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I'm currently writing a story set in a post apocalypse. It would be based in the current year . This story takes place from the perspective on one person who is speaking to a group of people, all in separate locations. Their main form of communication is radio. As far as I can remember any time I've seen characters use radio communication in any form of media it would always be a situation where only one person could talk at a time. So I wanted to know if there are any radio devices, or any other devices that could be easily attainable by the average person, relative to the setting, that would allow multiple people to speak simultaneous in a post apocalyptic setting.

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    $\begingroup$ I mean...your cell phone is a radio system that allows simultaneous conversation, but requires a level of technology that probably isn't available in your post-apocalyptic scenario. $\endgroup$
    – larsks
    Sep 20, 2022 at 11:30
  • $\begingroup$ Everyone can talk at the same time, but anyone talking cannot be listening while they do so. $\endgroup$
    – JamieB
    Sep 20, 2022 at 15:39
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    $\begingroup$ Any duplex system would allow this for analog. Any digital system would be able to allow this. $\endgroup$
    – stix
    Sep 20, 2022 at 20:02
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    $\begingroup$ @AlexanderNied That's basically what a duplex system is. $\endgroup$
    – stix
    Sep 20, 2022 at 20:15
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    $\begingroup$ You can kind of get this "one the same frequency" by transmitting on one side-band and receiving on the other (and the opposite on the other end). This is just a trick of filtering/mixing though, and the two side-bands still take up their own area of the spectrum. en.wikipedia.org/wiki/Single-sideband_modulation $\endgroup$
    – Steve
    Sep 20, 2022 at 22:50

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A simple 2 way radio only works when one person is talking at once. Modern technology has given us many ways to work around this limitation, at the cost of added complexity unnecessary for simple 2 way voice communication.

Similar to how it's difficult to hear someone talking when you're also talking, sending and receiving on the same channel, at once, doesn't work well. On a simple 2 way radio operating on one channel, any station that starts transmitting while the channel is in use will obstruct communications. This is why radio operators follow established voice procedures to coordinate who is speaking when on a single channel. If your group of survivors are having one conversation, then by taking turns talking, similar to how they would communicate if speaking in person, they can all use a single channel at the same time.

Of course being crafty monkeys, humans have come up with all sorts of creative techniques for working around this restriction. A simple one being duplexed communication. By using 2 channels one for sending and one for receiving, it's possible to have simultaneous 2-way communication. We've also come up with many ways to smoosh more operators into the same channel, using complicated encodings, and transmission strategies.

If you're old enough to remember when cell phones started being popular, before they became primarily devices for accessing the internet, that used a more complex system to enable multiple devices sending and receiving data on a single channel simultaneously. Code Division Multiple Access uses some fancy math to be able to extract single signals from many on a single channel. Wendover Productions has a great explainer should you want to know more of the details. While CDMA and more advanced technologies can allow for large numbers of people to share the same channel at the same time, this requires dedicated technology like cell phones and towers which it seems like your survivors don't have access to.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Sep 22, 2022 at 1:36
  • $\begingroup$ More generally, two channels are needed for Full Duplex communications. These can be two channels under (Time/Space/Polarization/Spin/Frequency or Code) Division Multi Access. More specific to OP's question however, is that most of these xDMA schemes rely on digital or quantum processing to achieve. $\endgroup$
    – Aron
    Sep 22, 2022 at 6:19
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This is how the good ol' radio (amplitude modulation) works. If everyone broadcasts on the same frequency, everyone hears everyone else. But you cannot broadcast while listening, you would overwhelm (and possibly destroy) your receiving tuner (amplifier). Simple solution would be automatic voice (sound) detection - while you are speaking, you are broadcasting, but as soon as you take a breath, you can hear everyone else. This is already complicated, but still early vacuum tube technology.

With technology, this becomes more difficult - FM exhibits capture effect, you will hear only the strongest sender. And with digital, everything unexpected is noise (unless carefully designed otherwise).

For this reasons, amplitude modulation is still used in aviation - if two pilots (or the control tower) speak at the same time, you at least know they want to say something (and can ask to repeat etc...) as opposed to not knowing someone wants to communicate.

And that is a solution for your post apocalyptic settings - your parties got hold of a bunch of aircraft VHF radios. Perhaps raided a cemetery or a former busy airport. The radios are in perfectly working order, just hook them to the power (and an antenna) and there you go (you do not even need a surviving ham to help you, just some basic competency with electricity and some experimenting).

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There is a very good reason that radio is usually one-person-talks. If both people have their microphone open at the same time as they are receiving, then you get feedback.

Headphone warning. This vid at this link has high pitched squealing noises on it.

https://www.youtube.com/watch?v=82scBZ3h62Q

This results as follows. A small sound is picked up by the microphone and amplified. The speakers put out a louder version of the sound. This louder sound is picked up by the microphone, and amplified again. And around and around.

With two radios this simply means the loop involves two microphones and two speakers. A sound is picked up by one microphone and transmitted to the other radio and amplified. It is then picked up by the other microphone and transmitted back.

There are two main ways of reducing this.

