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A cave-bound society with radios.

Consider that:

  • This is a three level network of caverns.
  • The radio is internal and doesn't need to connect to the outside.
  • The level of technology is sort of what you'd get from a not very historically consistent steampunk(ish) novel.

In an isolated, underground society, would having a radio with radio stations work? (I know the cave's going to be horrible for radio waves) But is there a way to make it work?

If not with radio waves, what could be used to make a radio like system?

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  • $\begingroup$ For those who aren't into steampunk, it would be better if you can edit your question to state the level of technology as, for example, "resembling Earth in 1970 but no spaceflight capabilities" (or whatever is appropriate). $\endgroup$ – a CVn Sep 7 '15 at 21:01
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Depends on the size of the caverns and the distances between inhabited places. There might just not be enough need for radio broadcasting to develop - just put the cable through the caves (cable radio has been especially popular in Soviet Union). If there is an electrical network, you can broadcast the radio channels through it, the cabling is already there.

Alternatively, if the caverns are big and you want portable sets, low power ubiquitous transmitters (one per cave, fed from the cable) is what you want. In any case, you are looking for long waves (if you want diffraction to neighbour caves) and reflection resistant system, which means FM, if not even more sophisticated norm (or outright digital, which needs rather high level of technology). These are unfortunately somewhat contradictory conditions, with long waves (~ hundred kHz) you won't get enough bandwidth for FM.

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    $\begingroup$ So-called "leaky feeder" or "leaky cable" technology is basically the ultimate form of your "low power ubiquitous transmitters" -- and it's used routinely in mines and tunnels to provide radio-based services. (Also, NBFM can be run on a few kHz of bandwidth.) $\endgroup$ – Shalvenay Sep 6 '15 at 17:24
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    $\begingroup$ +1 to what @Shalvenay said about bandwidth. For example, the European PMR446 squeezes FM into 6.25 kHz channels, and I think the US FRS system is very similar. That's about as narrow as you can reasonably go for FM, but it does give you roughly telephone-quality audio which is often quite sufficient for voice and even supports modest fidelity music (shortwave AM is similar in bandwidth and fidelity, but FM gives you a cleaner demodulated signal once you get past the capture signal-to-noise ratio of the demodulator). Amateur radio FM is commonly 12.5 kHz, using a 3 kHz modulation bandwidth. $\endgroup$ – a CVn Sep 7 '15 at 20:53
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    $\begingroup$ Now, you can't really (readily) transmit a 6-7 kHz wide signal with reasonably sized antennas in the MF region, but if you go with the "leaky feeder" concept, you don't need to do that. In that case, because you aren't relying on diffraction to get signals between places, you can probably use VHF range frequencies, which makes this absolutely trivial with a technology level of approximately Earth 1950s onwards. $\endgroup$ – a CVn Sep 7 '15 at 20:59
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While a low-frequency radio could be used in a "through-the-earth" configuration, such communications are extremely narrowband -- real systems that do this are limited to one-way text paging or perhaps an "emergency alarm button" for a return channel. (These systems are limited to sub-3kHz ELF frequencies in order to get adequate ground penetration.)

More realistic communications (NBFM voice, two-way short messaging, narrowband data) require the use of "leaky feeder" or "radiating cable" technology, using higher frequencies run over a special coaxial cable that's fabricated with slots in the shield so it acts as a continuous antenna. This would require repeaters in the cable every-so-often, but depending on the frequencies at hand, this is not difficult to solve -- they can be powered by superimposing DC on the feeder cable, and operation at VHF frequencies was feasible with WWII-era vacuum tube technology (if not slightly before then, even).

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Assuming no other changes to radio technology, then the best way to move radio waves between caverns would be to use wave guides. Waveguides are simply metal pipes or ducts that confine the waves within and allow them to propagate over long distances without attenuation (the spherical waves emitted from a typical dipole antenna fall off due to the inverse square law, besides being absorbed by the cavern materials).

In practical terms, a transmitter would have one or more wave guides fanning out towards the various connecting passageways to the other caverns it is meant to service. the ends of the waveguides can be left open at the entryway to the other cavern, to allow the waves to propagate into the cavern and let people pick up signals. This would be fairly efficient for one way broadcast radio (much like real radio even today), but you would not realistically be able to transmit two way traffic like WiFi or VHF voice radio in this manner, unless the transmitters and receivers are placed at each end of the waveguide. At that point you may as well use wire as the transmission medium rather than radio waves. The other issue is the waveguide will have to extend into each cavern you want to transmit to, so if you have a string of caverns, then the Cave Broadcasting Corporation "World News Service" would need a maze of waveguides leading into each cave. This would be rather inefficient, and eventually there would be signal loss at the end of the longer waveguides. Depending on the frequency being used, the radio waveguides might also be rather large, which could be a problem if the passageways are small or have very sharp corners, and also if multiple radio stations are trying to broadcast (imagine a service corridor with lots of pipes and ducts to get the idea).

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  • $\begingroup$ Waveguides become too large to be efficient below oh, 1-2GHz, unfortunately -- coax with antennae on each end is a far better way to get a radio signal from point A to otherwise-inaccessible point B below that mark. $\endgroup$ – Shalvenay Sep 7 '15 at 21:19

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