Why is frequency modulated (FM) phone not typically used below 28.0 MHz?

Frequency modulated (FM) phone is typically not used below 28.0 MHz primarily because of the challenges associated with harmonic generation. When operating in the FM mode, the transmission can generate harmonics, which are unwanted frequencies that are multiples of the fundamental frequency. These harmonics can interfere with other communications, especially in crowded or lower frequency bands.

At frequencies below 28.0 MHz, the ability to attenuate these harmonics to acceptable levels becomes difficult. The nature of FM signals, combined with the characteristics of the lower frequency bands, means that the harmonics can fall within other allocated communication bands, causing interference with other users. Thus, the potential for harmonics to cause unwanted interaction in these bands makes the use of FM phone less practical and often discouraged in these lower frequency ranges.

Overall, this limitation highlights important considerations for amateur radio operators when selecting modulation types and operating frequencies to ensure effective and responsible communication.