Why doesn't fm go as far as am?

Question

Why doesn't fm go as far as am?

Answer 1

frequency.

The frequencies used for FM broadcast are primarily useful as line-of-sight, which is why the transmit antennas are placed on moutain tops if possible. The propagation is very consistant, day or night and only slightly affected by normal weather conditions.

AM frequencies have 2 propagation modes. During the day they have groundwave, which is why AM towers are often in bogs or low lying ground, never on mountain tops. At night skywave goes up and is reflected back to earth at great distances.  in oder to prevent chaotic interference, many AM stations have to stop broadcast at sunset, otherwise you would hear too many stations on the dial.

the method of modulation AM or FM by itself has no factor on the RF propagation characteristics.  For very weak signals neither is used as SSB modulation is prefered.

Answer 2

it is the frequency, the lower the frequency the further it can go.