One is directional microphones that pick up sound only from sources directly in front of them. This allows you to do such things as have a person at the front of an auditorium speak into one microphone, and another person in the audience asking a question into a second microphone. You can still get feedback. To avoid it the audience mic is usually fixed in place. There will also often be clever speaker placement so that the speakers point at the audience not the microphone's pickup location.

The other possibility is headphones. That way the available sound is kept very much lower in volume, and microphones tend not to pick up enough sound to produce feedback. It may not be convenient to make everybody use headphones.

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Simultaneous transmission is how radio works. The button is there for economy and convenience.

There's nothing stopping simultaneous transmission on any radio frequency. The one-way communication you're accustomed to seeing exists for two reasons.

  1. Some radio systems use the speaker as a microphone. Speakers are like generators/motors, whether or not it's a microphone depends on how it's hooked up. Attach an active signal circuit to it and you get a speaker. Attach a detector circuit to it and it's a microphone. Using the speaker as a microphone is cost effective, especially in units where the quality of the signal is less important.

  2. More importantly, most radio has mediocre fidelity. You can get around this (concert-quality music can be transmitted via radio), but it doesn't change the fact that unless you have a complex microphone setup, the audio is flat. There's no dimension (there are some really cool tricks game designers use to give game sound a sense of direction and depth — but you don't have that in most radio broadcasts). As a result, when multiple voices are overspeaking on the frequency, it's very difficult for the listener to distinguish the speakers.

Don't under-estimate what your ears and brain can do. It's amazing. In the 3-D space your two ears and fearsome brain occupy you can easily distinguish unique voices in a room full of chatting party-goers. But all the indicators your ears and brain use to do that don't exist on broadcast radio. Using radio, unless you have a complex speaker setup, all the sound is coming from a single point in space with.

And to make matters worse, it's whompingly hard to filter your voice out of the conversation. So if simultaneous transmission is occurring, you're hearing your voice along with everyone else.

But is that really a problem?

Heck, no! Do a little research into party line telephony. Rather than a single line running to your house, your very one telephone line that could connect to anyone else, a party line was a loop that had multiple people attached to it — all chatting away like there was no tomorrow never mind how many conversations where happening all at once.

In other words, we've forced radio to non-simultaneous transmission for the same reasons we've left party lines behind: privacy and convenience. Nothing more.

If you want simultaneous transmission, you've got it. We actually go to an extra effort to stop that.


Edit: Some folks have really missed the point. Imagine a conference call on your cell phone. Three or more people talking simultaneously over radio. You don't even need a conference call. Put both cellphones on speaker phone and a dozen people can chat away just fine. There's nothing magically different between MHz and Ghz carrier frequencies. If there is a difference, it's that digital signal transmission is simpler because signal discrimination is normalized and packet identification is easier to deal with than separating modulations. Independent branch circuits for the microphone and speaker are also much simpler — but they're not required. Even the receiving and transmitting events (which occur at different power levels) can be done simultaneously with the right circuitry — it's just cheaper and easier to separate the circuits. But we live today in a world filled with simultaneous radio communication. We have done for quite a long time now.

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    $\begingroup$ It is very hard for a radio to be receiving a signal on the same frequency as it is transmitting at the same time... And party-line telephones allowed only one conversation at a time, because there was only one link to the branch switch; yes, the parties themselves could talk between them, but only one connection to the outside world could be made at any given time. $\endgroup$
    – AlexP
    Sep 20, 2022 at 7:08
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    $\begingroup$ "most radios use the speaker as a microphone" please cite a consumer radio that does this. While a dynamic microphone uses a coil in a magnetic field, that only superficially resembles a speaker and I have a hard time believing there is a microphone/speaker that can be used for both purposes. $\endgroup$
    – cms
    Sep 20, 2022 at 13:00
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    $\begingroup$ @cms when me and my brothers where teenagers, we built an intercom system at home exactly like that, with just a speaker at each end. $\endgroup$
    – ths
    Sep 20, 2022 at 14:28
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    $\begingroup$ Your suggestions will only work for cable based communication. $\endgroup$
    – user24582
    Sep 20, 2022 at 14:50
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    $\begingroup$ @AlexP You're talking to an electrical engineer. It's not hard at all for a radio signal to receive a signal on the same frequency it's transmitting. What's hard is getting it to NOT receive signals you don't want. The radio frequencies don't care. It's only an issue of modulation discrimination - which is how party lines worked. And my mother-in-law tells wonderful stories of she and her mother chatting away with a half-dozen other conversations all at the same time on their party line connections. You're correct that only one connection to the outside world could be had at (*continued*) $\endgroup$
    – JBH
    Sep 20, 2022 at 19:38
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While there are multiple convenient solutions, like the one your phone uses, the simplest solution works with even the earliest radio technology: have everyone use different frequencies for transmission.

This can't scale to thousands of transmitters in the same area - there are not enough sufficiently apart frequency bands. But a few dozen people could speak at will into their fixed-frequency transmitters.

