Ham-designed Gear Used In Thailand Cave Rescue

Unless you live in a cave, you’ve probably heard a little about the thirteen people — mostly children — trapped in the Tham Luang Nang Non cave in Thailand. What you may have missed, though, is the hacker/ham radio connection. The British Cave Rescue Council (BCRC) was asked for their expert help. [Rick Stanton], [John Volanthen] and [Rob Harper] answered the call. They were equipped with HeyPhones. The HeyPhone is a 17-year-old design from [John Hey, G3TDZ]. Sadly, [G3TDZ] is now a silent key (ham radio parlance for deceased) so he didn’t get to see his design play a role in this high-profile rescue, although it has apparently been a part of many others in the past.

The HeyPhone is actually considered obsolete but is still in service with some teams. The radio uses USB (upper sideband, not universal serial bus) at 87 kHz. The low frequency can penetrate deep into the ground using either induction loop antennas like the older Molephone, or — more commonly — with electrodes injecting RF energy directly into the ground.

You can find a very detailed article about the radio from 2001 if you want more details. The system is somewhat dated, but apparently works well and that’s what counts.

What we find interesting is that in today’s world, people take wireless communications for granted and don’t realize that cell phones don’t work underground or in the face of widespread disasters. We would imagine most Hackaday readers know how cell phone towers use “cellular reuse” to support more than a handful of phones. Ask some non-technical friend if they know how a cell phone works and you’ll be surprised how few people understand this. Ham radio operators and hackers are vital to building and deploying specialized radio systems in times of disaster or — in this case — where people need rescuing from an odd environment.

We were glad to see a nod to some hacker gear in the popular press. But we almost wish there had been more reporting on the volunteer divers and their hacked radio gear.

We’ve talked about VLF radios before, but not for caving. Of course, in the old days, all radio was VLF and it might have even had some unintended consequences.

41 thoughts on “Ham-designed Gear Used In Thailand Cave Rescue

  1. Would be nice to see an updated, open-source, new take of this technology !
    No doubt it can be highly miniaturized, simpler design with DSP and image transmission / text capabilities…

    1. If you follow the first link you’ll see that there is an updated version “micro heyphone” which I assume is just miniaturized… links to get more information require signing up for an account somewhere, and I didn’t bother to do that.

        1. But according to the article it was already used to transmit pictures. Although they had to be encoded by a phone app (and probably were taken by the phone’s camera).

        2. Based on the same design, but not possible to be *exactly* the same as a number of key components have been withdrawn as obsolete. …also, paying £15 for a custom cut crystal (as used in the original design) didn’t fit well with trying to keep costs down, so this has been replaced by a more generally available crystal, and then divided in hardware with the peripherals of a microcontroller (not used on the original design).

          The AGC of the original design was never its strongest feature and prompted a number of discussions on the subject in the CREG journal at the time. This has been replaced by a more robust and functional AGC circuit, along with provision to readily swap from USB to LSB without having to unseal enclosures and modify the circuit (as per the original design).

          Currently work on the project is looking to replace the analogue receiver board with a dsPIC digital receiver. The prototype is working fairly well, but power consumption is rather higher than the basic analogue original. Once that’s running alright attention can turn to digitising elements of the transmitter…

  2. Thanks for the plug for ham radio.

    Something else people don’t realise about cell phones: the towers usually have batteries (for short power outages) and generators (for longer outages). But if power’s out for a week, the cell tower will probably go down, because the generator has propane or diesel for only about 3-5 days of continuous operation. And, if you’re depending on that tower as a backup for your VOIP line, you’re going to be out of luck. This happens every so often here in the Boston suburbs, when a storm brings down enough trees that the trucks can’t get to the cell tower to refill the fuel.

    Ham radio’s a good backup to have.

      1. Reminds me of the earliest morse code cables being unshielded also. Seems and area of opportunity to shield the utilities lines to prevent losses… though maybe that has something to do with the ULF or really VLF field around the earth changing the Van Allen Belts HaD article and other nefarious malicious activities for the prison and related industries to maintain crime.

