What is the relationship between high standing wave ratio (SWR) and transmission line loss?
Why On a mismatched line the reflected wave travels back down the feed line and is re-reflected, so power makes multiple trips through the cable and each trip suffers the cable's normal attenuation. The extra loss caused by SWR is always added on top of the matched-line loss, and it grows with how lossy the line already is. A theoretically lossless line would show no added loss no matter how high the SWR, which is why the effect is described in terms of a lossy line.
Watch out Saying there is no relationship ignores the fact that reflected power retraces the line and is attenuated again; the idea that high SWR reduces the effect of loss has it backwards, since mismatch always makes total loss worse, never better.
SWR does not create loss by itself, it multiplies the loss the line already has: low-loss line, small penalty.