457 KHz ELECTROMAGNETISM AND THE FUTURE OF AVALANCHE TRANSCEIVERS

John Hereford1

Rescue Technology, Inc.Bruce Edgerly2

Backcountry Access, Inc.

ABSTRACT: The standardized frequency for avalanche transceivers, 457 kHz, presents many interest­ing, important, and confusing issues, especially related to receive range, flux lines, near field, interfer­ence, point sources, receiver design, searching, and specifying and measuring transmit power andreceive range. Improved standards and the possible addition of a higher frequency will help in prOVidinga sophisticated, yet uncomplicated beacon in the future for the expeditious rescue of avalanche victims.

1. BENEFITS AND DISADVANTAGES

An advantage of using the (656 m) longwavelength 457 kHz signal for companion rescueis that there is little attenuation or effect by objectssuch as snow, the body, metal, trees, and rocks.There is no "multi-path," which means that the sig­nal does not bounce or reflect off of objects in thebackcountry, which would present confusion inlocation systems. [Multi-path is what causes"ghost" images on a television set using an anten­na (at about 50 - 200 MHz)].

For a small antenna as used in avalanchebeacons, the (near) fields transmitted and receivedare predominantly magnetic. This is why objectslike aluminum shovels don't significantly limit thetransmit field strength of a beacon (unless it isplaced so closely that it affects the Quality of theantenna and circuitry); the blade may only"block" the small part of the electric field. Theearth and its grounding do not attenuate or affectthe signal as mUCh. Ferrous objects, however, dohave an effect (e.g. steel towers, iron framework).

As the appendix below points out. theboundary between near field and far field is 'A/21t.At 457 kHz, this distance is at about 100 meters(656rn16.28). so the operation for companion res­cue is definitely within the near field. (For refer­ence, the wavelength of a 60 Hz power line is 5million meters or about 3000 miles and the near­field boundary is 833 km).

'2400 Central Ave., Ste. B-1, Boulder, CO80301; (303)415-1890; rescue@csd.net22820 Wilderness Place, Unit H, Boulder, CO80301; (303)417-1345; edge@bcaccess.com

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A disadvantage of the 457 kHz frequen-cy is that, in its near-field application. the shape ofthe signal can be quite complex. In the near field,as compared to the far field, the flux patterns aredependent on the distance (r) from the transmitter,mathematical analysis is very complex, antennasize and type is important. field strength decreasesby up to r 3 versus r-1, magnetic and electric fielddependence varies, and the fields are curved (afar-field application would directly point to thesource). This curved shape looks like a figureeight or the wings of a butterfly. Another analogyis that the flux pattern in the near field appears likewater coming from a fountain.

far field

Near field! far field. In the near field, the shape of thesignal can be quite complex. A far-field applicationwould point directly to the source.

2. FAR-FIELD EXAMPLES

The question is often raised why GPS(Global Positioning System) technology has notbeen applied in the field of avalanche rescue. Itsfrequency is 1.6 GHz, which gives a wavelength of-0.19 meters or about 7 inches, which allows forsmall, efficient antennas. Because of this high fre­quency, the transmitting satellites need less than50 Watts to provide usable signals down to earth.There are about 24 satellites in orbit and for the tri­angulation needed, several satellites are needed.The signals require line-of-sight orientationbecause the small wavelength signals are blockedby buildings, mountains, canyons, tree, etc. andare severely attenuated or limited by snow.Furthermore, a GPS receiver will tell you whereyou are, but there is substantial added technologyto relay that information to a person searching foryou.

Another example of a far-field applicationis the Recco system currently in use for locatinglost individuals. It uses a 1.6-kg transmitter/detec­tor to bounce microwaves at 917 MHz-off a specialreflector-a thin printed circuit card that doubles thesignal frequency-that is attached to an individual'sequipment or clothing. One limitation, due to itshigh frequency, is that the user should alwayshave two reflectors so the body does not interferewith the signal.

