EUREKA NOVEMBER 2000 COVER FEATURE STORY

Magnet waves speed signals through rock and under water

A new method of communication will allow free swimming divers and underground rescuers to communicate with ease. Tom Shelley reports

Divers and workers underground will soon be able to communicate with each other using magnetic waves - electromagnetic signals without the electric part.

The idea comes from experiments performed before the invention of radio, and by amateurs since.

Range is limited, but magnetic waves go through anything except iron and other ferromagnetics.

The idea of transmitting telephone signals between coils of wire by magnetism dates back to the 1880s, when it got as far as being used on a routine basis to communicate between the mainland and one of the Isles in the Bristol Channel.

The present development, however, only dates back a few years and stems from the scuba diving interests of Ken Hawes, managing director of Class Instrumentation, an electronics development company based in South London.

"I used to like to go diving, particularly in the Cayman Islands, but after umpteen near accidents and crises under water, I became aware there was an acute need for some reliable means of communication between free swimming divers. I went to all the shops and diving schools, and found nobody sold anything remotely suitable, and the only one who did have a system, which was based on an ultrasonic carrier wave, told me it was of no practical use."

He then went to work to see if he could solve the problem. He knew that radio waves used to communicate with submarines have to be at such low frequencies, around 16kHz, that they can only be used to transmit digital Morse type messages, and require antennae with enormous lengths of wire in order to function as tuned circuits.

He also knew that the problem with ultrasonics under water is that unless you are a whale and speak in clicks, you require immense signal processing to sort out all the signals coming over different path lengths and echoing off obstacles.

He began with the idea of using conduction, with two electrodes spaced apart at the transmitting end, and two electrodes at the receiving end. He managed to make the system work, but soon found by experiment that the 50V and fairly hefty electric currents needed to drive it at the transmitting end posed a considerable risk of electric shock.

He then proceeded to try magnetism, since magnetism penetrates conducting media as well as air, provided the medium is not ferromagnetic.

The only problem, he says, is that magnetic fields fall off with the inverse cube of the distance to the receiver, whereas electromagnetic waves only fall off as the inverse square of the distance.

"This means", he says, "You need a lot of gain at the receiving end and transmitting and receiving aerials with a Q (selectivity) of at least 100"

"In order that the aerial stay tuned, you have to take away any stray capacitance and permeability."

He does this by enclosing the coils in a conductive tube. The screens fix the capacitance and suppress any electric vector in the emanating waves. In the case of the prototype system shown to Eureka, the coils consist of screened coaxial cable wound onto square frames, although in a production version, there would be likely to be multiple turns of wire within a single screen.

Transmitters and receivers use SSB (single side band) for maximum efficiency. At present, the operating carrier frequency is 87kHz, because this is an amateur band, not used by anything critical likely to lead to complaints in the event of inadvertent interference. Input power is only 9W, and development efforts are being focussed on improving the signal to noise ratio at the receiver. Transmitting distance is presently limited to 15m, although it is hoped to soon increase this to 30m, which because of the cube law, requires an eight fold improvement in sensitivity.

Tests have been conducted mainly in air, but also in water, where it was soon established that there is no change in efficiency compared to working in air. The transmitting amplifier presently uses class B, which is 63% efficient, but Hawes expects to soon go to Class D or H, simulating sine waves by digital switching in a similar manner to electric motor inverter drives. Audio modulation is currently in the range 300 to 3kHz.

The first stage of the development work has been supported by a DTI SMART Award, and the study has already had significant input from DERA, who doubtless have their own interests in the project. The project has been given the name, 'Water Talker'.

Nonetheless, all Mr Hawe's target markets are civilian, and the aim is to produce a transceiver which can go on sale for about $300 to end users. As well as recreational scuba divers, there is already much interest from the caving community, who at present have no reliable means of communication underground or between underground and surface. Another interested group of people are those who have to try to rescue people from light aircraft which have crashed on water and sunk. A problem has also been identified in the Channel Tunnel, where no means presently exists to communicate between tunnels in an emergency. The system may also prove useful in a number of industrial situations, including communication with systems inside nuclear reactors, because it is so good at penetrating thick concrete.

One person a week dies in under water accidents in the UK alone. The development could save many lives, as well as improving the fun factor for more than a few people who enjoy their sports under water or under ground.

www.classltd.com

Design Pointers

System allows communication under water or under ground

Target distance capability of the current development is 30m

Sale price per transceiver is expected to be about $300

The beginnings of magnetic wave communication

In 1880, eight years before Hertz's discovery of radio waves in Karlsruhe, a professor John Trowbridge of Boston, USA, suggested that it might be possible for ships at sea to communicate with each other using large coils consisting of ten or twelve turns of wire stretched from the yard arms. Experiments were carried out on board ship, and it was noticed that signals were strongest when the two coils were parallel to each other. In 1885, William, later Sir William Preece, the Divisional Engineer to the General Post Office, laid two separate squares of insulated wire, 440 yards on each side, a quarter of a mile apart on the Town Moor at Newcastle. Communications were easily established, and found to still be possible when the squares were 1,000 yards apart. Similar trials were held in 1886 in different parts of the country and in March 1898, Preece's system was permanently established as a means of communicating between Lavernock Point and Flatholm Island in the Bristol Channel, 3.3 miles away and was handed over to the War Office. The main limitation of the method was that is was soon realised that in order for the system to work, the length of wire in each coil had to be at least roughly equal to the distance between the two coils. Preece ultimately sponsored Marconi when he came to England with his much more promising invention in 1896.

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