What happens to power lost in a feed line?

As radio frequency (RF) energy travels through a feed line, such as a coaxial cable, a portion of that energy encounters resistance in the metal conductors and dielectric loss in the plastic insulation. According to the laws of physics, energy cannot be destroyed, so this lost electrical power is simply dissipated as heat. While a higher SWR can lead to increased power loss (and thus more heat generation), the lost power itself does not cause the SWR to rise, nor does it radiate as harmonics or distort the audio signal. The fundamental mechanism of signal attenuation in any feed line is strictly the conversion of RF energy into thermal energy.