3. ANTENNA AND TRANSCEIVER LIMITATIONS

Of course, the avalanche rescue transceiv­er for companion rescue needs to be a portableproduct. Therefore, the antennas are electricallysmall and the (battery) power available is very lim­ited. These are two main reasons, along with theoperation in the near field, for little increased rangepotential at 457 kHz.

For optimum antenna size, its circumfer­ence or equivalent height should be ~*A, or 327meters at 457 kHz. Therefore, the avalanche bea­con antenna is a very small portion of the wave­length. There are things that can be done toincrease the effective height of the loop antenna,such as adding a ferrite core and increasing thenumber of turns of the wire, but the efficiency isstill less than 0.1 percent, and this is a limitationwith both the transmitter and the receiver.

Transmission power for a beacon is less

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than 0.1 Watts. Compare this to AM radiotions, which are slightly higher in frequencyhave a power typically greater than 10kW (100,000 times more powerful) - and the tting antennas can be hundreds of meters .

Atmospheric and man-made noiseduced by such things as power lines and 'phenomena, is very high in the region of 451and can be aggravated in an urban environFor all types of receivers, extensive filteringprocessing (e.g., mixing) is done to reduceextraneous noise and to help isolate thetransmission signal, which gets very weakfrom the transmitter. This partially explainsso-called analog receivers appear to havereceive range: with analog transceivers, this",ing is done by the user's ear rather than theceiver's microprocessor. Consequently, the..ness of this weak signal at long range is heaYidependent on the ability level of the user.

This difference in receive range is dueexclusively to the noise filtration process of thedigital receiver, and has no relationship to thenumber of antennas used in receiving the si~as suggested in other literature (Peterson, 2Olq.On the contrary, the number of antennasa~increases the search strip width. In the case oUtTracker DTS, which uses two receiving antennas.the search strip width is increased by a factorcf15 percent (Meier, 2000). Since search strip"defines the primary search path, not maximumrange, this has a stronger effect on the primarysearch time than a beacon's maximum range.

. However, while receive range and seaJdIstrip width are often perceived as an importantproduct benefit, they may have more marketingvalue than technical significance. The receiverange of an avalanche beacon has no significIReffect on the speed of a search or the probabilyof a live recovery - and can actually prolong thesearch when performed by recre~tionists(A_1999). On the periphery of an analog beacon'~receive range, the searcher must cover a rel_large distance before making a determination oRsignal strength and direction. For recreationislS.this can be extremely time consuming, resultintin unnecessary backtracking and signal interprelttion. For this user group, it might very well beless time consuming to continue with the prin8Ysearch until the signal data can be presentedwit~ enough resolution to make quick decisions.

This is where the signal-to-noise filtration in digitalsystems can be seen as a major benefit: it elimi­nates the "gray area" which can frustrate noviceanalog beacon users at longer range.

4. ANALOG VS. DIGITAL TECHNOLOGY

A better term for analog beacons would be"audible-based." The human ear is a powerful sig­nal detector out of noise. An example of this isthat, in a noisy room it is possible to detect andhear a known voice. It is difficult for a digital signalprocessing system in the room to detect, recog­nize, and isolate the speaker, especially if thevoice is as loosely defined as it is by the presentintemational standards for an avalanche beacontransmitter. For example, the present broad stan­dard for the on- and off- time may tell a listener orreceiver that the speaker in the room is feminine,but a tighter definition would better describe thetransmitter's specific speaking characteristics toallow isolation of a specific person.

The greater perceived range of the audi­ble-based transceiver is not due to better design ornecessarily better signal-to-noise ratio, but due tothe power of the human ear. But the human ear isa very poor judge of loudness (volume) changes.That is why it is difficult to determine the directionof a transmitter based on audio level changes,especially at low signal levels and especiallyamong non-professional users. However, the earcan recognize very fine changes in pitch.

A "digital" beacon can take several forms,but basically it takes the Radio Frequency signalthat has been filtered, mixed, and amplified usinganalog technology and then digitizes this to allow amicroprocessor to process it. This provides formany advantages, such as determination of direc­tion (from a dual antenna system), distance calcu­lation, audio interface improvements (such as pitchvariation), improved algorithms for signal detec­tion, mUltiple transmitter isolation and location,automatic sensitivity adjustment, digital filter imple­mentation, and other user interface improvements.

