Communication in underground or confined environments has suffered historically from a range of serious problems:

• The impossibility of using conventional RF equipment due to the attenuation of electromagnetic waves when passing through mediums with electrical conductivity (rock, floors, walls, etc)
• Solutions used up to now, based on laying out wires, require high deployment times and complex transportation of heavy items. In addition they are vulnerable to collapse.
• Operation in hostile environments (humidity, dirt, etc.)

How does TEDRA™ work?

How are TEDRA™ devices capable of interchanging voice transportation signals through the “rock channel”, if radio signals are unable to penetrate the earth? And what kind of signal is transmitted?

The answer is that when you talk into a TEDRA™ device, an electrical current is injected into the earth by a pair of electrodes. This current transports the voice signal, which is propagated into the surrounding ground while the digital TEDRA™ device in receive mode reconstructs the signal of the emitted voice.

The drawing shows how each TEDRA™ device is equipped with two electrodes inserted into the ground. When one of these devices is in emit mode (when the user is talking through the device), the amplified signal is transmitted by injecting a current into the terrain. The lines of electrical current flowing between both electrodes (A & B) find their way through the surrounding earth. The TEDRA™ device in receive mode then detects the small variations of tension in its own electrodes (C & D), caused by the current injected by the other device and this results in the reconstruction of the signal and, in turn, enables communication.

So the more the lines of electrical current expand, the greater the distance that they can cover through the earth. So far, tests have been carried out in different caving environments, reaching a distance way beyond 1,000 metres between the two points of communication and with surprisingly high audio quality. However, the presence of faults, an excessive variety of stratums, etc. in the earth between the two TEDRA™ devices in communication mode, may result in signal reduction. For this reason, it is not possible to guarantee, at first sight, a specific transmission range for all cases which could potentially arise.

Reduced installation time

The first step for installation is to ensure that each electrode makes good electrical contact with the terrain (by staking them firmly into a compact area, or laying them in pools of water or humid areas) and always trying keep the maximum distance possible between both electrodes (the wiring supplied allows up to 50m). The electrodes are then connected to the main electronic device via the wires. From this point, it is now possible to turn on the device and start establishing voice communication through a PPT (Push To Talk) handset, as the type of communication is “Half Duplex”.

Low weight & high mobility

Another aspect of this device, greatly appreciated by users is its low weight and ease of transport. Incursions in underground or confined environments normally require the lightest equipment possible, and in any case the maximum acceleration of deployment. Each complete Tedra™ device weighs approx. 2.5 kg, including two 25m wires and two 25m stainless steel electrodes.

The main electronic device is transported in a watertight case (IP 67), approx. 21x17x9 cm in size. But there are also other, less visible aspects which explain the clear and powerful sound offered by Tedra™. The heart of the device is the digital signal processor (DSP) which obtains, for example, much cleaner signals than with a purely analog solution. The improvement capacity of a DSP is such, that most radio systems are implanted with DSP software.

All in all, this device will no doubt revolutionize communication in sectors, which have previously depended on wire solutions, and the disadvantages which these entail, such as transport and high laying out time, not to mention the vulnerability of wire communication when faced with falling rocks in a confined space.

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