How does an avalanche beacon work?

A beacon isn’t a locator in the GPS sense. It’s a low-frequency radio doing one of two things at any moment: shouting, or listening. Everything else on the spec sheet — antenna count, range, marking, Bluetooth — describes how well it does the listening half.

What follows is the mechanism and the numbers, not training. A beacon is one part of a beacon-probe-shovel set, and using that set under time pressure is a trained skill taught in person. Nothing here substitutes for a course.

What does an avalanche beacon transmit?

Every certified avalanche beacon transmits a pulsed signal at 457 kHz, and the frequency is set by standard rather than chosen by the manufacturer. ETSI EN 300 718-1 defines the band as 456.9 kHz to 457.1 kHz, allows a frequency error of no more than ±80 Hz, and pins down the pulse itself: on time of at least 70 ms, off time of at least 400 ms, and a period of 1000 ms ±300 ms. The standard also describes the device as containing “in one unit at least a transmitter/receiver including antenna and battery.”

That is why brand doesn’t gate compatibility. A Mammut unit receives a BCA unit, and a Pieps unit receives a Black Diamond unit, because all four are certified to the same narrow slice of spectrum. It’s the opposite of the fit problem in crampon and boot compatibility, where the pairing decides whether the two parts work together.

Worth being clear about what the standard covers: radio behavior, environmental sealing, spectrum access. Ortovox lists “Waterproof (EN 300718)” on the Diract Voice spec sheet. It does not certify how quickly a given search interface gets a user to the right spot — a distinction that also separates the tested condition from the field condition in climbing helmet standards.

What’s the difference between transmit mode and search mode?

Transmit mode broadcasts the 457 kHz pulse; search mode reassigns the same antennas to receiving and processing other units’ pulses. A beacon spends nearly all its life in transmit and is only switched to search by someone who has stopped being a transmitter themselves. That trade is the whole design tension: a unit in search mode is no longer findable, which is why current models from Pieps and Ortovox list an automatic switchover that returns a stationary searching unit to send if a second avalanche runs.

In search, the unit compares the signal picked up on each antenna and turns the comparison into two outputs on the display: a direction indicator and a distance figure.

What does the third antenna actually fix?

A third antenna fixes close-range accuracy, not range. With one or two antennas, the strongest received signal can appear at a spot that isn’t above the transmitter — a spike, which beacon range testing describes as occurring “when the strongest signal is up to two meters away from the transmitting beacon.” A vertically mounted third antenna gives the receiver coverage “in three dimensions (i.e., forward/back, left/right, and up/down)” and greatly reduces or eliminates it. Arva puts the same point in one line: the “third or Z antenna provides depth information during the fine search to avoid the multiple spike effect,” per its transceiver selection guide.

The effect is invisible on a shallow test burial and obvious on a deep one, which is why shop-floor demonstrations rarely show it.

Don’t confuse the third receive antenna with the auto-switching on the send side. Pieps describes an “Auto Antenna Switch” that “always uses the antenna with the strongest signal,” and Black Diamond’s Guide BT lists an “auto antenna switch that automatically adjusts to send the best signal.” Those features exist because a buried unit’s orientation changes how loudly it reaches a searcher — which leads straight to the range number.

What does the advertised range number mean?

Advertised maximum range is a best-case figure, and the “search strip width” printed beside it is not defined the same way by every manufacturer. Mammut’s Barryvox reference manual is blunt about the shape of the problem: “The 457 kHz transmitter of the transceiver has a kidney shaped transmit distribution.” A receiver’s reach into that lobe depends on how the buried antenna happens to be lying. Range testing found that with a single-antenna unit “positioned 90° to the transmitting transceiver (i.e., perpendicular), the reception range decreases significantly,” and that beacons “tend to overstate the distance (when at their maximum range) by about ten percent.”

