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> From: keir@vi*.su*.ed*.au* (Keir Vaughan Taylor) > I was wondering if it would be possible to make a cave diving line > made up of lots of antennas that resonate to a radio signal. The idea > being that a diver would carry a transmitter/receiver which would have > the power to penetrate a few meters of water to reach the dive line. > The diver might be using a throat mike rather than use a full face mask. > The line would then carry a signal back to a surface team through > the line. Apparently an antenna broadcasts perpendicular to its length so > the line may have to have many mini antennas about 1mm imbedded in the line > and perpendicular to its long axis. Reception by the diver may not be > possible with such a system unless they clip onto the line itself > It might be that metal flakes embedded in the line could work ? Unfortunately RF engineering and electromagnetics isn't my speciality, but I will give my opinion on this proposed system and a possible alternative. As mentioned in the above posting, radio signal transception isn't of much use under water because of the high rate of signal attenuation. Some calculations I've done a few months ago show that the amount of signal power needed to achieve any realistic transmission distance above a few inches is prohibitive (size/power source). Low frequency transmission under water is used indeed, but low frequency=high power+large size. Miniature antennas (small loops/metal flakes) is only useable at high frequencies, else it can't be small! Thus this isn't an option. BTW, the radiation pattern of antennas isn't necessarily perpendicular to its length, it depends on the type and design. A neater option is to look at ultrasonic wave transception, the Buddy- Phone (tm), which achieves a distance of about 150ft, comes to mind. Ultrasonic wave transmission also isn't restricted to line of sight propagation (theoretically, yes), because of reflection from cave walls, bottom etc. A possible system can consists of a transceiver on the diver and repeaters at, say every >100ft on the guide line. Probable, but not unsurmountable, problems will be the isolation of each repeater in the correct direction. About the throat mike, I'm not familiar with this technology, but I'm wondering about speech garbling at high pressures. I'd rather go for a mouthpiece that replaces the regulator's, like the buddy-phone - for an interface in air between voice and mike. Moving into unfamiliar ground, which is technical diving (particularly cave diving), I was under the impression that the diver is clipped/attached to the line. Please correct me if needed. In this case the need for tranceivers won't be there as only two pieces of wire has to be imbedded in the guide line with some unknown way of tapping the signal from the line. Rethinking, this may be an unsurmountable problem! Communication between divers will then be a natural by- product. - Gerrit ****************************************************************** G. Conradie Phone: +27 21 808 4452 (W) Dept. E & E Engineering +27 21 903 4306 (H) University of Stellenbosch Fax: +27 21 808 4981 (W) Stellenbosch, 7600 South Africa Email: conradie@fi*.su*.ac*.za* ******************************************************************
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