AM (amplitude modulation) is a way to encode audio onto a radio carrier by changing the signal amplitude while the carrier frequency stays fixed.

It is older broadcast tech, but still useful for long-range radio because AM waves can travel far, especially at night.

Why night helps: during the day, the [[Ionosphere|ionosphere]]'s [[D Layer|D-layer]] absorbs a lot of medium-wave AM energy, so you mostly get local ground-wave coverage. After sunset, that [[D Layer|D-layer]] weakens, so more signal reaches higher ionospheric layers and reflects back to Earth ("skywave"), which can extend reception to hundreds of miles.

Compared to [[FM]], AM is usually more vulnerable to noise and static, but it takes less channel bandwidth.

## See also
- [[Electromagnetic Spectrum]]
- [[Electromagnetic Radiation]]
- [[FM]]
- [[D Layer]]
- [[Ionosphere]]