Directional radio antennas tend to be rather large, and I'm not sure it's even feasible to get rid of side-lobes to the point where they're undetectable (or at

Directional radio antennas tend to be rather large, and I'm not sure it's even feasible to get rid of side-lobes to the point where they're undetectable (or at least, with a signal too weak to interpret). Directional optical links might be feasible, but probably a lot more expensive than a simple radio.