Coax cable loss calculator
Coax loss is the share of transmitter power a feedline turns into heat before it reaches the antenna. It grows with frequency and in proportion to length: 25 m of LMR-400 costs 5.4 dB at 2.4 GHz, so only 29% of the power arrives. This calculator interpolates published attenuation curves for common cable types.
Times Microwave · foam dielectric, 10.3 mm
Each line is one cable type: attenuation in dB per 100 m against frequency, on logarithmic axes, which is why the curves are almost straight. Lower on the chart means less loss. The selected cable is drawn in green and the dot marks its attenuation at the frequency you entered.
Cable data comes from manufacturer datasheets — the Times Microwave LMR series — and from published RG attenuation charts: typical values for new cable at 20 °C into a matched load. Age, moisture ingress and connectors add loss. RG-type figures vary by 10–20% between manufacturers and cable variants, and above 1 GHz the RG curves are extended from their published points with the standard √f + f loss model.
How it works
- 01
Know where the power goes
Two mechanisms turn signal into heat. Conductor loss rises with f, because the skin effect squeezes the current into an ever thinner layer; dielectric loss rises in direct proportion to f. Their sum, L ≈ k1·f + k2·f, is why every attenuation curve bends upward, gently at HF and steeply above 1 GHz.
- 02
Read the attenuation at your frequency
Manufacturers publish attenuation at spot frequencies — LMR-400 is specified at 50, 150, 450, 900, 1500, 2400 and 5800 MHz. Between those points the curve is a straight line on log-log axes, so interpolating there is accurate to a few tenths of a dB.
- 03
Scale it to your length
Attenuation is quoted per 100 m or per 100 ft, and loss in dB is strictly proportional to length: L = L100m · l100 m. A 25 m run costs a quarter of the 100 m figure; 1 dB per 100 m is 0.01 dB per metre. Divide dB/100 ft by 3.281 to get dB/100 m, or multiply the other way.
- 04
Count what it costs
Every 3 dB halves the power. 5.4 dB leaves 29% (10−0.54), 10 dB leaves 10%. And it costs twice on a duplex link: the same loss cuts the transmitted power and degrades the received signal. At 2.4 GHz and above, the cure is a short, fat cable — or moving the radio up to the antenna.
Formulas
- L100m(f) — attenuation from the cable’s spec curve at f, dB per 100 m
- l — feedline length, m
- Pin, Pout — power into and out of the cable, W
- Ltotal — total loss, dB
Worked example
- datasheet: 6.6 dB/100 ft · 3.281 = 21.7 dB/100 m
- 25 m → 21.7 · 25100 = 5.4 dB
- 10 W in → 10 · 10−0.54 = 2.9 W out
- → 71% of the power heats the cable; at 2.4 GHz, keep coax short or move the radio up
FAQ
- How do I convert dB/100 ft to dB/100 m?
- Multiply by 3.281, since 100 m is 328.1 ft: dB/100 m = dB/100 ft · 3.281. LMR-400 at 2.4 GHz is 6.6 dB/100 ft, which is 21.7 dB/100 m. Going the other way, divide by 3.281. Loss in dB is proportional to length, so the conversion is a straight scale factor with no other correction.
- Why does coax loss grow with frequency?
- Two reasons stack up. The skin effect confines the current to a thinner shell of the conductors as frequency rises, raising resistance with f, and the dielectric absorbs a share that grows in direct proportion to f. A cable that loses 3 dB per 100 m at 150 MHz typically loses four to seven times that at 2.4 GHz.
- Does transmitting more power change the loss?
- No. Attenuation is a percentage, so the dB figure is the same at 1 W and at 100 W — you just lose a hundred times more watts as heat. Power ratings are a separate limit: they are set by the temperature the dielectric and jacket can survive, and they fall as frequency rises.
- What do connectors add?
- A well-installed connector pair costs roughly 0.1 to 0.3 dB at UHF, more at microwave frequencies and much more when it is poorly fitted or has taken in water. Count each transition in your budget, and remember that a corroded connector or a wet cable end can add several dB that no datasheet predicts.
Feedline loss settled at the mast — now check the path itself. Waveshed computes line-of-sight and coverage over real terrain, free in your browser.
Simulate the link →Related guides
- Plan radio coverage (RF / ITM) — the Power field takes radiated power — subtract the feedline loss you get here before you enter it
Sources & further reading
- Coaxial cable — Wikipedia ↗ — construction, loss mechanisms and attenuation tables
- Pasternack — RF coaxial cables ↗ — per-cable manufacturer datasheets for cross-checking the attenuation data used here
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