Cave Radio – Talking Through Solid Rock

Photos and diagrams by Mike Bedford, except where otherwise stated. Featured photo by Bartek Biela.

Radio signals don’t pass through rock to reach caves, mines, and tunnels, right? You’d be excused for thinking so, because you’ve surely lost your favorite radio station, or your phone conversation, when you’ve driven into a tunnel. But things aren’t that simple. By using special techniques, radio communication between the surface and underground really is possible, and you can try it for yourself.

Commercial equipment is available for communicating with miners, but these systems are massively expensive and not really portable. But cave explorers also need to talk to their colleagues on the surface — we’ll see why later — and cavers’ needs have been met, in the main, by the maker community. What’s more, there’s still plenty of scope here for devising new equipment and new techniques. So if you want to take up a new and unusual challenge, or if you just want to learn how cave radio is even possible, join us on this voyage of discovery into subterranean communication.

Why Cave Radio?

Why would cavers want to chat with the world above their heads? Surely getting away from the busyness of everyday life is one reason they engage in their remote outdoor activities?

There are other uses for cave radio, but by far the most important application is in cave rescue. Imagine that a rescue team find an injured caver a long way from the entrance. He’s in a bad way due to a fall, and it will take hours to get him back to the surface by manipulating him on a stretcher through tortuous narrow passages and up vertical pitches. With his condition deteriorating, it’s essential to make sure a doctor is present when they reach the entrance. Calling the surface party using a cellphone or walkie-talkie just isn’t an option but, by contacting the surface controller using cave radio, that all-important medical assistance can be ready to treat the casualty at the earliest opportunity. There’s no doubt that cave radios have saved lives.

Through-the-Earth Radio

Let’s come back to our earlier comment about radio signals not passing through rock. The reality is that radio signals do penetrate rock, but are attenuated at a rate that depends on their frequency. So, by using a low enough frequency, it is possible to transmit through the ground. In limestone, the rock in which most caves are found, a frequency in the LF band (30kHz–300kHz), commonly below 100kHz, works well.

However, there’s a problem. At the common cave radio frequency of 87kHz the wavelength is almost 3.5km, but efficient antennas need to be at least half a wavelength long — that’s 1.75km, or a mile-long antenna! This isn’t really feasible for a temporary antenna on the surface, and pretty much impossible underground.

For this reason, through-the-earth (TTE) cave radios originally used multi-turn loop antennas, usually around 1 meter in diameter. Such small antennas barely generate a true radio signal, but they do generate a so-called magnetic near field. This can be received by a receiver equipped with another loop antenna, but the signal decays with the cube of distance. This means that to double the range, the transmitted signal power has to be increased be a factor of eight, and to transmit 10 times further you need 1,000 times more signal. This would be a show-stopper for long-distance communication — but it offers a solution for cave radio, because most underground locations aren’t more than a few hundred meters below the surface.

Loops are still used today, but an alternative type of antenna is now more common because it provides much better performance. Often called an earth array, this antenna is a pair of wires run from each cave radio along the ground for 50 meters or so in opposite directions, each terminated with an electrode to inject a signal into the ground.

On the surface those electrodes can be metal tent pegs driven into the ground; in a cave where the floor is solid rock, they’d probably be lengths of bare wire trodden into mud or water.

The transmitter generates an electrical current that flows along the wires and through the ground. Some people suggest that it works as if it were a huge vertical loop in the ground, while others say it works by conduction, but the true situation is more complicated. Ranges of up to 1km are achievable, giving users more flexibility in where to locate the surface station. This is good news for a surface radio operator sitting comfortably in a vehicle, who might otherwise have been on a mountainside at night in a blizzard.

Communication Along Passages

An alternative to TTE radio is communicating along the cave passages. In fact, single wire telephones (SWTs) pre-date cave radios. They use a connection to the ground as the second conductor, and because the single wire has half the weight and bulk of a twin-conductor cable, it’s much easier to carry and reel out underground. A major advantage over TTE is that the equipment is much cheaper and simpler to use, and it’s still used today in some scenarios. It doesn’t involve radio, though, so let’s move on.

Another solution, which we can think of as a cross between telephone and radio, is guidewire radio. Like single-wire telephones, this requires a wire to be installed along the cave passage but, unlike an SWT, it doesn’t require the handsets to be physically connected to the wire. Instead, an ordinary radio — perhaps a 27MHz CB handheld — is held within a meter or so from the cable. The wire receives signals from the radio and transmits them along the cable so they can be received by another radio further along the passage. This allows communication over a range of several kilometers, even in passages with lots of bends and hence no line-of-sight path between transmitter and receiver.

