The power control sets how much RF the radio's final amplifier delivers. SWR protection is the circuit that turns that power down when the antenna system is a poor match, so the transistors in the final stage are not overstressed.
Setting your power
Power is a control, not a goal. Turn it down for a local contact; turn it up only when the path needs it. Doubling power is only 3 dB, about half an S unit at the far end (see Decibels), so extra watts help far less than a better antenna.
Speech peaks are brief, so SSB heats the final amplifier less than continuous-carrier modes such as FM, RTTY or digital. Lower the power for those, as the manual advises.
Use a low-power tune mode when adjusting an antenna tuner or an amplifier, so the radio is not sending full power into a bad match.
A final amplifier is designed to see about 50 Ω. A mismatched antenna reflects part of the wave back, and the amplifier then sees a different load. The extra current or voltage can overheat or destroy its transistors. The fraction reflected depends only on SWR:
SWR
Power reflected
Mismatch loss
1:1
0 %
0 dB
1.5:1
4 %
0.18 dB
2:1
11 %
0.51 dB
3:1
25 %
1.25 dB
5:1
44 %
2.55 dB
Moderate SWR is not a catastrophe: at 2:1 the mismatch itself costs only about half a decibel. The danger is mainly to the radio. See SWR and reflections for the mechanism.
Illustrative curve for a 100 W radio. Where protection starts and how steeply it cuts varies by radio.
Output allowed50 W
Reflected at the mismatch9.2 W
How the protection works
A directional coupler inside the radio samples the forward and reflected waves and calculates SWR. A control circuit then lowers the drive to the final amplifier, a response called foldback, or shuts the transmitter down altogether. Radios also protect against heat and overcurrent.
A meter outside the radio does the same measurement and shows it to you:
Meters go in the feed line, not on the power cable.
The meter goes in the feed line between radio and antenna. What it reads at the radio is not always the SWR at the antenna:
Loss makes the input SWR look better than the antenna's. A tuner makes the radio happy but cannot change the SWR on the line.
SWR measured at line input2 : 1
Pitfalls
Never transmit with no antenna or load attached. An open connector is the worst mismatch there is. Use a dummy load for testing.
A tuner at the radio makes the radio see 1:1, but any mismatch beyond the tuner is still there on the feed line.
Protection is a backstop, not a tuning method. A radio that keeps folding back is telling you the antenna, a connector or the feed line needs attention.