Why do most solid-state transmitters reduce output power as SWR increases beyond a certain level?

Most solid-state transmitters reduce output power when the Standing Wave Ratio (SWR) increases beyond a certain level primarily to protect the output amplifier transistors. When the SWR is high, it indicates that there is a significant mismatch between the transmitter's output and the antenna system. This mismatch can lead to increased voltage standing waves, which can result in excessive reflected power.

The output amplifier transistors are designed to operate within specific voltage and current limits. High SWR conditions can cause the transistors to overheat or become damaged if too much reflected power is delivered back to them. By automatically reducing the output power in response to rising SWR, the transmitter helps ensure that the transistors remain within safe operational parameters, thereby extending their lifespan and maintaining the reliability of the transmitter itself.

This protective measure is vital in solid-state designs, as they are often more sensitive to such conditions compared to older vacuum tube designs. It is also worth noting that while there are reasons related to compliance and performance, the primary concern with increased SWR is often about protecting the hardware.