Feedline Loss Calculator

Tells you how much of your transmitter power actually reaches the antenna — and how much is heating up the dielectric in your coax. Pick your cable, your run length, your band, and your TX power. The numbers update as you go.

How To Read The Result

Line Loss (dB). Under 1.5 dB across your run is excellent. Between 1.5 and 3.0 dB is acceptable for most stations. Above 3.0 dB you are losing more than half your power before it leaves the shack — time to rethink the cable or shorten the run.

Watts at Antenna. The number that actually matters. If you're running 100W from the rig and only 50W reaches the antenna, you have effectively cut your station in half. Doubling power to compensate (100W to 200W) costs significantly more than upgrading to better coax.

Watts Lost as Heat. This power is gone — converted to heat in the dielectric and never radiated. It's the cost of running the cable you chose at the band you chose for the length you chose. Sometimes that cost is acceptable. Knowing the number is the first step.

Pick The Right Coax The First Time

The single most important variable in the result above is the cable you chose. Larger diameter coax = lower loss. The cost difference between RG-8X and LMR-400 on a 100-foot run is roughly $60. The dB difference is significant on the higher bands.

If you want the full physics — why feedline loss happens, how SWR interacts with line loss, what to do about common-mode noise on receive — the long-form lives on the Signal Lab.

About The Numbers

The loss values used in this calculator are derived from manufacturer specifications and field measurements for each cable type at standard HF amateur bands. Calculations assume a matched (low-SWR) load; actual loss will be slightly higher in installations with significant SWR on the line. For a deeper treatment of SWR-loss interaction and feedline behavior generally, see Walter Maxwell W2DU's Reflections III — cited in the Greyline Authority Sources.

Greyline Performance · Sun Valley, Idaho · 435-200-4902