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 and receive range.
Improved standards and the possible addition of a higher frequency will help in providing a sophisticated,
yet uncomplicated beacon in the future for the expeditious rescue of avalanche victims.

1. BENEFITS AND DISADVANTAGES A disadvantage of the 457 kHz frequen-

cy is that, in its near-field application, the shape of
An advantage of using the (656 m) long the signal can be quite complex. In the near field, as
wavelength(λ) 457 kHz signal for companion rescue compared to the far field, the flux patterns are
is that there is little attenuation or effect by objects dependent on the distance (r) from the transmitter,
such as snow, the body, metal, trees, and rocks. mathematical analysis is very complex, antenna
There is no “multi-path,” which means that the signal size and type is important, field strength decreases
does not bounce or reflect off of objects in the by up to r-3 versus r-1, magnetic and electric field
backcountry, which would present confusion in dependence varies, and the fields are curved (a far-
location systems. [Multi-path is what causes field application would directly point to the source).
“ghost” images on a television set using an antenna This curved shape looks like a figure eight or the
(at about 50 – 200 MHz)]. wings of a butterfly. Another analogy is that the flux
pattern in the near field appears like water coming
For a small antenna as used in avalanche from a fountain.
beacons, the (near) fields transmitted and received
are predominantly magnetic. This is why objects
like aluminum shovels don’t significantly limit the
transmit field strength of a beacon (unless it is
placed so closely that it affects the Quality of the
antenna and circuitry); the blade may only
“block” the small part of the electric field. The earth
and its grounding do not attenuate or affect the
signal as much. Ferrous objects, however, do have
an effect (e.g. steel towers, iron framework).

The boundary between near field and far

field is related to the wavelength λ and is λ/2π (see
appendix). At 457 kHz, this distance is at about
100 meters (656m/6.28), so the operation for
companion rescue is definitely within the near field.
(For reference, the wavelength of a 60 Hz power line
is 5 million meters or about 3000 miles and the
near-field boundary is 833 km).

13020 Third St., Boulder, CO 80304;

(303)415-1890; herf@ uswest.net Near field/ far field. In the near field, the shape of the
22820 Wilderness Place, Unit H, Boulder, CO signal can be quite complex. A far-field application
80301; (303)417-1345; edge@ bcaccess.com would point directly to the source.
2. FAR-FIELD EXAMPLES which are slightly higher in frequency – they have a
power typically greater than 10kW (at least 100,000
The question is often raised why GPS times more powerful) – and the transmitting anten-
(Global Positioning System) technology has not nas can be hundreds of meters high.
been applied in the field of avalanche rescue. Its
frequency is 1.6 GHz, which gives a wavelength (λ) Atmospheric and man-made noise, pro-
of ~0.19 meters or about 7 inches, which allows for duced by such things as power lines and weather
small, efficient antennas. Because of this high phenomena, is very high in the region of 457 kHz,
frequency, the transmitting satellites need less than and can be aggravated in an urban environment.
50 Watts to provide usable signals down to earth. For all types of receivers, extensive filtering and
There are about 24 satellites in orbit and for the processing (e.g., mixing) is done to reduce this
triangulation needed, several satellites are needed. extraneous noise and to help isolate the beacon’s
The signals require line-of-sight orientation because transmission signal, which gets very weak quickly
the small wavelength signals are blocked by from the transmitter. This partially explains why so-
buildings, mountains, canyons, tree, etc. and are called analog receivers appear to have more receive
severely attenuated or limited by snow. Further- range: with analog transceivers, this filtering is done
more, a GPS receiver will tell you where you are, by the user’s ear rather than the transceiver’s
but there is substantial added technology to relay microprocessor. Consequently, the usefulness of
that information to a person searching for you. this weak signal at long range is heavily dependent
on the ability level of the user.
Another example of a far-field application is
the Recco system currently in use for locating lost This difference in receive range is due
individuals. It uses a 1.6-kg transmitter/detector to exclusively to the noise filtration process of the
bounce microwaves at 917 MHz off a special digital receiver, and has no relationship to the
reflector–a thin printed circuit card that doubles the number of antennas used in receiving the signal, as
signal frequency–that is attached to an individual’s suggested in other literature (Kroell, 2000). On the
equipment or clothing. One limitation, due to its contrary, the number of antennas actually increases
high frequency, is that the user should always have the search strip width. In the case of the Tracker
two reflectors so the body does not interfere with DTS, which uses two receiving antennas, the
the signal. search strip width is increased by a factor of 15
percent (Meier, 2000). Since search strip width
3. ANTENNA AND TRANSCEIVER LIMITATIONS defines the primary search path, not maximum
range, this has a stronger effect on the primary
Of course, the avalanche rescue trans- search time than a beacon’s maximum range.
ceiver for companion rescue needs to be a portable
product. Therefore, the antennas are electrically However, while receive range and search
small and the (battery) power available is very strip width are often perceived as an important
limited. These are two main reasons, along with the product benefit, they may have more marketing
operation in the near field, for little increased range value than technical significance. The receive range
potential at 457 kHz. of an avalanche beacon has no significant effect on
the speed of a search or the probability of a live
For optimum antenna size, its circumfer- recovery – and can actually prolong the search
ence or equivalent height should be one half of a when performed by recreationists (Atkins, 1999). On
wavelength (λ/2), or 327 meters at 457 kHz. There- the periphery of an analog beacon's receive range,
fore, the avalanche beacon antenna is a very small the searcher must cover a relatively large distance
portion of the wavelength. There are things that can before making a determination on signal strength
be done to increase the effective height of the loop and direction. For recreationists, this can be ex-
antenna, such as adding a ferrite core and increas- tremely time consuming, resulting in unnecessary
ing the number of turns of the wire, but the effi- backtracking and signal interpretation. For this user
ciency is still less than 0.1 percent, and this is a group, it might very well be less time consuming to
limitation with both the transmitter and the receiver. continue with the primary search until the signal
data can be presented with enough resolution to
Transmission power for a beacon is less make quick decisions. This is where the signal-to-
than 0.1 Watts. Compare this to AM radio stations,
systems can be seen as a major benefit: it elimi- avalanche transceivers. However, trying to stan-
nates the “gray area” which can frustrate novice dardize or explicitly define how the beacon should
analog beacon users at longer range. operate is counter-productive. User inferface issues
are most efficiently addressed in the marketplace,
4. ANALOG VS. DIGITAL TECHNOLOGY based on the needs and wants of the consumer.