Model	Published max range	Published search strip width	Battery specified
BCA Tracker4	50 m	55 m	3 × AAA alkaline
Mammut Barryvox S	70 m digital	70 m digital, 100 m analog	3 × AAA alkaline or lithium
Pieps Pro IPS	70 m	80 m	3 × AAA, 600 h alkaline / 800 h lithium
Pieps Powder BT	60 m	60 m circular	3 × AAA, min. 300 h
Ortovox Diract Voice	not published	max. 50 m	built-in rechargeable lithium-ion

Read that table as a warning about vocabulary, not a ranking. Three of these makers publish a search strip width equal to or larger than their own maximum range, Ortovox publishes a search strip width and no separate maximum range figure at all, and Arva defines the term a third way — “the search bandwidth is twice the useful range,” so a 30 m useful range becomes a 60 m strip. Cross-brand range comparison from marketing copy is therefore close to meaningless. What the numbers are good for is reading one manufacturer’s own model line against itself.

How far do phones and radios have to be from a beacon?

Keep interference sources at least 20 cm from a beacon in send mode and at least 50 cm from a beacon in search mode. BCA states it as “50 cm (20 inches) of your Tracker while performing a transceiver search” and “at least 20 cm (8 inches) from other electronic equipment or large metal objects” in transmit. Mammut publishes the identical pair — “20 cm in SEND; 50 cm in SEARCH” — and extends it to “metal objects (pocket knives, magnetic buttons), or other transceivers.” Black Diamond’s lab writeup calls it the 20/50 rule and notes it isn’t absolute, since some powerful devices interfere beyond 50 cm.

So the published distances agree; what differs is scope. BCA names large metal objects at 20 cm, Mammut adds other transceivers, and Pieps sells an “Interference Protection System” it says “compensates for the influence of electronic and metallic interference” — a hardware claim its competitors don’t make under that name. The ISSW 2013 recommendation behind the rule also puts a ceiling on the effect: range reductions of more than 30% are “seldom.”

Radios matter here because they’re carried on the chest, exactly where a beacon lives. If you run a Rocky Talkie or any other handheld, the 50 cm figure applies the moment someone switches to search. BCA’s own writeup on electronic noise also flags headlamps, GoPros, keyless entry fobs and snowmobiles, and recommends running a headlamp on high rather than a pulsing lower setting.

What batteries can go in an avalanche beacon?

The battery rule is manufacturer-specific and the manufacturers do not agree. BCA’s Tracker manual says “Use only alkaline batteries of identical age and brand. Do not use rechargeable, lithium, Oxyride, PowerPix, or any other non-alkaline battery.” Mammut’s Barryvox S lists “Alkaline and lithium 3 x AAA 1.5 volt” and quotes 300 hours in send on alkaline against 550 on lithium, while the standard Barryvox is alkaline only. Both Mammut manuals rule out rechargeables. Pieps rates the Pro IPS at 600 hours alkaline and 800 lithium. Ortovox went the other way entirely and built a rechargeable lithium-ion cell into the Diract Voice.

One rule is common to all of them: three new cells of the same type, replaced together, never mixed brands or ages.

What does a group check confirm?

A group check confirms that every unit in the party transmits and that at least one unit receives, before anyone commits to terrain. Mammut’s manual describes the leader switching from off to send and pressing the button within the first seconds to enter group check, with participants staying in send; the check passes when the leader hears each participant’s beacon within the range shown on the display. BCA’s manual describes the same idea by number — the leader approaches each member “until each member’s distance reading is less than 0.3 meters.”

It’s a device function test. It tells you the electronics work; it says nothing about the snowpack.

The short version

Every certified beacon speaks 457 kHz because EN 300 718 makes it, which is why interoperability isn’t a purchase decision. Antenna count matters at close range: two antennas give direction, and a third fixes the close-range spike that otherwise puts the strongest reading up to two meters off. Advertised range is a favorable-orientation maximum and “search strip width” means different things at different companies, so compare figures within a brand rather than across brands. Keep powered gear 20 cm away in send and 50 cm away in search, and use exactly the battery type the manufacturer prints. The beacon is one third of the kit and none of the training.