      2. Not sure where you’re going to get a line-powered analog phone line like that. The telcos don’t offer it any more, and even the old ones are rapidly being pulled out of service.

      3. I had this for a long time, but abandoned it as the monthly bill for a bare-bones land line (no caller ID, no call waiting, no nothing) neared $40. The “more likely to work in an emergency” feature just doesn’t seem worth that much.

  3. ” in the old days, all radio was VLF”

    That’s not even remotely true.
    When Heinrich Hertz started with his spark gap experiments, he used 9m dipoles, and 60 cm dipoles with parabolic reflectors to increase the range of the electromagnetic transmissions. All far from VLF.

    1. I think it was a fair statement. Some exception doesn’t negate the statement.

      Once radio got out of the lab, it pretty much was all under the top of the current AM broadcast band. The higher frequencies were deemed useless, so everyone wanted the prime spot. That created problems because there was so little space. Rules came along to prevent interference, and to segment the spectrum for various uses. Ham radio, because people had been playing with radio since Marconi spanned the Atlantic, became formal and fairly early relegated to the “useless” frequencies, “200 metres and down”.

      So in December of 1921, they had their own tests, and spanned the Atlantic using those useless frequencies. Suddenly shortwave became valuable, and others moved up, ham radio having to live with smaller bands spread around.


    2. Ok maybe “all” wasn’t the right word, but in fact, all of Marconi’s commercial radios were very low frequency primarily — I think — because he wanted bigger and bigger antennas. Even the CW crowd had to use alternators because there was no good amplification method yet and that was 10kHZ and below although there were improvements made (we did a post on that) that got it up by a factor of 8 or so. In 1901, I think Marconi’s frequency was the highest at like 850 kHz. And very quickly dropped down to 45 kHz well before 1910. There is a pretty good article about this: https://www.ieee.ca/millennium/radio/radio_differences.html

      Interestingly, too, Marconi’s first transatlantic transmission was probably not actually received. He knew what to expect and heard it in the noise, something a lot of hams will attest can happen.

      So yes, there were isolated experiments but commercial wireless was primarily a VLF affair for the first part of the 1900s.

      1. I also would like to mention that terms such as High Frequency, Very High Frequency have change definition over the years (defining the frequencies they span). So, technically it wasn’t VLF!


  4. “Unless you live in a cave, you’ve probably heard at a little about the thirteen people — mostly children — trapped in the Tham Luang Nang Non cave in Thailand.”

    I see what you did there! :D

  5. “people take wireless communications for granted and don’t realize that cell phones don’t work underground”

    It makes me sad when I hear non-hackers talk about cellphones and I realize how little most of our fellow humans understand about the world they live in every day. I’ve heard from more than one that we are living in the future because we now have “Star Trek communicators”. Really? Geeky enough to make a Star Trek reference and don’t even know the difference between a cellphone which relies on always being within a couple miles of a tower and a plot device that works from the surface of a planet to orbit? They even worked indoors and underground except when the plot called for them not working. Those things would be magic if they were real. Did anyone notice the episodes where away teams lost contact at regular intervals because the ship’s orbit took it to the other side of the planet? No? Me neither.

    The thing that amazes me about cellphones is not the technology in them. With enough money we probably could have had basic functioning bag phones in the 1950s. It’s the fact that it is financially viable to build and operate enough cell sites to make them work that I find incredible!

    Actually… I suppose some ham who lived within range of a repeater with an autopatch probably did put his radio and a lead acid batter in a bag once.

    1. not sure about back-then, but in the 80’s people did just that, complete with Z80-based pulse-dial-DEcoding and tone-dial-ENcoding

      ever sent a FAX (from a NON-modified FAX-machine) while ALSO (at the same time; 2nd radio) on a long-distance voice call while you just happen to be stopped along a deserted desert highway while riding a (modified, recumbant) bycycle???

      did i mention the Z80 was paired with an LCD screen and DIY keyboard and lots of other “stuff” relating to the fact that the Z80 can be made to use very low (average?) power compared to a real/modern-at-the-time desktop computer.

      i have NOT done that, but it HAS been done, in the 80’s, using 80’s technology/parts.

