Seeing how zombies do not use radio (except when a piece of shrapnel that doesn't kill them gets stuck in their head!), and there are a lot fewer people than zombies left -- otherwise, it wouldn't be an apocalypse --, I don't think this is much of a problem.
Just use 20 or 30 deliberately chosen frequencies per "radio type" (such as CB or short-wave) so it's a bit of a challenge of actually finding someone talking. There are few people left, remember, so there are few people sending, too. Of the 30 channels, at least 28 to 29 should be empty on the average.
Add a few automated "Military blockade Number 5 in Manchaster is safe, we have shelter and food" type of messages (or which number was it?) on some fixed frequencies and make half of them fake or outdated, just to add a bit of risk.
TV and radio broadcasting would either have nothing at all, or a test screen, or an endless loop of governmental disaster directions. You don't even need to worry about frequencies, just make 5 channels numbered from 1 to 5, and you're good.
If you're hardcore, and your world is big enough so it matters, give different bands different ranges. I've never done amateur radio or such myself, but I think they have ranges of maybe 10-20 kilometers? Typical ultra-short-wave radio (the stuff you have in your car) has a range of approximately 150km, as everybody has experienced. Short-wave broadcast radio, on the other hand can easily be received on a different continent -- when I was a child, my father used to play with that kind of thing (the range has to do with reflection on the ionosphere as he told me).
Medium wave radio (like in the 1950s) works much better at night (around 1000 km), I've never quite understood why, but I think it has to do with the ionosphere too. That might be interesting to implement if you want a day-night cycle and put timely contraints on communications.
