Clear Channel didn't really expand in a major way until about the mid-1990s, owning 43 radio stations in 1995, following legislation relaxing station ownership limits (for both radio and television) in 1992.
For those recalling 1980s or earlier radio in the US, the situation was rather more diverse, particularly in ownership, though fairly narrow segment programming was becoming the norm as professional "programmers" entered the field.
More largely, the history of popular music has run through multiple phases of diversification and consolidation as new performance, recording, and distribution technologies emerged. I covered this referencing the work of Charles Perrow some years ago: <https://news.ycombinator.com/item?id=24930562>.
A curious one is truly independent as it's parent University closed WFMU https://en.wikipedia.org/wiki/WFMU
WSOU Pirate Radio a heavy metal and hair metal focused station at Seton Hall U.