5. STANDARDS

Beacon development is not just limited byelectronic technology, but also by down-level stan­dards that do not define the signal characteristicsvery well, specifically on- and off-times of the 457

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kHz carrier. Modernizing these standards couldsignificantly improve the future performance .ofavalanche transceivers. However, trying to stan­dardize or explicitly define how the beacon shouldoperate is counter-productive. User inferfaceissues are most efficiently addressed in the mar­ketplace, based on the needs and wants of theconsumer.

There is no one international standard.The European standard is ETS 300 718 (currentlyundergoing revision), with the EN 282 standardstill being used in some cases. The only standardfor avalanche beacons in the United States is setby the American Society for Testing and Materials(ASTM F1491-93); it sets only the frequency at457.0 kHz, with no other requirements.

Standards should be modernized so thatthe signal is better defined to allow better digitalsignal processing and isolation. Also, productdesign is challenged by direct tradeoffs betweentraditional wants and assumptions, ''feature bloat,"and simplicity. For example, a standard thatrequired a minimum receive range or search stripwidth might suit the needs of the snow safety pro­fessional, but would be counter-productive for therecreational consumer, who generally does nothave the skills required to make use of a weak sig­nal at longer range. These conflicts should not beaddressed in the standards, but the product devel­oper and (ultimately) the consumer are best suitedto determine the best device at the lowest cost.

6. HIGH FREQUENCY AND 10 LOCATOR

We propose to significantly improve bea­con operation by adding a higher frequency signalto this 457 kHz carrier. With digital technology,this is now more feasible than in the past. Thiswould increase the detection range and wouldallow giving each transmitter a unique identifier(10) so that multiple victims can be even better iso­lated and located.

Since there is more power explicitly in ahigher frequency, this would increase the detectionrange, but without the inherent limitationsdescribed above regarding the (non)usability of aweak signal in the near field by the recreationist.Since the operating range would be in the far field,the transmitter could be seen as a point source,initial detection would "poinf' in that direction,antenna systems could be more optimally

designed, and there would be less effect fromatmospheric noise. Finally, this higher frequencysignal would allow giving each transmitter a uniqueidentifier so that mUltiple victims could be evenbetter isolated and located. Of course, this fre­quency would have to be carefully selected basedon issues related to snow depth, multi-path,human body effects, radio spectrum allocations,and other considerations.

Adding this higher frequency to the pre­sent 457 kHz carrier would not interfere with down­ward compatibility, or the ability of a newlydesigned transceiver to detect an "older" transmit­ter. The higher frequency would "ride~ on the 457kHz signal much like DSL or ISDN data rides onan analog telephone line. The 457 kHz signalwould still be used for fine and pinpoint searchingin the near field.

7. CONCLUSION

Avalanche beacon design has improvedmarkedly in the past three years, but progress hasbeen limited by the issues stated above.Constraints for future development are not just lim­ited by technology, but by poorly defined standardsfor the signal and by the need for downward com­patibility with existing beacons. Professional use isan important aspect of transceiver design, but onemain goal should be to make effective avalancherescue transceivers accessible to as many usersof the backcountry as possible, especially thosewho are most at risk: recreationists. By leavinguser interface issues up to the designers andallowing for a higher frequency in addition to thecurrent 457 kHz standard, transceiver technologycould see even greater improvements than thepresent, yet maintain downward compatibility withthe transceivers of the past.

APPENDIXRF Electromagnetism

There are some essential physics and definitionsthat help highlight the issues. Electromagneticsentail just what the word says: With any RadioFrequency (RF) signal (which is produced andradiated by a changing, time-varying currentthrough a wire or antenna), there is a magneticfield, H - a vector signifying the current densitymeasured as Amperes per meter (Aim) - andthere is an electric field, E, also a vector quantity

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that is a voltage density denoted as Voltsmeter (Vim).

