Not an expert, but some lines of sight stretch from the top of one building to the top of another. Chicago doesn't have any mountains nearby, so those lines would just extend out into space and never touch the ground.

The frequencies involved would be blocked by buildings, so the "line of sight" would necessarily be above them. Thus, rooftop.

Yeah, I just assume "roof" cuz that's where antennas will be. That kind of thing is as close to "tight beam" as can be managed with antennas, usually; and that can be quite small.