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My story is set in an apocalyptic Earth (occurring during the story), and the main character is stationed out alone on a sea fort in the middle of the ocean.

He receives a radio call/signal from a stranger on his radio (or something similar). I don't know enough about radio waves to know if this is plausible - could an SOS call show up on a radio like this? It's basically a voice in the wilderness - someone sending a one-way message out, hoping someone will receive it.

Further, is there a plausible way by which he might 'fix' the device he uses so that he could be able to send a signal out back to the recipient, but that doing so would then permanently prevent him from receiving a signal again?

I'm open to new technology (or old rediscovered tech) being used, as well as interference/jamming being caused by the environmental effects of the fall of civilisation around him.

Thanks so much - the downside of my only studying arts/humanities is a real lacuna in my knowledge of science and physics!

EDIT: So, a crucial plot point is that the main character essentially sacrifices his ability to receive transmissions in order to attempt to find a way to transmit back to the broadcaster. It's essentially an act of love and sacrifice, whereby he gives up the consolation and hope that receiving the messages gives him, in order to instead gives those things to the woman whose voice he hears. Therefore, it's crucial that a) the 'fix' to his radio means it can't receive messages anymore and b) the fix could conceivably allow him to transmit long-term. It wouldn't be worth it if he could only send one message, as with the battery/satellite phone idea below.

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  • $\begingroup$ I'm slightly confused by the specifics of the question. So does the broadcaster have something like a radio station antenna they're sending from, and the receiver have a regular radio, or something like a CB or ham radio setup? A bigger problem would be that the sender might not be listening for a return signal. AM vs FM would also make a big difference. $\endgroup$
    – DWKraus
    Commented Apr 19, 2020 at 15:49
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    $\begingroup$ The specifics of the broadcaster's technology are never answered in-story. The protagonist only knows that a) she is able to transmit to him, b) she is able to receive transmissions (but noone has responded so far) and c)he is not able to transmit to her at this stage. $\endgroup$
    – Swank
    Commented Apr 19, 2020 at 16:30
  • $\begingroup$ These technologies all use different frequencies, so you'd need to have them specified somehow. Does the sender give a message about how to respond? She'd need the equipment to receive the reply. The traditional approach (from books/movies) is to rig up a transmitter on repeat, specify a time you will be monitoring a set frequency for a response, or give a location and ask for people to come if the transmitter doesn't have receiving equipment. $\endgroup$
    – DWKraus
    Commented Apr 19, 2020 at 18:57

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So, the crucial point, as I understand it, is that the hero must knowingly destroy his receiver capability in the act of transmission.

Two-way radios actually have to put a fair amount of effort into not having the transmit and receive sides blow each other up. This is typically done by disconnecting the receiver from the antenna while transmitting.

It's plausible that this component of the hero's radio has broken, and therefore if he attempts to transmit (by overriding the interlocks), he will destroy the low-noise amplifiers at the front end of his receiver.

This has the benefit of allowing your hero to already possess a radio capable of transmit/receive, while explaining his reluctance to use the transmit features. It seems to fit the storyline much better than a failed repair job, as it is intentional rather than accidental. Also, I would hope that it would be understandable to a non-technical reader that shoving several kilowatts of power into just about anything that isn't designed for it isn't going to end well.

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This is my time to shine (I'm a ham radio enthusiast). Ok, let’s break down the question:

He receives a radio call/signal from a stranger on his radio (or something similar). I don't know enough about radio waves to know if this is plausible - could an SOS call show up on a radio like this?

Ok, so he gets a signal, and the message is not important, but before I get into the complex stuff: By all means, yes, this is very plausible. You want him to receive a signal, but he is not yet be able to reply. So here's what I think could happen:

  1. he is stranded with a basic AM/FM/weather radio
  2. he modifies it so that it can receive other frequencies
  3. he is scanning and picks up a transmission

Further, is there a plausible way by which he might 'fix' the device he uses so that he could be able to send a signal out back to the recipient, but that doing so would then permanently prevent him from receiving a signal again?

  1. here is where it could split into two paths
    1. when he was modifying it, he gave it the ability to transmit, but it's not strong enough
    2. he has to modify it so that it can transmit
  2. then the story progresses and he is now in contact with whoever it was

You could make it so the modification needs a part that he also needs for receiving and once he changes it, he won't be able to put it back or he accidentally breaks something and can no longer receive anything.

So for not being able to receive, the first path wouldn't work because how could he know if it wasn't strong enough and how would he have received the message, so you would need to use the second path? This may not, however, be the most plausible solution, because he might not have the required parts or expertise to pull it off, so some hand waving might be necessary if you choose that. But this is not the only way; most other users have also had great suggestions, and it's a matter of what you think fits the story best. To help you decide, I've added a list of the other answers and some other possibilities too.

  • modifying standard radio (this answer)
  • It's a satellite phone that's low on battery by John Dvorak (here)
  • Drone boat by Willk (here)
  • Patricia Shanahan's answer (here)
  • fixes old broken radio
  • I will update this list when i think of more

PS: Welcome to Worldbuilding SE!

