FM, pretty much line of sight. And, theoretically, you are supposed to voluntarily register them with the FCC. I wonder if anyone has ever done so?
Importantly, they are charged with a/c house current, a consideration if there is none available.
Hmm, I live in the foothill with mountains all around. We were thinking of getting some, we will take that into consideration. I wonder if any can be charged with a solar battery charger.