What is the relationship between high standing wave ratio (SWR) and transmission line loss?

In a real-world, lossy transmission line, a high Standing Wave Ratio (SWR) means that a significant amount of RF power is not absorbed by the antenna. Instead, it is reflected back toward the transmitter. This reflected energy bounces back and forth between the transmitter (or tuner) and the antenna. Because all practical transmission lines have some electrical resistance and dielectric loss, a small amount of energy is converted into heat every time the signal travels through the line. When high SWR causes the power to make multiple trips back and forth, it encounters this attenuation multiple times. As a result, a high SWR compounds the natural attenuation of the feed line, significantly increasing the overall power loss. This effect is especially pronounced when using coaxial cables at higher frequencies.