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    $\begingroup$ The idea of botching a repair is good. Perhaps is as simple as losing one of those pesky tiny passives which he has no extra? I don't know if you worked soldering modern electronics, but in manual soldering it's damn easy to just have those tine things fly away, somewhere. Good luck looking for something that's 2x1mm $\endgroup$
    – jaskij
    Commented Apr 19, 2020 at 18:38
  • $\begingroup$ @jan-dorniak i have done some soldering projects not big and im not very good at it, but its really not too hard but when you do lose something small, its lost forever. i don't solder tiny things usually because im just not that good but even with normal size parts it can be a pain to find them. $\endgroup$
    – Topcode
    Commented Apr 19, 2020 at 18:44
  • $\begingroup$ Lowest I can go is 0603 and we are agreed on this ;) now, modern radios commonly use even 0402 capacitors and inductors in their radio frontends... $\endgroup$
    – jaskij
    Commented Apr 19, 2020 at 18:46
  • $\begingroup$ This sounds perfect :) S/he could mess up the only spare small part in a first go, then make the choice to sacrifice the part from the receiver in the knowledge that some step - perhaps taking it out - would cause irreversible damage $\endgroup$
    – Mike M
    Commented Apr 19, 2020 at 20:14
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    $\begingroup$ Another way to lose capability (with the possibility of repairing it further on once he figures out what's going on) is for a trace to break when pulling out a component to put somewhere else. Without a multimeter, it may be next to impossible to visually identify a broken trace, but if he later gets clever with a bit of wire, a battery, and a small bulb, he might be able to make a primitive continuity tester to find (and ultimately bridge) the break. $\endgroup$
    – Doktor J
    Commented Apr 20, 2020 at 15:18
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Drone boat.

It's basically a voice in the wilderness - someone sending a one-way message out, hoping someone will receive it.

A drone boat runs aground on his island. He wakes up and sees it there. This would be exciting too. He is not sure if anyone is on it. As it turns out there is a cat on it who is very happy to see him. There is also a message in an envelope taped to the inside of the windscreen. The message has coordinates. The drone boat is like a message in a bottle.

Your sea fort has flying drones and drone boats. Or did have drones; they all got used during the apocalypse. But there are spare parts for drones, and the tech to steer them. Your character is handy with this sort of thing and has built some silly stuff just to pass the lonely hours. Now he has the cat to help him. He fixes the drone boat, charges up its battery and installs a spare, and sets the boat to travel autonomously to the GPS coordinates in the message. He also loads up some supplies requested in the message, and he tapes his own one-way message on the inside of the windshield.

The cat does not want to go back on the boat, so your character sends a robot frog he built.

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    $\begingroup$ i really like this answer because it seems like it could really progress the plot in multiple ways, the cat adds an extra character which is important so the story doesn't seem boring and having the main character send back a pet adds a similarity between these two people that have never met. honestly i think this would be really good to see in a story, i would definitely read a book with this basic idea. $\endgroup$
    – Topcode
    Commented Apr 19, 2020 at 18:02
  • $\begingroup$ @Willik Nice answer! Slightly on the story-based side, but still good. $\endgroup$
    – In Hoc Signo
    Commented Apr 21, 2020 at 10:47
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It's a satellite phone that's low on battery

Far from any man-made destruction, immune to every disease known and unknown, designed to draw endless power from its environment, and always falling but never hitting the ground, these chunks of metal in the sky are here to stay, at least for a decade or so.

Unfortunately for your hero, his last solar charger that still has a USB 4.5 port available, has just bitten the dust - and it's impossible to fix with the technology they have. They can convert it to a DA port (double alligator) solar charger, but the darn phone refuses to charge through a DA port - at any appreciable speed, at least... or if you wish, the hero doesn't even need to tinker with the battery, but they do have only enough battery for one message, and then the phone turns off for good, or at least until a charger is found.

Your hero can disable the battery's undervoltage protection to squeeze out enough juice for one more outgoing message, but the battery will be unable to charge after that juice is squeezed out. Li-Ion batteries have the unfortunate tendency to violently combust if operated outside of standard conditions, which include stuff like giving it too much power, taking away too much power and then giving it some, giving it power too quickly, taking away power too quickly...
Video of a battery exploding due to a short circuit

And sure enough, it's just at this moment that the phone makes a loud ding, and sure enough, it's one of those cases that require a response.

It's up to you to decide if it's a cell broadcast style message, something akin to the amber alert thingie, if your hero is subscribed to a message board (it was a miracle to find one that still works) or if the hero's phone line is one of official public utility, one of his many, many tasks being to provide a rescue service in the general area.

Alternatively, it's not the electric charge your hero is low on, but credit. Perhaps it was originally sponsored by the government but there's nobody anymore to pay the bills and the AIs left to run it now do not care about emergency.

Satellite phones are very much a thing even today, and even today one of their benefits is that they continue to work after a natural disaster, as well as that their coverage includes such remote areas as a sea fort in the middle of an ocean. And even today, satellite phone services include those that can only receive as well as those that can only send.

