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If you inject a 19 kHz pilot tone on a mono transmitter, assuming that the audio circuitry will pass it, that will cause the stereo decoder in the receiver to switch on. The stereo light will go on, but it will also reduce the signal to noise ratio of the detected signal. Why is that? Mainly because most of the noise that is added in the stereo mode comes from the difference channel between 23 and 53 kHz (30 kHz worth, to be exact). Further adding to the S/N deficit is the fact that the post-detection FM noise spectrum is parabolic, which means that there is significantly more noise per Hz at higher frequencies. Stereo transmission incurs a penalty of approximately 23 dB S/N degradation as compared to mono.
So, the bottom line is: if you want to maximize your range by transmitting in mono, do NOT turn on the stereo decoder of the receiver by transmitting a 19 kHz pilot.