What is the typical effect of placing a low-pass filter in the antenna feed line of a transmitter?

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A low-pass filter in the antenna feed line is used to limit the frequency content so only the fundamental carrier (and its required modulation spectrum) passes with minimal loss, while the higher-frequency harmonics are greatly attenuated. Those harmonics would otherwise radiate and cause interference and violate emission limits, so the filter cleans up the transmitted spectrum. It doesn’t improve modulation stability, voltage regulation, or increase power; in fact, some insertion loss may slightly reduce transmitted power. By suppressing harmonics, the filter helps ensure compliant, cleaner radiated signals.