What indicates that a transmitter is radiating harmonic signals?

When a transmitter radiates harmonic signals, these are typically multiples of the fundamental frequency at which the transmitter is operating. In this context, if you hear signals on a different frequency than the transmitter's designated frequency, it indicates that these signals are likely harmonics being emitted by the transmitter.

Transmitter harmonics occur because, in addition to transmitting at the fundamental frequency, any non-linearities in the transmitter can cause it to also emit signals at even and odd multiples of that frequency (like twice, three times, etc.). As a result, receiving these additional frequencies means that the transmitter is indeed creating unwanted harmonic signals that may enter the amateur radio bands.

The other options do not accurately reflect the behavior of a transmitter with harmonic emissions. If it were to be heard only on its designated frequency, then the harmonics would not be present or would be too weak to detect. If it is not heard at all, this would indicate that the transmitter is not functioning properly or is turned off, unrelated to harmonic transmission. Lastly, a scenario where the signal improves signal strength would suggest that something is positively affecting the transmission, which does not relate to the presence of harmonic signals. Hence, the ability to hear transmission on a different frequency is the clear indicator of harmonic