Q.What is the purpose of the guard bands for broadcast FM? How wide is an FM broadcast channel?

Guard bands are small unused frequency gaps left between adjacent FM stations, and each FM channel is allotted a fixed width that includes these gaps. …

[!TLDR]

Guard bands keep adjacent FM stations from overlapping; the FM broadcast channel is 200 kHz wide.

An FM broadcast station has a maximum permitted frequency deviation of $\pm 75$ kHz, so the wanted signal spreads over 150 kHz. If two stations were placed exactly 150 kHz apart, the edges of their significant sidebands could overlap and cause interference.

To avoid this, an extra vacant frequency space — a guard band — is left on each side of every station's occupied bandwidth. These guard bands separate one channel from the next so that the sidebands of adjacent stations do not interfere. …