Many of you in childhood, for sure, have tried to establish radio communications with a rope and a couple of cans. This is one of the most primitive devices for transmitting a voice at a distance, but it has a pretty good potential if you develop the idea correctly. Michael Rainey was so imbued with this idea that he brought it almost to perfection, though the rope in his concept is no longer used. Instead, a vote converter into electric system and a pair of bipolar transistors (MPS6521), which transmit the signal through the transformer to the antenna, acts. The maximum range of the signal that the creator of the device was able to achieve was 1486 km, but the transmission quality does not allow the use of words, so the radio amateurs “speak” with the alphabet Morze.
Source: Hackaday