Coax Cable Signal Loss Calculator

Why coax loss matters:

Every foot of coaxial cable between your radio and antenna absorbs some of your transmitted signal, and it weakens the received signal by exactly the same amount. That second half is the part people forget. At HF (3 to 30 MHz) the losses are modest. At VHF and UHF (144 to 440 MHz and above) they get severe, and a bad enough run consumes more power than it delivers.

Loss is measured in dB (decibels):

Common coax cable loss (dB per 100 feet):

Loss formula:

Total loss (dB) = (Cable length / 100) × Loss per 100 ft at frequency

Power at antenna (W) = Transmit power (W) × 10^(-loss_dB / 10)

Worked example:

Running 75 feet of RG-213 at 144 MHz with 100 watts:

• Loss rate: 2.8 dB per 100 feet
• Total loss: (75/100) × 2.8 = 2.1 dB
• Power at antenna: 100 × 10^(-2.1/10) = 100 × 0.617 = 61.7 watts
• Power lost as heat: 100 - 61.7 = 38.3 watts wasted

Upgrading to LMR-400 at the same length:

• Loss: (75/100) × 1.5 = 1.13 dB
• Power at antenna: 100 × 10^(-1.125/10) = 77.2 watts
• Saves over 15 watts just by changing cable type

Note the 1.125 in that last line rather than the 1.13 printed above it. Round the decibels first and you get 77.1 W instead of 77.2 W. It does not matter here, but it is the reason a hand calculation and this page can disagree in the last digit.

Connector losses:

Each connector pair (PL-259/SO-239) adds approximately:

• HF (below 30 MHz): 0.1 dB per pair
• VHF (144 MHz): 0.2 dB per pair
• UHF (440 MHz): 0.5 dB per pair
• N-type connectors: about half the loss of PL-259 at UHF and above, and a good one does better than that

Enter how many pairs are in the run and the calculator adds them in. Count both ends, plus any barrel joins, patch panels and lightning arrestors in between. On a short UHF run they are not a rounding error: four PL-259 pairs at 440 MHz cost 2 dB, and 2 dB is about 74 feet of LMR-400. The connectors can outweigh the cable.

Practical advice:

• For HF (below 30 MHz): RG-213 or even RG-8X is fine for most runs under 100 feet
• For VHF/UHF (144/440 MHz): Use LMR-400 for runs over 25 feet
• For microwave (1296+ MHz): Use LMR-600 or hardline
• Keep cable runs as short as practical. Moving the radio closer to the antenna is free, and no cable you can buy beats not needing it