    2. I have a scene in mind: Picard to Enterprise!

      I’m sorry, you have reached the B’Xara IX wireless consortium. As we do not yet have a peering agreement with Federation Cellular, you will need to provide funding for this call. If you do not wish to complete this call, please disconnect now. Otherwise stay on the line and an AI will be with you shortly. Please have your Galatic Latinum card number ready for the AI. Have a nice day!

  6. The idea of using LF/VLF in a cave is even older. In 1975 or 1976, I sat through a presentation at the Baltimore Grotto (caving club) by some radio amateurs who were also cavers. Their system used frequencies just below the AM band, and was packaged in small ammo cans. They had to get a special, temporary license from the FCC. I don’t remember where the presenter was from. I think another Eastern state.

  7. This transmitter could be received using a high end audio card coupled with the right antenna and software.
    I wonder if recent development in SDR technology could help to build a better receiver section with DSP assisted demodulation to cover even more distance.

  8. DSP today is certainly appropriate at the surface. But underground (or in any tough spot), at least for now, it’s still hard to beat the simplicity and power budget (total system miliwatts consumed per word transmitted) of dumb, discrete analog SSB voice circuits. My push for an update to this system would be to develop the surface stations with DSP, but retain simple SSB on the portables. The former should allow portables to use less transmit power for the same distance. As our DSP devices get smaller and less power hungry (a real trend now), there might be some new advantages. Digital images, however, require relative mountain of either bandwidth, or power and time. Remember, we’re talking about a 2.5kHz wide channel. Nyquist says….. ;)

  9. Saw this elsewhere but he doesn’t want to make a comment. He thinks it should be it’s own update to the article but isn’t willing to write it:

    So you guys covered the cave rescue ‘HeyPhone’ amateur radio today.. But completely missed the actual ‘hack’ in the design. And I think you should release another followup story to do it justice.

    The craziest part about the HeyPhone design, to me, is that they’re using a regular old 10 watt car audio amplifier on a chip for the RF power amplifier — The TDA2003 (http://bcra.org.uk/creg/heyphone/pdf/heyphone-schematic-tx.pdf, https://www.st.com/resource/en/datasheet/dm00028077.pdf)

    That’s pretty brilliant. Intended for audio frequencies up to 20 kHz or so, but hey, let’s push it a wee bit up to 87 kHz for radio transmission purposes and hey, it works just fine, it’s now a 10 watt PA. For anyone that’s tried to design their own PA up into HF or VHF range, it’s not trivial. I’ve not played around with LF or VLF but the coolest part is that it’s so low that you can start pulling off clever hacks like this.

    The TDA2003 is basically perfect for this application. And what’s better, they’re using the chip beyond what the datasheet lists, so it’s a hack in the truest sense.

    All in all it’s a great design. They’re using Tayloe mixers (SSB modulation for dirt cheap with no purpose-built parts), a little tank circuit for harmonics suppression, a VERY simple user interface (two knobs — Mode and volume/on/off). You can’t even change the frequency, which is good for this application. And the mode switch does some really clever things that would be needed for a cave rescue team, like beacons. All through-hole parts, easy to make and maintain, no microprocessors (for this application, lack of complexity is good). The volume isn’t a potentiometer, but a stepped attenuator on a rotary switch. Again, very durable.

    All in all this a great design for its intended purpose. It deserves a story to actually, you know, cover the hacks.

  10. There is now a github repo for design files for the original Heyphone.


    The design files are based on the scanned artwork at the British Cave Rescue Association.

    It is a work in progress but aims to include design files for the three boards for the original Heyphone, allowing gerbers to be generated easily, and modifications to be made.

  11. Based on the original circuit, but not exactly the same as some of the original components are obsolete and there are better and cheaper ways of achieving some things. The original AGC was never the HeyPhone’s strongest feature, so that’s been upgraded as well.

    Current work is focussed on using a dsPIC to replace the receiver PCB with a digital version.

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