actually arrive at the far end.
Attenuation figures come straight from manufacturer datasheets: Times Microwave publishes an attenuation formula for each LMR cable, and Belden publishes nominal attenuation tables for its RG-58 (8240), RG-8X (9258), RG-213 (8267) and RG-6 (1694A) cables, which the calculator interpolates on a log–log scale. It is handy for amateur radio operators choosing a feed line, installers planning Wi-Fi, LTE or GPS antenna runs, and anyone comparing a cheap thin cable with a thicker low-loss one.
Datasheet values assume a matched line (VSWR 1:1) at about 25 °C. Real-world loss is a little higher with age, moisture, high SWR and poor connectors, so add your connector loss and keep a margin. Generic cables sold under the same RG number can differ from the Belden figures used here.
With the default inputs, the total loss is 1.52 dB. Change any value above to recalculate instantly.
How to use the coax cable loss calculator
- 1Pick the coax type (or Custom if you have your own dB/100 ft figure).
- 2Enter the operating frequency in MHz.
- 3Enter the cable length and choose feet or metres.
- 4Add the total loss of connectors and adapters, if known.
- 5Enter your transmitter power to see how many watts reach the antenna.
- 6Compare cables in the chart to see whether an upgrade is worth it.
Formula and method
Coax loss has two parts: conductor (skin-effect) loss, which grows with the square root of frequency, and dielectric loss, which grows in proportion to frequency. Times Microwave captures both with its published formula α = k1·√f + k2·f (f in MHz, α in dB per 100 ft), and this calculator uses those exact constants for each LMR cable. For Belden RG cables it interpolates the nominal attenuation table on a log–log scale between published frequencies.
Attenuation in dB scales linearly with length, so the run loss is α × length ÷ 100 ft, plus whatever you enter for connectors and adapters. The fraction of power that survives is 10^(−dB/10): 3 dB leaves half, 10 dB leaves a tenth. All figures are for a matched (1:1 SWR) line at room temperature.
- α
- Attenuation in dB per 100 feet
- f
- Frequency in MHz
- k1, k2
- Conductor and dielectric loss constants from the datasheet
- L
- Cable length in feet
- P_in, P_out
- Power into and out of the cable (W)
Worked examples
100 ft of LMR-400 on the 2 m band (146 MHz)
α = 0.12229 × √146 + 0.00026 × 146 = 1.478 + 0.038 = 1.516 dB per 100 ft. Over 100 ft that is 1.52 dB, and 10^(−0.1516) = 0.705, so about 70.5 W of a 100 W transmitter reaches the antenna.
50 ft of RG-58 at 440 MHz with 0.3 dB of connectors
Belden lists 8.4 dB/100 ft at 400 MHz and 11.7 dB at 700 MHz; log–log interpolation gives 8.89 dB/100 ft at 440 MHz. Fifty feet loses 4.44 dB, plus 0.3 dB of connectors makes 4.74 dB, so only about 16.8 W of 50 W arrives.
15 m of LMR-240 for a 2.4 GHz Wi-Fi antenna
α = 0.24208 × √2400 + 0.00033 × 2400 = 11.86 + 0.79 = 12.65 dB per 100 ft (41.5 dB per 100 m). Fifteen metres is 49.2 ft, so the run loses 6.23 dB and a 1 W radio delivers only about 0.24 W — a good reason to keep microwave feed lines short.
Frequently asked questions
How much loss is acceptable in a coax feed line?+
Many operators aim for under 1–2 dB. A 3 dB loss means half your transmit power and half the received signal never reach their destination, so at VHF, UHF and microwave frequencies a lower-loss cable or shorter run usually pays off.
Why does coax loss increase with frequency?+
Skin effect pushes current to the surface of the conductors as frequency rises, increasing resistance roughly with √f, and the dielectric absorbs more energy in proportion to f. Both terms appear in the k1·√f + k2·f formula.
Is LMR-400 better than RG-213?+
At VHF and above, yes: LMR-400 loses about 1.5 dB per 100 ft at 146 MHz versus roughly 2.3 dB for RG-213, thanks to its foam dielectric. At HF the difference is small and RG-213 is cheaper and tougher.
Does high SWR increase coax loss?+
Yes. Datasheet attenuation assumes a matched 1:1 line. A mismatch makes power travel back and forth, adding extra loss on top of the matched figure; the extra is small for low-loss lines but grows quickly on lossy cable at high SWR.
How much loss does a connector add?+
A properly installed N or PL-259 connector typically adds about 0.05–0.2 dB at VHF/UHF, more at microwave frequencies or if water gets in. Adapters and lightning arrestors add similar amounts, so count every junction.
Can I use 75 Ω RG-6 for a 50 Ω radio?+
You can for receiving and short runs, but the impedance mismatch causes a 1.5:1 SWR before any other mismatch. RG-6 is designed for TV, satellite and SDI video systems, which are 75 Ω throughout.