Every metre of coax turns some of your transmit power into heat before it reaches the antenna. Choose your cable, length and band to see how many watts actually make it.
| Cable | Loss | Watts out |
|---|
Where the numbers come from. The losses are typical published figures for each cable type at a perfect 1:1 match. Brands differ, and old or water-soaked coax is much worse, so check your cable's datasheet.
SWR adds loss. When the antenna isn't matched, power bounces back and forth along the line and some of it turns to heat each trip. A tuner at the radio makes the radio happy but doesn't remove that loss in the coax.
Rule of thumb. Losing 3 dB means half your power. On HF, short runs of almost anything work. At 2 m and 70 cm, cable choice matters a lot. Ladder line is very low-loss on HF, but it needs a tuner and has to be kept away from metal.