Coax Cable Loss Calculator
Enter the cable loss per 100 feet and the run length to find total feedline loss in decibels. Add your transmitter power to see the watts that reach the antenna and the percent lost as heat.
How to use the coax loss calculator
- Enter the cable loss per 100 feet from the datasheet at your frequency, for example 2.5 dB.
- Enter the total cable run length in feet.
- Optionally enter transmitter power in watts to see output power and percent lost.
Examples
75 ft of 2.5 dB/100ft cable
lossPer100Ft = 2.5, cableLengthFt = 75, inputPowerWatts = 100
total loss = 1.875 dB, output = 64.94 W, lost 35.06%
100 ft of 4.5 dB/100ft cable
lossPer100Ft = 4.5, cableLengthFt = 100, inputPowerWatts = 50
total loss = 4.5 dB, output = 17.74 W
Frequently asked questions
What is the coax loss formula?
Total loss in decibels is the loss per 100 feet times the run length divided by 100, so totalLossDb = lossPer100Ft * (cableLengthFt / 100). A 75 foot run of 2.5 dB/100ft cable loses 1.875 dB.
How do I turn cable loss in dB into watts?
Multiply the input power by 10 raised to minus the loss over 10, so output = input * 10^(-lossDb / 10). With 100 watts in and 1.875 dB of loss, about 64.94 watts reach the antenna.
How much power does 3 dB of loss waste?
3 dB is about half the power. Each 3 dB roughly halves what reaches the antenna, so half of your transmitter output is dissipated as heat in the feedline at 3 dB of loss.
Why does cable loss depend on frequency?
Loss rises with frequency, so the same cable that loses little on HF can lose several dB per 100 feet at VHF or UHF. Always read the loss per 100 feet from the datasheet at your operating frequency.
Does this include connector or mismatch loss?
No. This figure is the cable loss alone. Connectors, adapters and a high SWR add extra loss, so real feedline loss can be a little higher than the calculated value.
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