I see it used a lot in old movies, people communicating over great distances with dots and dashes
If the signal is bounced off the ionosphere how does the person receiving the message know which message is theirs?
I’m assuming many messages were sent during the war … with all those messages bouncing around how did we zero on the one specifically for the receiver ?
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From the 1890s to about 1920 radios used ‘spark gap’ transmitters that could produce pulses of radio waves. These could be picked up by a receiver as a sound that was either ‘on’ (beep!) or ‘off’ (a pause).
So you’d press the key and generate a beep. How long you held the key (how long the beep was) and how fast you pressed the key (the pause between beeps) encodes information just like in a telegraph. Spark gap radios could not transmit voice, so these were the only way to send messages.
You’d know who the message was for because they would start and end a message by encoding the sender’s ID. A garbled message might have to be repeated several times to hear who sent it, though.
When transmitting on a radio you adjusted to frequency you transmit on and listen to. If someone else is transmitting, you would wait for them to finish before sending, then send an identifier and who the message is for in code, then your message, then close the message with your ID and who you are sending to.
So when there are a lot of people ‘talking’ in one area you might have to wait a while to send your message.
Messages sent by ‘skywave’ (bouncing off the ionosphere) can go really, really far, so you’d have to be careful with who you send to and the frequency you use. You’d want to pick one that won’t be busy near where the message will be heard, in case someone around there is sending at the same time and your receiver can’t hear your message over the local traffic.
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