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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First, I assume you are talking about using telegraphs? If so, they were literally connected via lines (wires, like for landline telephones). One person sent a message over the line and anyone on that line could hear it. They would then transcribe it and deliver the message to the intended person. It’s like “snail” mail, but via electrical lines.
When they were concerned about people intercepting them, they would send them via a code.
In a standard electrical telegraph system, the machines used by the sender and receiver were directly linked by a wire that could carry electrical current. This meant that the system was very much not portable/flexible. In early days, these wires led into fixed telegraph offices, with the “last mile” of getting the message to/from the people actually communicating was still done with paper and couriers. Depending on the number of wires connecting two places, it also had terrible bandwidth (i.e. no ability to send/receive more than a message or two at once), which is why telegraphs were so short and expensive.
The transmitter would send at a specific frequence and the reviecer would be set up to recieve at that same frequency. This is similar to how an old radio can recieve multiple different radio stations, but only one at a time.
If multiple people use the same frequency at the same time, they would need procedures for not talking over eachother. Walky talkies have this same problem, that is why they say over when it is time for the next person to seak.
Other than using the same frequency, radio signals are boradcast to everyone. This is the reason codes were used, as the enemy would need to break the code th actually know what was being sent.
A more amusing example is jamming: sending noise so that the opponents cannot hear eachother. In ww1 both the british and germans were jamming using their own anthem, in an attempt to demoralise the other. That is until they realised that their anthems were both set to the same tune.
Morse was often sent by wire. You’re only ever sending one message on that wire, so there is no trouble there.
Now, morse could be sent by radio once radio became more accessible, but in world war 1 it was copper all the way. Sending morse over radio is also pretty easy – you just use a frequency that nobody else is. As transmitters got better, receivers could afford to be more selective, allowing more channels in the same bandwidth for even less competition.
Not long after that we got voice over radio and it was something of a moot point. Controlling frequencies is how we keep different signals from interfering to this day.
Sometimes, the sender and receiver have agreed in advance on a time and radio channel to be on. The sender will transmit, and the receiver will listen at that time and channel. Other times, someone who is transmitting is expecting multiple people to receive it. This is especially the case in emergencies: you want as many people to hear it as possible.
But most importantly, in most cases anyone sending and receiving via radio has call signs or other ways of identifying themselves. Someone may say “XYZZY, this is WXYZ, are you there?” and everyone knows who the message is for and from, even if they don’t know exactly who XYZZY and WXYZ are.
It was a directly connected key and buzzer on the far end. Usually, only telegraph offices had the tools to listen to the electric signals on the wire. The message would be either kept by the receiving office if the person was in that town or sent on to the next office down the line. Eventually, it got to the intended recipient. The current was supplied by batteries at each office.
[the electric telegraph](https://www.history.com/topics/inventions/telegraph#the-electric-telegraph)
Modern radio communications can use an ics 213 form which has a recipient field. Anyone with a properly tuned radio can hear the message and copy it down. The message will then be copied down and delivered to the recipient.
Much like telecommunications today, many were routed through wired connections. Just they had to use switchboards to connect endpoints. Radio communications were (and still are) tuned to specific frequencies. In this case, it wasn’t directed at a specific receiver, as anyone else tuned into that frequency could hear it too. If the communication was sensitive, it could be encrypted with a cipher shared by both sender and receiver. The enigma machine was used for this by WW2 Germany and famously broken by the Allies who could listen in to otherwise secure German communications.
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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 ?
by the use of call signs. you would be assigned one, and told the other call signs you’d need to speak to, and everytime you transmit, you’d ID yourself and call for a specific answering callsign (“ground control to major tom. Ground control to major tom….”). Standard practice is to annouce who you are when you speak, so people can keep track of who says what (“This is major tom to ground control, I’m stepping though the door…”)
to prevent impersonation, thier might be additional elements such as pre-arranged code phrases (ie the classic spy, “the winds blow strongest in june” type stuff), or just straight up encryption, where whats sent over the air is just a random set of letters (“ehtaasdfadfitgnvhesuhfjdsgkndsgohs”), and only with a decryption machine like an Engima or its allied equivalent, and the appropriate key settings, could you decipher that into a message.
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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