Coax Cable Loss Calculator
Calculate total signal loss through a coax feedline from its attenuation rating and length, and see how much power reaches the antenna.
How it works
Loss(dB) = attenuation(dB/100ft) × length ÷ 100 · Power surviving = 10^(−loss ÷ 10) · Output(dBm) = input − loss
Coax attenuation is specified in decibels per 100 feet (or per 100 m) at a given frequency, so total loss scales linearly with length. But because decibels are logarithmic, the power that survives falls off fast: 3 dB of loss throws away half the power, and 10 dB throws away 90%. That is why long runs at high frequency use low-loss cable — or the antenna gets a fraction of what the transmitter sends.
Worked example
50 ft of cable rated 6 dB/100ft loses 6 × 50 ÷ 100 = 3 dB. That leaves 10^(−3 ÷ 10) = 0.50, so half the power reaches the antenna; a 30 dBm (1 W) input arrives as 27 dBm (0.5 W).
Frequently asked questions
Why does the cable rating depend on frequency?
Loss rises with frequency because of the skin effect and dielectric losses. A cable that loses 2 dB/100ft at 100 MHz might lose 8 dB/100ft at 2.4 GHz, so always use the attenuation figure for your actual band.
How many dB of feedline loss is acceptable?
A common target is to keep total feedline loss under 3 dB (half the power). Above that, upgrading to lower-loss cable or shortening the run usually beats increasing transmit power.
Does loss matter for receiving too?
Yes — feedline loss attenuates incoming signals by the same number of decibels and adds directly to the receiver noise figure, so a lossy cable hurts weak-signal reception as much as it hurts transmit power.
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This calculator is for educational purposes. Double-check important results before acting on them.