What is the effect of transmission line loss on SWR measured at the input to the line?
Why SWR at the input depends on the ratio of reflected to forward power seen there. The forward wave is attenuated on its way to the antenna, and the reflected wave is attenuated again coming back, so the reflected power arriving at the transmitter is reduced more than the forward power. That makes the measured SWR at the line input lower than the actual SWR at the antenna feed point, and the loss can hide a serious mismatch. In the extreme, a very lossy line reads close to 1:1 no matter what is at the far end.
Watch out The idea that loss raises the reading gets the physics backward; loss always attenuates the reflected wave and pulls the reading toward 1:1. And loss makes the reading less accurate as a measure of what the antenna is doing, not more.
Loss hits the reflected wave twice, so it hides mismatch: a lossy line always reads closer to 1:1 at the shack.