What is the main adverse effect due to operating with high SWR?

When you have a high SWR, power is reflected from the antenna back down the transmission line toward the transmitter. As this unradiated energy travels back and forth along the coaxial cable, it encounters the cable's natural resistance multiple times. This turns a significant portion of your valuable RF energy into heat. This phenomenon results in increased transmission line loss, meaning less power actually makes it out of the antenna. While some operators worry about high SWR damaging modern radios, most modern equipment will automatically reduce power to protect itself; therefore, the primary universal penalty is simply wasting your transmit power as heat in the feedline.