As a caveat, you'll need a separate receiver for each person you'll be simultaneously listening to. (Good engineering could reuse some parts, but only some parts, and then we're no longer talking about the earliest radio technologies.) Such costs would be dear in any respectable post-apocalypse scenario. One could add a relay station to mitigate this. The relay station mixes all signals there. Everyone else just needs one transmitter (on his own frequency) and one receiver for the mixed signal sent by the relay.

Another caveat: interference and propagation limits works a little differently at different frequencies, so its effect would be selective.

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  • $\begingroup$ In a very limited scenario, I wonder if everyone could have a single receiver with a wide bandpass filter so that it picks up a very wide swath of frequencies. I remember building a crystal AM radio receiver that had such poor selectivity that would receive four or five different radio stations at the same time. $\endgroup$ Sep 20, 2022 at 15:22
  • $\begingroup$ You probably could do that (single receiver) if you want to talk to everyone with a transmitter. If you want to be more selective, you'd have to have individually tunable channels so that you can exclude the guy in the middle of your frequency range who just broadcasts fart noises all day. Or, depending on the particular kind of apocalypse, there could be other types of noise blocking certain frequencies from being useful. $\endgroup$
    – Miral
    Sep 21, 2022 at 3:21
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    $\begingroup$ If you add a relay station, you can mix all signals there. Everyone else just needs one transmitter (on his own frequency) and one receiver for the mixed signal sent by the relay. $\endgroup$
    – user24582
    Sep 21, 2022 at 8:09
  • $\begingroup$ @user24582 good idea, I'll edit it in. $\endgroup$ Sep 21, 2022 at 16:52
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Experienced radio telegraph operators are entirely capable of simultaneously sending a message with one hand and receiving and writing down a different message with their other hand. This might be close enough to 'talking' for your purposes.

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  • $\begingroup$ How many of those are there now, and how many will survive the apocalypse? And how many working CW radios and keys do you think there will be? $\endgroup$
    – Zeiss Ikon
    Sep 20, 2022 at 12:40
  • $\begingroup$ That's an impressive feat. Do you know if their telegraphs were sending and receiving on the same frequency? An important part of the OP's question is the single channel requirement. $\endgroup$
    – sphennings
    Sep 20, 2022 at 16:32
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Don't forget that cell phones use radio waves to work.

It is feasible to have your own personal cell tower. Depending on where you live these cost like a thousand US dollars. There are more expensive options that are mobile, called C.O.W.'s (cell on wheels). With one such tower around, you can use telephones instead of walkie-talkies. Also you don't need smartphones, any dumbphone will do as long as it is not something pre-historic. Even those old, indestructible Nokias should support group call.

A GSM cell tower can cover anywhere from 3 to 80 km (~2 to 50 miles) depending on its power rating and how it is set.

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The term you are looking for is full-duplex. Push to talk systems like most hand held radios are half-duplex where only one person can talk at a time. For full duplex communication (such as is used in cellular networks) you typically need a separate resource for transmit and receive. The most straight forward method is to have different frequencies for transmit and receive, but it can also be done with very short time slices that people don't notice, or with more advanced coding mechanisms.

https://en.wikipedia.org/wiki/Duplex_(telecommunications)

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How far past apocalypse is the setting and how long distances there are?

For relatively short distances WLAN and various levels of networks can allow this. Just need enough power to be around. And someone skilled enough to configure the network to work. With somewhat better organised societies even long distances might be possible as long as hardware continues to work.

Packet networks(like Internet and WLAN) where data is transmitted to both directions at same time in small chunks or alternating really fast do exactly seemingly simultaneous talking. Just need working network(some routers, some solar panels and some batteries) and right software. The software can be run on your average laptop, tablet or smartphone.

Most of this can likely be scavenged unless the apocalypse was type of frying all electronics, in that case all bets with radio are mostly off anyway. With simple population reduction, there is likely enough gear around for a years to decade or two.

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  • $\begingroup$ You probably have a decade until bit rot kills the firmware. two if you're lucky. $\endgroup$
    – Jasen
    Sep 21, 2022 at 11:15
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HAM Radio operators do this all the time. We can certainly hear and understand multiple people talking at the same time.

If we are operating with only one antenna, then when we transmit we don't hear the other voices until we stop transmitting. But, many transceivers can have more than one antenna attached and even accommodate a transmitting antenna versus a receiving antenna. Or, we simply run two transceivers each with their own antenna. Using two rigs allows us to transmit and receive simultaneously - even on the same frequency.

Of course, it is easy to establish a convention where you transmit on one frequency and receive on a frequency just slightly higher or lower on the dial. We call this using an offset and helps to divide the cacophony of voices.

These are all techniques we use today.

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This is actually a common problem that had to be solved for modern video chat services. The way they do it is basically the time-sequenced digital transmissions (a.k.a., packets) that all go to a central server for combining and rebroadcast as a unified wave form.

The problem with multiple people talking on a single frequency at the same time is that your radio will only pick up the "loudest" signal. That's almost always your own. This is how radio jammers work: they broadcast a much louder signal that drowns out all of the other signals.

For small areas (like a few blocks), you could use existing mesh network technology to collect and combine multiple signals. If you're talking about uncoordinated groups, like refugees who don't know who you are, then this isn't a thing. It requires coordination.

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