The common set of all of those regions where it's impossible to pick up anything on the radio. Usually noticed only on the highway, as if you're not in a car, there are other things to do to amuse yourself. Commonly encountered in the mountains.
Note that the definition of "nothing on the radio" is extensible, as anyone with a satellite dish for their TV can attest (the phenomenon of "five hundred channels and nothing's on"). You can be in radio limbo if the only music stations you can pick up have formats that you can't stand.
Radio limbo becomes radio hell when the music stations playing the music you can't stand are superimposed over stations broadcasting in a language that you don't understand, or alternately talk radio stations broadcasting hosts that you can't stand.