Which of the following might be the cause of low RF power output from a solid-state transceiver?
Why Solid-state final amplifiers have no easy way to tolerate reflected power, so modern transceivers include a protection circuit that senses SWR and automatically reduces (folds back) drive when the load is badly mismatched. A high SWR on the feedline means a large portion of the power is being reflected back toward the radio, and the protection circuit throttles output to keep the finals from overheating or failing. So an antenna system fault, bad coax, or wrong-band antenna usually shows up first as reduced power out.
Watch out Low SWR is the goal, not a fault, since it means the antenna is accepting the power. Noise figure describes receiver sensitivity, and poor linearity would produce distortion and splatter rather than a drop in power.
High SWR equals power foldback: the rig protects its transistors by turning itself down.