This tiny 5w Chinese $100 bucks clone HF radio you can reach a good part of half the world without any infrastructure. And could be used for the the transmission of a digital data.

A bitcoin transaction typically consists of 500–1000 bytes, and it would need to use a digital mode that is specifically designed for large data transmissions, such as:

PACTOR: a digital mode that can transmit up to 10 KB (10240 bytes) of data, but it requires a specialized modem and a strong signal.