These fields generate their respective f1uxe"flow" of electric and magnetic energy. Thes,field strength gives rise to the electric flux d0, with units of Coulomb per square meter.magnetic field produces a magnetic flux dedenoted by B, with the unit T, for Tesla or vOltond per square meter. Magnetic field strengthindependent of the medium, but its flux dens'force field, does depend on the material's peability. This is true also for the electric fieldits flux is related by permittivity. '

For magnetic fields, which the avalanche DeaCOlftantenna mainlv radiate, the relationship is H=Bille, where flo· is the permeability of air.materials, such as snow, rocks, and trees, all hilia relative permeability close to one, or the sameas air; Le., they're non-magnetic. Different pemi­tivities means electric waves may be attenuated,reflected, refracted, scattered, and diffracted bythe changes and depth of the media through whicIlthe RF signal propagates. The permittivity of snowis about 80 times that of air.

Some essential field vector equations froma predominantly magnetic source for any distance,given in spherical coordinates, are:

~3 [ 1 j 1] .& ojp-rHe =- 4-ft I'h·~.r - (~r/ - (~.r)3 ·sm c

~3 [j I I -Hl~rH :=-I·h· --+-- ·cos9·e

r 2.ft (~r)2 (~r)3 -

3 [1 j]..iP-rE~ :=30·~ ·I·h· ·sm~e

l3.r (~.r)2

Where:

2·7tA --f-' --

')..

"j,.= wavelength; the distance the beginning of a RFsignal covers before the beginning of the next;given by elf, where.c is the speed o~ Iig~t (which isthe velocity the radio wave travels (m air or freespace)) and f is the frequency.

Therefore, at 457 kHz:1= c/f= (3XI08 mls) I (457XI03 Hz (cycles/s» =.

656 mlcycle or over 0.4 miles longI = electric current flowing through antenna looph = equivalent height or area of antennar = distance from transmitterj = imaginary number that is used in definingvectors

Notice that the terms with the coefficients

will all be equal at the distance r = liP =')J2rr.. Thisdistance is defined as the boundary Detween near­field and far-field.

When r «').)21t, totally in the near field, only thethird term in each equation is significant. Thewave impedance or resistance to the field, is theratio of the electric field to the magnetic field. Fora small, loop antenna, the above equations reduceto:

Zw = E. (Vim) I He (Aim) = Zo*(1J2m) ohms

Zo (the free space impedance) = 1201t = 377 ohms

This means that the resistance to the magneticfield, especially in relation to the electric field, isless as one gets closer to the transmitter. Themagnetic field is reactive or inductive and is con­centrated near the source.

When r » A/21t,which is in the far field, only thefirst terms of 1/r are significant and the field com­ponents reduce to:

~2 . ~'~rE$ ·=3Q.-·I·h·smBer2 .

~ -J'~'r~:=-~I'h'sine'e

4·rr.·r

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The wave impedance is the constant relationshipof Zo (1201t or 377 ohms), and the fields areorthogonal to each other and produce planewaves. Hr reduces to zero, so the curved flux pat­tern phenomenon goes away. Therefore, the fieldswould directly "point" to the source. They are radi­ated fields and their effects extend far from thesource. The field strengths fall off as 1/r and therelationship between the electric fields and mag­netic fields is a constant.

In terms of power, the relationship at any distanceis a Poynting vector, or EX H (cross product), andthe power density would be He2*Zw[(Alm)2(ohms)). Therefore, terms are squared:e.g., power roll-off of r 6 in the near field.

REFERENCES

Atkins, D., 1999. Companion Rescue andAvalanche Transceivers: The U.S. Experience.MAP Avalanche Review, 1999.

Meier, E, 2000. Determining the Search StripWidth for Avalanche Beacons. ISSW Proceedings.

Peterson, M., 2000. Avalanche Transceivers:Uses, Limitations, and Standards. ISSWProceedings.

The Radio Spectrum and Frequency Allocations

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the Radio Spectrum compares to the audible and visible range