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    $\begingroup$ I like the idea of using battery as the limiting factor, as it's easy to understand by both the reader and the protagonist. However, there's no need to make it a satellite phone unless you want to. Radio signals are inherently public, anyone listening can receive them. Receiving radio signals takes very little power. Sending radio signals takes an amount of power proportional to the distance you send them. So perhaps the protag has tried answering before on a low-power setting without success, and now has to use up the last of the power to send a message as far as possible and hope it's enough $\endgroup$
    – Matthias
    Commented Apr 19, 2020 at 16:40
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    $\begingroup$ @Matthias satellites for two reasons: the infrastructure is much more long-lasting than anything earthbound and sending anything from it is far more energy consuming than for any other system. Mobile is out as it needs extensive infrastructure which quickly degrades and would most likely not be available in that fort anyways. On industrial ships mobile signal is only available when close to shore for example. $\endgroup$
    – jaskij
    Commented Apr 19, 2020 at 18:35
  • $\begingroup$ @Matthias "Sending radio signals takes an amount of power proportional to the distance you send them." This is not so simple. Especially on the short wave signals might bounce on the ionosphere and end up in places that are hard to predict. On some frequencies, there's a zone that's too close to make contact, while at the same time you could easily talk to some favourably positioned stations 1000 km away with less power than a cellphone uses. You ARE right though in noting that receiving typically consumes MUCH less power than transmitting. $\endgroup$
    – EdvinW
    Commented Dec 7, 2022 at 21:36
  • $\begingroup$ @Matthias You could even receive with no power at all! See for instance en.wikipedia.org/wiki/Foxhole_radio $\endgroup$
    – EdvinW
    Commented Dec 7, 2022 at 21:41
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The sender has no access to electronics, but knows enough to create a battery or generator, and has some pieces of wire. That is sufficient to build a spark gap transmitter. For this purpose, a very noisy, wide bandwidth, transmitter is good because the sender does not know what frequencies will have anyone listening.

The hero is going to get an intermittent buzzing noise on a simple receiver. They will then have to find a reference for Morse code to interpret the messages.

The messages include information about the location of the sender. The hero has the materials, fuel, and knowledge to get an autonomous drone aircraft working and programmed to land at the sender's location, but will need to use some of the components of the hero's radio receiver. Once the drone is sent with a piece of paper containing the one-shot outbound message, the hero will no longer be able to receive messages.

Modification following an edit to the question. There is only enough fuel for the drone to make one trip. The hero puts his only radio receiver in the drone, irrevocably transferring his ability to receive messages, and builds his own spark gap transmitter to send messages.

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Yes, this is very plausible. A transmitter sends signals. As long as the transmitter is transmitting a signal and as long as the transmitted signal is powerful enough to be received, your character should be able to detect it so long the radio being used is compatible with the modulation of the signal. Provided the signal is sent by a radio transmitter it should be able to be picked up by a radio.

While it may be difficult to convert a modern day IC radio to transmit a signal, with access to basic electronic devices, building a really simple transmitter can be done with the appropriate skill. Note that to transmit a signal, a power source is required. It is much easier however to build a receiver than a transmitter (see foxhole radio [ https://en.wikipedia.org/wiki/Foxhole_radio ])

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    $\begingroup$ This does not answer the question - would changing the receiver into a transmitter (or using vital parts from it to make a transmitter) permanently prevent it being used as a receiver. $\endgroup$
    – KerrAvon2055
    Commented Apr 19, 2020 at 14:08
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Radios use hundreds of times as much power when transmitting than when receiving. Furthermore, due to how batteries work, output voltage sags under load. The more power you draw, the more the voltage drops.

As a result, a two-way radio with a mostly discharged battery can happily receive transmissions for many hours, but rapidly die as soon as you try to transmit.

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  • $\begingroup$ What about LoRa? $\endgroup$
    – Peter Mortensen
    Commented Apr 20, 2020 at 17:44
  • $\begingroup$ LoRa or analogous ham QRSS digital modes like WSPR or FT8 are way more efficient, but can't be used to transmit realtime voice because the bitrate is so low. (Shannon coding theorem, after all) You could theoretically transmit over DMR, but if you wanted to reach as many people as possible, you'd prefer boring old inefficient AM or FM over oddball proprietary digital voice codecs. $\endgroup$
    – Samuel Bierwagen
    Commented Apr 21, 2020 at 8:45
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Imagine an old, tube-powered, pre-heterodyne, short-wave radio.

In general, you can rewire the local generator of the receiver in order to use the audio-frequency amplifier as a last stage radio-frequency transmitter.

In the transistor case you generally can't, because audio-frequency power transistors are quite different from RF power transistors and you have a PCB with etched wiring.

With tubes, in practice any tube is good at least up to 30MHz and the wiring is 3D soldered links and elements. You just need to know what you are doing and be careful not to damage some element when resoldering. For half an hour you are good. Maybe.

For the needs of your plot, the character may break something in the process.

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