Guidewire works in a similar way to the leaky feeders that are installed in some underground mass transit systems including the London Underground and the Hong Kong Rapid Transit Railway, and this leads us to another method of cave communication. Professional leaky feeders are hugely expensive, bulky, and heavy. However, cavers have discovered that some of the cheap coaxial cables intended for use with domestic TVs actually work as leaky feeders and offer some advantages over regular guidewires.

Next up we’re going to look at radio along cave passages without a guidewire or leaky feeder. And that’s a definite advantage since it can be a tricky job laying cables along passages, and those cables are at risk of being broken if cavers walk over them. The range is much reduced but, depending on the diameter of the passage and the number of bends, it might be sufficient for some applications.

Unlike TTE, which needs low frequencies, un-guided radio along cave passages needs high frequencies. Radios must operate above the cutoff frequency, which depends on the passage diameter but is typically above 1GHz, otherwise the signal won’t propagate. The maximum range can be considerable, but if it isn’t adequate, it can be increased by dropping repeaters along the passage each time the signal drops below a threshold.

Using the license-free LoRa standard for low-speed data transmission, cavers have achieved a range of almost 300 meters using 10 repeaters. This can be through bends and tight sections, over piles of boulders, and down vertical drops, all of which block a line-of-sight path.

Try It Yourself

So how about trying out cave radio yourself? Here’s a suggestion. In the early days of TTE cave radio, signals were transmitted at audio frequencies from 300Hz to 3kHz instead of LF (30kHz–300kHz). But because the loop antennas had to be tuned to resonance, the ultra-low frequency audio meant that they had to be large and have lots of turns. A 1,000-turn, 10m-diameter loop was common — heavy, bulky, and expensive to build. But if we use the now more common earth array antennas, audio frequencies can become a viable option, using cheaper, off-the-shelf hardware. This is an area where you could make a real contribution.

The DIY TTE transmitter shown here is just an audio amplifier with a microphone input, plus an impedance-matching transformer. The transformer allows the amplifier to inject an adequate current into the earth, which will have a much higher impedance than the speakers that the amplifier was intended to drive. I used a cheap Pulse MP20 10W megaphone with the speaker removed and the matching transformer wired in its place. Identical-looking models are available with different names. The transformer was an Eagle P038 15W 100V audio line transformer wired back-to-front, that is with the amplifier connected to its secondary windings and the earth array to its primary — see the schematic. (In the U.S. and other countries that use 70V instead of 100V lines, you can use a 70V transformer but you’ll have to experiment in choosing the best taps.)

The receiver is even simpler. Use an audio amplifier with a microphone input and the earth array wired to the input instead of a microphone. You could even use the microphone input of a laptop, configured to route the signal to the speaker. However, to prevent possible damage to your laptop from high-voltage spikes, wire two oppositely polarized diodes across the input and, ideally, use a cheap USB sound card instead of your internal sound card.

Using this system, I was able to communicate from 100m underground to the surface. You’ll probably find that communication is hindered by a constant hum caused by the electricity distribution system. Some form of filtering could be used to reduce this interference at 60Hz and its multiples in North America, or multiples of 50Hz in Europe or Asia. Here’s a block diagram for a transceiver, instead of a separate transmitter and receiver, that includes a hum filter. This could be your next step.

Stay Safe

We’ve seen that cave radios can help save lives, so it would be ironic if your experiments put you in harm’s way. Caving is potentially dangerous, so how can you stay safe? First, remember that the surface is a suitable test bed for your first tests. Only when your pride and joy is working on the surface would it be appropriate to take it underground. And you might be able to do your first underground tests somewhere safer than a cave. One option is to use an abandoned rail tunnel that has been converted to a trail for cycling or hiking.

When you really need to venture into a wild cave, the golden rule is that, if you’re not an experienced caver, don’t be tempted to go underground by yourself. If you know any cavers, perhaps they’ll introduce you to the subterranean realm. Otherwise, you should make contact with a local caving club who can teach you to cave safely. Caving organizations can help you find a nearby club; try caves.org/find-a-grotto in the U.S. or british-caving.org.uk/about-bca/caving-clubs in the U.K.

Learn More

If you want to keep up-to-date with cave radio, the quarterly CREG Journal published by the British Cave Research Association is highly recommended. It covers the application of technology — mostly electronics for communication — to the exploration and study of caves. It costs only £5 per year for the online version. Take a look at bcra.org.uk/pub/cregj.

You might also be interested in my book Cave Radio, ISBN 9781913995768, recently published by the Radio Society of Great Britain. It covers much the same material as this article, but in quite a lot more detail. However, it’s a popular science book aimed at makers, rather than a textbook aimed at academics. You can buy it from rsgbshop.org/acatalog/Cave-Radio-2394.html.

This article appeared in Make: Volume 96.