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The cave radio

Abstract: description of the first wireless cavephone ever build in Belgium, according to a British Design by John Hey. It gives the possibility to establish voice-communications between the surface and a team in the cave, up to a depth of well 100 m (actually, depths of 500 m are in within reach!!).
Author: Herman Jorens.


Everyone who has ever tried to establish a connection between cavers underground and a surface-team, using Walkie-Talkies, knows that this does not work! Even communicating IN the cave itself with Walkie-Talkies is often not possible (except in big rooms).

Special cave-radios have been developed, that can send their signals through solid rock. In Great-Britain, they are being used quite often, but on the continent their use is still exceptional. In Belgium, as far as I know, they have never been build nor used before. In France some experiments have been conducted.



Radio-communication through solid or liquid substances is not that simple! The higher the frequency, the better the surrounding matter will absorb the radio waves. This is why Walkie-Talkies won't work, unless the transmitter and receiver can "see" each other.

The lower the frequency, the better the waves penetrate in the solid or liquid matter. Submarines are using since many years low frequencies to establish communications with surface-stations, by trailing long antenna's behind them. This illustrates one of the main disadvantages of low frequency-radios: the antennas get real big. That's why cave-radios use antennas that are made of cable, wound in a small loop.

In the 70ties, some British cavers started experimenting with VLF (Very Low Frequency) transmitters to try to establish radio-communications between the surface and a cave. After some years of trying and inventing the right antennas, they succeeded in their plans. These first cave-radios were pretty big and used a lot of (battery)power, so cavers were dragging around considerable weights! But things soon got smaller and lighter. Two popular designs were the Ogophone and the Molephone, capable of reaching depths around 150-180 m.

The Molephone was commercialized and can still be bought. The design is secret. The design of the Ogophone however was published, and amateurs can still rebuild this type of radio. The antenna is a loop with a diameter of 1 metre, made out of 64 turns of computer- flat cable. The radio is build into an ammunition box, since British cavers like to drag these things around. The battery is put in a separate box.



Currently, cave radio's are commonly used in Great-Britain, especially by the Cave Rescue Organisation. No more dragging telephone wires around: the Rescue team leaves with two ammo-boxes, plugs in the microphone and antenna, and they have a vocal communication with the surface-team. If necessary, the receiver (surface) is connected to a normal telephone or G.S.M. so the underground Rescue-team can speak to a doctor or hospital!

In Great-Britain there is a user group, called the CREG (Cave Radio & Electronics Group). They are a division of the BCRA (British Cave Research Association). These guys like to experiment with cave radios. In fact, the predecessor of the cave radio, a loop-wound antenna that gave a 3kHz "beep-beep" signal, was born here. It was used to for radio-positioning (positioning an underground point at the surface). One of our members, Jan Geboers, build one and we use it quite often. You'll find a description of it a bit further in these pages!



Since a couple of months, I had the idea of trying to build a cave radio. The Ogophone is pretty complicated to construct (since one has to wind several coils, and that is not easy at all). In the winter of '96 I discovered the "CREG" and saw in their publications the design of a new cave radio. It was small, compact and made with modern components. Not one single coil in it! I contacted the maker of it, John Hey, a caver, radio-amateur (G3TDZ), engineer in electronics and, thank God, a very helpful guy.

Constructing these radios was not so simple. Although I had copies and descriptions of the circuit-boards, a lot of problems had to be solved. Some of my friends did not believe in my project, but this stimulated me even more. I worked every evening, sometimes 'till 2 o'clock in the morning, to get the transmitters/receivers working. John and I faxed each other 2 times a week.
On June, 2 1996 the first underground test was conducted. Patrick (another Avalon-member) and I made voice-contact through 60 m of solid rock! We had a very clear communication. As far as I know, this was the first time ever (in Belgium) that wireless voice-contact was established between a cave and the surface.

Making a phone call at -100 m!After some modifications in the receivers a second test was held. This time we chose one of the deepest caves in Belgium, Trou Weron. We established a voice-contact between the surface and the final sump of the cave, 106 m deep! (see picture). The cave was unfortunately situated under some high-tension powerlines and these interfered with my radios.  So we held a third and final test in the Sainte-Anne cave. Crystal-clear communications between the cave and the surface (95 m higher) was made.

In October '96, our club stayed for a week in Yorkshire and participated at a "field-meet" of the CREG. We experimented with different kinds of radios in an abandoned cupper-mine near Richmond. My radio reached a (horizontal) distance of 140 m through solid rock.



The transmitter/receiver has more or less the dimensions of a car-radio. At the front there are some knobs and switches, and the connectors for the microphone, antenna and battery. All this fits in an ammo-box. The battery also fits in this box, so do the microphone and some cleaning-attributes. Everything is surrounded by shock-proof foam. The ammo-box is waterproof. There are two identical sets.

The antenna is a hexagonal loop, made out of 22 turns of isolated wire (0.75 mm2). It can be mounted on 3 wooden laths. Everything can be plied together and be transported in a PVC sewer pipe, 110 mm diameter.

The transmitter functions on 87KHz, in SSB modulation and has an output of 10Watts. In fact we should better call it an "induction transmitter", because it emits induction waves.



The transmitter/receiver has a build-in "beacon". When this is activated, it will emit a powerful "morse-like" signal that enables the surface team to track the transmitter over long distances (several hundreds of metres).

