In an HF station that includes an external RF power amplifier, where should a low-pass filter be located?

When you add an external RF power amplifier to your station, the amplifier becomes the final active component generating the high-power RF signal. Because amplifiers can introduce their own distortion and harmonics into the amplified signal, placing the filter before the amplifier (at the transceiver) will not stop the amplifier's own harmonics. Therefore, to effectively prevent these new harmonics from reaching the antenna system, the low-pass filter must be installed as close as possible to the external amplifier's output. This intercepts any unwanted high-frequency signals right at their final source. Distractors suggesting placement near the transceiver output or antenna tuner fail to account for the fact that the amplifier is the final device capable of generating harmonic interference before the signal goes to the antenna system.