Avalanche Transceivers 101: Understanding Function

What is an avalanche transceiver?

An avalanche transceiver is an electromagnetic radio signal emitter and receiver. The radio waves they emit have a 457 kHz frequency, which is inaudible and a few hundred kHz below your typical AM radio signal. Think as a beacon as a super low power radio station that plays some music. When switched from “Send Mode” to “Search Mode”, it goes from being the radio station antenna to the radio in your car, expect at a much much smaller scale. 

Why a 457 kHz frequency?

Radio waves are undulating with a certain frequency (measured in Hertz). The bigger the frequency, the more the wave undulates. For example, a GPS uses a very high frequency, which produces a super short wave length (let’s say 20 cm). In comparison, a single wavelength at 457 kHz is 656 meter long. Short wavelengths (like your GPS) are easily blocked by buildings, mountains, vegetation, etc. and snow. For this reason, a longer, less undulating wavelength is much better at passing through objects. As a result, an avalanche transceiver signal does not bounce or reflect off objects in the backcountry. This allows for a victim to be found under the snow. 

On a 457 kHz frequency, an avalanche transceiver works in a “near field” situation. This means that below 100 m distances, the signal transmitted is predominantly magnetic since the wavelength is so long. This leads to a complex signal that has a curved magnetic field pattern. The variation of this circular magnetic signal depending on distance, along with complex mathematical analysis, is what allows your beacon, in search (receiving) mode, to locate victims and display approximate distances. 

A disadvantage of the 457 kHz frequency is that “noise”, or interference is high in this frequency. For avalanche transceivers, this means that extensive filtering needs to be done to isolate a transmission signal.

Why aren’t transceivers more powerful? Simply because they’d need huge antennas (optimum antenna size at 475 kHz would require a 300 meter magnetic coil) and a lot more electricity to produce more powerful signals. At the end of the day, it needs to be portable and convenient for backcountry skiers. This greatly limits the antenna and battery size. 

Analog vs Digital

An analog beacon is a much more simple device than a digital beacon. Think about the car radio. An analog beacon is like an AM radio with a dial that is fixed at a 475 kHz frequency. When it captures a radio signal (magnetic in the case of a beacon), it filters and amplifies it before broadcasting at an audible frequency. With you car radio, you can sometimes hear partial sounds of radio stations when the station’s antenna is very far away. An analog beacon works in a similar way. It’s up to the human ear to listen to the amplified signal and follow it in the right direction based on the strength of the broadcast. Analog beacons achieve the greatest search range because the “listening” is done by the human ear. But keep in mind that a weak signal at a long range needs a well-trained user. This is where digital beacons simplify everything. 

A digital beacon takes that signal, filters it and amplifies it just like an analog beacon. The signal is then digitized to allow for a microprocessor to run algorithms based on the signal strength, shape and direction. The result is an audible and visual display that includes a direction and a distance to the victim. All current beacons are digital in order to offer a much more user-friendly product that doesn’t require as much training to be used properly.

What’s the deal with having multiple antennas?

A transceiver antenna only emits in one plane. This is why, in order to calculate the distance and the direction of a victim, a digital beacon needs to triangulate based on the flux pattern of two antennas. The main two antennas are “X” and “Y” and they are orientated perpendicularly inside your transceiver like an “L”. These two antennas are used for coarse search, but as you get closer to an emitting beacon, the signal of its antennas become tangled and cause bursts of erratic signals, called spikes. This is especially true in deep burial situations. In those situations, the strongest signal won’t be directly on top of the victim because of the circular flux lines that the buried transceiver emits. In order to solve this problem, modern avalanche transceivers have a third, vertically mounted, “Z” antenna that only turns on when the beacon gets closer to the signal it is receiving. Calculations in all three dimensions help greatly with locating a victim with more accuracy.

An internal view of an avalanche transceiver, with the X and Y antennas visible

Why does search range vary?

For an avalanche beacon in “Search Mode” to locate a victim, its antennas need to achieve some coupling with the signal broadcasted by the transceiver in “Send Mode”. As a result, the transceiver in “Search Mode” won’t necessarily capture a good signal from as far away if both beacons are in different positions. The optimal orientation for transceiver search to work at its best is if the antennas of the receiving and the emitting beacon are aligned together. On the contrary, if the three antennas of the “Send” transceiver are all perpendicular to the receiving one, the coupling will be greatly reduced. This explains the variability in search range. Most beacons do not achieve full “advertised range” performances even in optimal orientation due to various reasons. 

Various coupling positions of transmitters and receivers. (1) Good (2) bad and (3) worst coupling position. Rx = Receiver – with three active antennae, Tx =
transmitter – has only one active antenna.