One transmitter/receiver set is put into the cave with the antenna flat on te ground. The beacon is switched on. The surface team will quickly pick up the beacon and now has to locate the point that is straight above the transmitter. At that location, the distance is at it shortest and voice-communication will be at it's best. The surface team proceeds as follows: they keep the antenna vertically and rotate it slowly around the vertical axis. When the signal (beacon) dies out, the antenna will be in a plane that points either to the underground transmitter, either in the opposite (180 degrees) direction. By repeating this test at 2-3 different places, each 20-30 metres apart, they can quickly tell where the transmitter is located. They'll hurry to this spot and just start ...talking in the microphone! If the depth isn't too deep, let's say 2/3 of the maximum reach (140 m), they'll have a crystal clear voice communication, sometimes better than on a GSM handheld telephone!



Here a list of things that you can use a cave-radio for:

- Cave rescues: this is probably the most common use of cave-radios. It is indeed a lot easier and quicker to carry a cave-radio through the cave, than it is to roll out telephone wires. It is also a lot more secure: telephone wires get tangled or cut easily, especially during a rescue. With the cave-radio, the underground rescuers keep in contact with the surface-team, or even remote sites: a doctor, hospital, or relative of the victim, if the surface team has a GSM hand phone available (just hold it in front of the loudspeaker of the cave-radio!)

- Exploring flood-prone caves: in such circumstances, it is vital that the underground team is Beam us up, Scotty!informed about changing weather situations. This is especially useful in case of explorations, where underground camps are used, and where cavers are staying for several days in the cave without having an idea about the weather outside!

- Cave-diving: the cave-radios also work between two parts of a cave or even different caves. One set can be taken along by divers, behind a sump. It will allow the diver to stay in touch with the team before the sump. Very re-assuring indeed! It will also allow him to report any problem that arises.

- Announcing your return: a team that returns from the bottom of the cave, can, as soon as they are at a depth of -140 m, announce their return to the surface-team. Very nice indeed, when you like a fresh beer when you get at the entrance of the cave!

- Making radio-locations or estimating depths: Using the cave-radio, one can make radio-locations or estimate depths (using the "beacon") in the same way as with the "Induction Loop" (see article nr 4 on these pages!) . One big advantage is that a vocal contact is also possible, thus permitting to do several radio-positionings in one day. Without voice-contact, you would have to agree on a very strict time-schedule (which often can't be kept).

Estimating the depth is done by measuring the angle of the waves. First the "ground zero" (= the spot just above the underground transmitter) is located, by holding the antenna vertically and doing a small triangulation. Then one will step away from this point for a short distance (depending on the depth: 20-50 m should do). One keeps the antenna horizontally and slowly tilts it downwards, pointing at "ground zero". The "beacon" will fade out at a certain position. Keep the antenna firmly in this position, and measure the angle between the frame of the antenna and the earth. An electronic inclinometer is just what you need for this job! Using this formula you can calculate the depth: The magnetic induction field

Depth = 2L / V((9tanA▓) + 8-3tanA))


V=square root
L= distance between ground zero and the centre of the antenna
A= angle between antenna and earth

A LOT EASIER is this method: step away from ground zero, holding the antenna horizontally and tilted 45 degrees downwards, until the "beacon" fades out. The depth of the transmitter is now 1.77 times the distance between you and ground zero! In the field, having no calculator available, you can easily make a good estimate by multiplying the distance by 2, and subtracting 10% (thanks, Stuart France, for this tip!). Example:

exact result: 22 m x 1,77 = 38,9 m

or appr.: 22 m x 2 = 44 m -10% = 39,6 m.



Why did I start this project in the first place? Well, because it interests me a lot, and because it was about time that a cave-radio was being build in Belgium! I also hope that this article could stimulate people to start experimenting with caving electronics.

In England, they now experiment with "Cave Pagers", small beepers that can be carried inside the oversuit and that can be used to warn an underground team (in case of changing weather, for instance). I'm confident that in 5 years, cave-radios or pagers will be commonly used in the caving world. . They'll always get smaller and reach bigger depths. One day, we will talk at -1000 m with the surface team. Actually, depths of +/- 500 m are already within reach!

If you would like any additional information, feel free to contact me:

Jorens Herman, Randerode 68, 2940 Stabroek, Belgium

tel: +32 3 568.16.45 fax: +32 3 561.40.56 (f.a.o. Jorens H.)


If you'd like to learn more about cave radios; in particular the "Nicola System" developped by Graham Naylor, then surf to:



Actually, we are experimenting with a totally different type of antenna. Instead of a loop, two wires (ordinary 0,75 mm2 electrical wire)) are used (both underground and at the surface) that are tended in opposite directions. At their ends they are fixed to a small steel pin that is driven in the soil of the cave and surface. The longer the wires, the deeper one can communicate! These antenna's are a LOT lighter and smaller than the bulky loop-antennas.

We recently tested this system in a French cave and were able to establish an incredibly crystal-clear communication (clearer than a GSM-handphone!) at a depth of 350 metres!
We used wires that were only 17 metres long. The conditions were ideal, since the cave was large and had a muddy soil. What a great feeling to hear Herman say that the sun was shining, and that he was lying in the soft grass listening to the birds... while we were still a long way from home!

French cavers also have build John Hey's cave-radio and they recently reached depth's of well over 600 m in the Dent des Crolles, using 80 metre-wires.

Its is clear that these new antennas give enormous possibilities for the exploration of deep-cave-systems. Establishing vocal communication with a team or underground camp, 500 m deep, is now only a matter of minutes. Cavers will be held alert of changing weather-conditions, will be able to report any problems or progress, or will just simply be comforted by the voice of their teammates at the surface...

Contacteer/contact us:  SC Avalon vzw
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