Growing up in St. Louis, Missouri I was aware that AM radio stations on "my" side of the Mississippi river began with the letter K and on the other side of the river started with the letter "W" but I never really knew WHY. So for The Voice of the Prairie I did a little digging.
It turns out it actually started with telegrams. Telegram operators created a series of short abbreviated call signs to identify the locations of telegrams - often to military and commercial ships. But because there was no standardization on these call signs, things got super confusing - operators could just make up their own call signs and some got re-used and it got messy. To attempt to alleviate the problem, the Bureau of Navigation (part of the
Department of Commerce), began assigning three-letter call signs to
American ships in the early 1910s. Ships in the Atlantic and Gulf of Mexico got a K prefix and in the
Pacific and the Great Lakes got a W. Why the letters W and K? Nobody actually seems to know any more.
In any case, in 1912 there was a big conference called the London International Radiotelegraphic Convention, and ranges of letters were assigned to each of the participating nations. The U.S. was told to keep using the W and most of the K range, as well as N and A (largely for military communications). Canada was assigned C and Mexico was given X. The call sign combination was allowed to be 3 or 4 letters in length.
It's at around this time that AM radio started to gain traction. Like we see in The Voice of the Prairie, some upstart mavericks began to set up radio towers and start their own radio stations. So in 1928 the Federal Radio Commission (now called the FCC - Federal Communications Commission) planned to assign licensed ground based commercial radio stations in the same way, except the directions somehow got flipped. Eastern stations got W call signs and the Western ones got Ks. Where exactly did the FRC draw the line between East and West? For a while it ran north from the Texas-New Mexico border, but shifted in 1923 to follow the Mississippi River and more accurately split the country in half.
It's never been a totally strictly regulated standard and a number of old radio stations got grandfathered in, but going forward radio stations have tended to follow in that standard. For about a year in the 1920s, the Bureau of Navigation that decided that all new
stations were going to get a K call sign no matter where they were
located. Still other exceptions were made by special request, station
relocations, ownership changes, and even human error. So, it's a standard except for when it's not.
The letters after the W or K? They're a bit more nonsensical. Some are named after their parent company, like WABC or KCBS. Chicago's WGN stands for "World's Greatest Newspaper" and Chicago's WTTW stands for “Windows to the World.” Some in the east actually reference the heir station's number, like KTWO and KFOR or the Roman numerals in call names like KXII or WIXT.