A better term for analog beacons would be There is no one international standard. The
“audible-based.” The human ear is a powerful signal European standard is ETS 300 718 (currently
detector out of noise. An example of this is that, in undergoing revision), with the EN 282 standard still
a noisy room it is possible to detect and hear a being used in some cases. The only standard for
known voice. It is difficult for a digital signal avalanche beacons in the United States is set by
processing system in the room to detect, recognize, the American Society for Testing and Materials
and isolate the speaker, especially if the voice is as (ASTM F1491-93); it sets only the frequency at
loosely defined as it is by the present international 457.0 kHz, with no other requirements.
standards for an avalanche beacon transmitter. For
example, the present broad standard for the on- and Standards should be modernized so that
off- time may tell a listener or receiver that the the signal is better defined to allow better digital
speaker in the room is feminine, but a tighter signal processing and isolation. Also, product
definition would better describe the transmitter’s design is challenged by direct tradeoffs between
specific speaking characteristics to allow isolation traditional wants and assumptions, “feature bloat,”
of a specific person. and simplicity. For example, a standard that
required a minimum receive range or search strip
The greater perceived range of the audible- width might suit the needs of the snow safety
based transceiver is not due to better design or professional, but would be counter-productive for the
necessarily better signal-to-noise ratio, but due to recreational consumer, who generally does not have
the power of the human ear. But the human ear is a the skills required to make use of a weak signal at
very poor judge of loudness (volume) changes. That longer range. These conflicts should not be ad-
is why it is difficult to determine the direction of a dressed in the standards, but the product developer
transmitter based on audio level changes, espe- and (ultimately) the consumer are best suited to
cially at low signal levels and especially among non- determine the best device at the lowest cost.
professional users. However, the ear can recognize
very fine changes in pitch. 5. HIGH FREQUENCY AND ID LOCATOR

A “digital” beacon can take several forms, We propose to significantly improve beacon
but basically it takes the Radio Frequency signal operation by adding a higher frequency signal to this
that has been filtered, mixed, and amplified using 457 kHz carrier. With digital technology, this is now
analog technology and then digitizes this to allow a more feasible than in the past. This would increase
microprocessor to process it. This provides for the detection range and would allow giving each
many advantages, such as determination of direc- transmitter a unique identifier (ID) so that multiple
tion (from a dual antenna system), distance calcula- victims can be even better isolated and located.
tion, audio interface improvements (such as pitch
variation), improved algorithms for signal detection, Since there is more power explicitly in a
multiple transmitter isolation and location, automatic higher frequency, this would increase the detection
sensitivity adjustment, digital filter implementation, range, but without the inherent limitations described
and other user interface improvements. above regarding the (non)usability of a weak signal
in the near field by the recreationist. Since the
5. STANDARDS operating range would be in the far field, the trans-
mitter could be seen as a point source, initial
Beacon development is not just limited by detection would “point” in that direction, antenna
electronic technology, but also by down-level systems could be more optimally designed, and
standards that do not define the signal characteris- there would be less effect from atmospheric noise.
tics very well, specifically on- and off-times of the Finally, this higher frequency signal would allow
457 kHz carrier. Modernizing these standards could giving each transmitter a unique identifier so that
significantly improve the future performance of
tims could be even better isolated and located. Of
course, this frequency would have to be carefully
selected based on 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 newly designed
transceiver to detect an “older” transmitter. The
higher frequency would “ride” on the 457 kHz signal
much like DSL or ISDN data rides on an analog
telephone line. The 457 kHz signal would still be
used for fine and pinpoint searching in the near field.

5. CONCLUSION

Avalanche beacon design has improved

markedly in the past three years, but progress has
been limited by the issues stated above. Con-
straints for future development are not just limited
by technology, but by poorly defined standards for
the signal and by the need for downward compatibil-
ity with existing beacons. Professional use is an
important aspect of transceiver design, but one
main goal should be to make effective avalanche
rescue transceivers accessible to as many users of
the backcountry as possible, especially those who
are most at risk: recreationists. By leaving user
interface issues up to the designers and allowing for
a higher frequency in addition to the current 457 kHz
standard, transceiver technology could see even
greater improvements than the present, yet maintain
downward compatibility with the transceivers of the
past.

APPENDIX:

REFERENCES

Atkins, D., 1999. Companion Rescue and Avalanche

Transceivers: The U.S. Experience. AAAP Ava-
lanche Review, 1999.
Meier, F., 2000. Determining the Search Strip Width
for Avalanche Beacons. ISSW Proceedings.
Kroell, F., 2000. Avalanche Transceivers: Uses,
Limitations, and Standards. ISSW Proceedings.