take a look at https://en.wikipedia.org/wiki/QRP_operation https://en.wikipedia.org/wiki/QRP_operation - there's an entire community dedicated to this pursuit.

take a look at https://en.wikipedia.org/wiki/QRP_operation https://en.wikipedia.org/wiki/QRP_operation - there's an entire community dedicated to this pursuit. Also take a look at https://en.wikipedia.org/wiki/LowFER https://en.wikipedia.org/wiki/LowFER - you can even use 160-190 kHz under FCC part 15 in the US. The data rates would be very low, but for experimental purposes it's viable. The cool thing is there's a ton of simple circuits you can build on your own that other enthusiasts have put online. The requirement for a small antenna is hard to accommodate, but a lot of people find creative ways to deploy something with a low profile. I also think the idea of using drones for microwave links has some potential - haven't seen much on that yet but I'm sure people are doing it.