This is an interesting debate and one that seems to open a few questions. Thinking in terms of 'non' aviation radio If I drive my car through the Hatfield Tunnel on the A1 which has a bend in the middle so you cant see right through, I can pick up am radio stations all the way through for example radio 5 on 909 Mhz. FM stations however (93.5 Mhz) do not work right through . . . but mobile phones, however which in this country are FM operating at either 900 of 1800 Mhz do work right through the tunnel. From this, I guess the conclusions would be that AM radio is more penetrating than low frequency fm, however as the frequency of FM gets higher it becomes more penetrating (or possibly better reflected?).
Ok, this has little to do with aviation (unless you want to try flying a Cessna through the Hatfield Tunnel), but if anyone can fully explain my observation in ther context of this debate, I would be interested to hear it.