OCR Text |
Show 2.6.14 High Frequency Radio Emission From the technical standpoint this is one of the most promising search methods. A miniature radio transmitter carried by the victim can quickly be located with a simple transistorized receiver. This technique has been tested and found effective. The transmitter can be very compact- even pocket size- but does require an antenna loop sewed into a parka or shirt. If the transmitter operates on a broadcast band frequency, an inexpensive transistor radio will serve as the receiver. The highly directional character of the ferrite core antenna found in most small radios aids in locating the transmitter. Only very low power from the transmitter is needed ( and in fact is severely limited by law), so it will operate for many hours from miniature batteries. Rechargeable nickel- cadmium batteries are preferred because they are much less affected by low temperatures than normal dry cells. Legally- permitted radiation at the lower end of the broadcast band can be detected with an ordinary transistor pocket radio up to about 50 feet from the transmitter. This is a substantial improvement in search radius over the magnetic methods. Because prolonged transmission is possible, the triggering problem is eliminated. The carrier of the transmitter can switch it on whenever he enters an avalanche danger area and leave it on until he leaves. Limitations of the radio detection method are the restricted amount of permissible radiated power and the necessity of carrying a separate radio receiver for detection. There are also problems of signal absorption In wet snow or by other buried objects. Like all Category B search methods, this one is limited largely to organized groups which can provide the necessary equipment to persons exposed to avalanche danger. 2.6.15 Audio Frequency Induction Field A research team at Cornell Aeronautical Laboratory ( Buffalo, New York) under the direction of Dr. John Lawton has recently proposed a search method which removes the limitations of the radio detection technique. This replaces the radio transmitter with a 2 kHz audio signal generator connected to a transmitting loop similar to the radio antenna. The receiver is a high- gain audio amplifier connected to the same loop and housed in the same case as the transmitter. Power is provided by miniature rechargeable batteries which permit 200 hours of transmitting and 1500 hours of receiving between charges. Working units ( Figure 19) of this system have been furnished by Cornell Aeronautical Laboratory for operational test by professional ski patrols and Snow Rangers during the winter of 1967 and 1968. The designation SKADI, derived from Norse mythology, is the name of the wife of Ullr, so-called patron saint of skiing. Use of audio frequencies removes the legal limitation on radiated power. The induction field is virtually unaffected by any surrounding medium. It is detectable through 300 feet of solid rock and presumably through any conceivable avalanche debris under which a victim might be buried. Tests to date have shown the system to be highly effective. The signal of a buried transmitter can be detected up to 300 feet away. A completely inexperienced operator can follow this signal to its source within a few 53 |