Why do most amateur stations use lower sideband on the 160-, 75-, and 40-meter bands?

The convention of using Lower Sideband (LSB) on bands below 9 MHz (160, 75, and 40 meters) and Upper Sideband (USB) on bands above 9 MHz is purely a historical tradition. It is a commonly accepted amateur practice rather than a regulatory requirement or physical necessity. This standard originated from the design of early SSB exciters, which often used a 9 MHz intermediate frequency (IF). The mixing schemes used to generate the final transmit frequencies naturally resulted in sideband inversion for the lower bands and non-inversion for the higher bands. Distractors suggesting that LSB is more efficient, legally required, or AM-compatible are incorrect. Both sidebands perform identically in terms of physics and propagation, and FCC rules permit either sideband to be used.