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Coax Cable Loss and EIRP Calculator

Enter the length of cable, its loss per 100 m (from the maker's table) and the power: get the loss in dB and the power that comes out. A second mode gives the EIRP from gain and losses.

Power out

81.2831 W

0.9 dB loss · 18.72 % of the power lost

Ideal values: connectors, SWR and the antenna's real efficiency are left out. Gain is in dBi, not dBd. This does not say what power is allowed where you live.

How it works

  1. Choose “Cable loss and power out” or “EIRP from gain and losses”.
  2. For the cable: enter the length, the loss per 100 m from your cable's data and the power at the radio.
  3. For the EIRP: enter the power, the antenna gain in dBi and the total losses in dB.
  4. Read the result and copy it into your plan.

What people use it for

  • Choosing a feed line

    Compare 20 m of two cables: the loss in dB and the watts that survive.

  • Estimating the power at the antenna

    Subtract the cable loss from the power your radio sends.

  • Working out an EIRP

    Add the antenna gain and subtract the losses to get the radiated power in watts.

Questions

Where do I find the loss per 100 m?
In the data sheet of your cable, for your frequency. This page has no cable table on purpose: loss changes a lot with the cable and the band, so the number must come from you.
What are the formulas?
Loss (dB) = length ÷ 100 × loss per 100 m. Power out = power in × 10 raised to (−loss ÷ 10). EIRP = power × 10 raised to ((gain in dBi − losses in dB) ÷ 10). 100 m at 3 dB per 100 m takes 100 W down to 50.12 W.
What does it leave out?
Connectors, adapters, SWR and the antenna's real efficiency. These are ideal values: measure the real station before you rely on them.
Is dBi the same as dBd?
No. dBi compares an antenna with an ideal isotropic radiator, dBd with a half-wave dipole; dBd + 2.15 is about dBi. Enter the gain in dBi. The page does not tell you what power is allowed where you live.