Citizen-Band radio
Amstrad CB901
An Amstrad CB radio emitter.
Using the CB radio as a modem
CB radio could be used as a modem through the cassette port of the Amstrad CPC.
Packet Radio
Packet Radio is a digital radio protocol that evolved from previous digital radio protocols like Morse code or Teleprinter. Packet Radio is frequently used by amateur radio operators.
See: Demonstration of Packet Radio transmission with an Amstrad CPC
FM radio diffusion
Some local or pirate FM radios diffused software over the air.
You simply had to record it on tape to use it later on your Amstrad CPC.