I'm going to presume there's was a really good (technical?) reason why Radio 1 and friends is allocated different frequencies throughout the country as opposed to being, say, Radio 1 on 97.9 everywhere like Classic FM is everywhere on 100.1FM?
Classic FM isn't on 100.1 everywhere, it too occupies a 'sub-band' between 99.9–101.9.
The reason they use a range of frequencies is because they are national stations transmitted from lots of transmitter sites across the UK. Adjacent areas cannot have the same frequencies or they'll interfere. It's the same with all radio transmissions, TV multiplexes use a range of about 40 frequencies to cover the country, and mobile phone networks are split into cells to avoid interfering.
There are such things as single frequency networks which are timed so they don't interfere, DAB muxes are as well as some smaller TV relays. The national medium wave networks are also synchronous - all the transmitters on 693MW for 5 Live are timed to each other, and all the 909MW ones are timed to each other for example.Otherwise those on the fringes that get more than one transmitter don't get a mush (I remember TalkSport doing split commentary in London by using their 1089 frequency there... and you could hear their other 1089 transmitters with different content fading in and out behind)
Why it's not done for the national FM networks I'm not sure, I imagine it's part historic and partly because of the scale, its normally only done on a small scale, synchronising hundreds of transmitters from the channel islands to the orkneys would be a challenge I'd have thought
EDIT: you'll notice that Classic has two frequencies for London, one 3Mhz higher than the other. The 4 BBC networks also have this, one is from Wrotham in Kent and the other is a lower powered fill in at Crystal Palace. Even being only 3Mhz away could cause interference problems so there's some system for keeping them timed to avoid this. Here's the R&D if anyone's interested:
Last edited by Inspector Sands on 25 January 2019 8:20am - 4 times in total