why did FM become the most popular radio frequency and what difference did it have over AM?
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FM radio (on VHF frequencies) was superior to AM (on longwave and mediumwave) because it allows higher audio quality (more bandwidth, audio frequencies up to 15kHz), including stereo, whereas AM on mediumwave only allows frequencies up to about 5.5kHz (and mono), which isn’t much better than telephone quality.
The VHF/FM also has better signal-to-noise, i.e. the quiet parts can go really quiet without as much background hiss as you get with AM. This is particularly important for classical music (and before they used as much dynamic-range compression as is common now). In particular, AM is sensitive to interference (crackles, hums, buzzes) from anything that makes sparks, including the lighters in gas ovens and gas boilers, and spark-plug leads in cars, and the switching on or off of anything electrical.
Further, the coverage and quality of VHF/FM is consistent and predictable at all times of day, whereas AM/mediumwave carries much further after dark, which means you could hear overlapping stations or annoying whistles from interference after dark (the AM/mediumwave signal reflects off the ionosphere, whose height and properties changes with time of day).
The VHF/FM signal is typically limited to about 30-50 miles, depending on terrain – it its close to line-of-sight. VHF/FM doesn’t get around mountains, so in hilly areas you need a lot of repeater stations. On the other hand, the actual VHF/FM transmitting aerial can be quite small for a local station (only around 1.5 metres long – although you might put it on a tall building or small mast). The VHF FM signal also lends itself to relatively low-cost local stations. For mediumwave/AM you really need a dedicated tower tens of metres high, or wires strung between two masts tens of metres high and tens of metres apart, but because the wavelength is long, it will diffract around mountains/hills to some extent.
A potential advantage of a powerful AM transmitter, especially on the longwave band, is that a single transmitter can have a coverage area several hundred miles in diameter. After dark, it may go 500 miles – used for national state stations (and propaganda stations!).
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