I was wondering that too, and that led to the question whether, without an atmosphere, all radio communications at any wavelength would be strictly line-of-sigh

I was wondering that too, and that led to the question whether, without an atmosphere, all radio communications at any wavelength would be strictly line-of-sight. It turns out, however, that the moon has something of an ionosphere, though I don't know whether it would support over-the-horizon radio (or, for that matter, whether it tends to interfere with the accuracy of GPS, as it does on Earth.) https://phys.org/news/2011-11-mystery-lunar-ionosphere.html https://phys.org/news/2011-11-mystery-lunar-ionosphere.html