What is the effect of transmission line loss on SWR measured at the input to the line?

When you measure SWR at the transmitter (input) end of a lossy feed line, the forward RF power decreases as it travels to the antenna. Any power not absorbed by the antenna is reflected back, but this reflected wave also suffers attenuation as it travels back to the meter. Because the reflected wave experiences line loss twice (once on the way to the antenna, and once on the way back), the meter sees proportionally less reflected power than what actually exists at the antenna feed point. Therefore, higher transmission line loss makes the SWR at the input appear artificially lower (better) than the true SWR at the antenna. This is why critical SWR measurements should ideally be taken directly at the antenna.