If using multiple converging lasers, the antenna need not be connected to the equipment. Two lasers could ionize the air in a line 2500m long, and a third could excite the center of it in a modulated fashion. That would be transmit only, unless you had some way to measure the energy levels at that center point from a distance.

It would be unlikely to glow brightly enough in the visual spectrum to see in the daytime, but it would be pretty cool at night.