Why do radio and tv stations in the United States begin with W in the eastern half and K in the western half?

Question

By the actuall call letters.

Answer 1

I believe all radio stations (with a transmitter above a certain power) need a licence from the FCC, who assign a unique "call-sign" to that station. As to why the call-sign is what it is, I don't know (in the UK the first letter of the call-sign indicates the type of licence, which decides how powerful your transmitter can be).

I also don't know if there's a difference between call-signs to differentiate amateur (HAM radio) radio stations from commercial ones.

Answer 2

that's not really true actually.. Here in NY we got stations with BOTH those letters.  Like, W104.3 FM, and W97.7, but we also got K-92.3, and K107.4....so, I don't think you're right.  

Answer 3

It's not really designated by halves, as much as it is the Mississippi River.  And, to add to the trivia, not all radio stations east of the Mississippi begin with "W."  I live in Pittsburgh, PA, and we've got KDKA.

I'm not sure how it (the naming) got started, though.