Frequency Specifications

Early avalanche beacons transmitted at 2.275 kHz. In 1986 the international standard of 457 kHz was adopted. The Ortovox F2 (which was released in 1980) transmitted on 457 kHz but received on both 2.275 and 457 kHz. This allowed it to locate both new and old-style beacons. That was handy in the 1980s, but it doesn't mean much today.

A 1997 standard specified that beacons transmit within ±100 Hz of 457 kHz. In 2001 this allowable variance was reduced to ±80 Hz. Unfortunately, older analog beacons that used ceramic oscillators (e.g., the F1 and F2, but not the M2) can drift outside this ±80 Hz range. Newer beacons use crystal oscillators and transmit very close to 457 kHz.

The drifting usually gets worse when temperatures change. For example, a warm beacon inside your jacket may transmit near 457 kHz, but when it cools it may drift outside the ±80 Hz range.

Frequency Testing

I tested 51 avalanche transceivers using a sophisticated spectrum analyzer to see if they were within specifications. The tests included 15 different transceiver models and the frequency was tested at 70^F and 0^F to see how temperature affected the transmitted frequency. I presented the results of this testing at the International Technical Rescue Symposium (ITRS) in 2007. Suffice it to say that 49 of the 51 beacons were within range. The others were only slightly out of range (-90 Hz  and -100 Hz, and only when at 0^F). Click here to see a plot of an Ortovox M2 that is transmitting 63 Hz below 457 kHz.

During these tests I compared the results of the spectrum analyzer with the built-in frequency tester found in the Pieps DSP.

Frequency Drift and Transceiver Performance

The downsides of a beacon transmitting outside this ±80 Hz range are twofold. Most importantly, some transceivers may not receive the wayward beacon's signal at all. Secondly, the range (i.e., distance) at which the wayward beacon can be located will be significantly reduced.

This was first brought to my attention by a friend who successfully located three buried Ortovox F2s during a beacon drill in the morning. That evening he went outside to retrieve the beacons and did not receive a signal from any of the beacons! They were eventually located with a different beacon. I assume the F2s drifted off-frequency as they cooled.

I tested the distance at which four different beacons could receive a signal from an Ortovox F2 that was transmitting +200 Hz above the normal 457 kHz frequency (i.e., at 457.200 kHz). The results are shown in the following graph. The maroon bars show the distance that the beacon could receive a properly transmitting 457 kHz signal when the transceiver was held perpendicularly (i.e., in worst-case orientation). The blue bars show the distance that the beacon could receive the wayward 457.200 kHz signal. Note that I only did these tests on one occasion using these four transceivers. The results should be viewed as indicative of the problem rather than as refined numerical data.

You can see that the Pieps DSP which could receive a 457 kHz signal at 48 meters did not receive the wayward signal until 20 meters. The Barryvox 3000 received the signal at 11 meters, but only in analog mode (i.e., no direction indicator). The Tracker had the smallest percent decrease in performance (although the actual distance was less than the DSP), but the distance of 12 meters is also unacceptable. Needless to say, having an old analog signal that transmits this far out of range is dangerous.

The Pieps DSP has a feature wherein it does not normally search outside of the ±80 Hz range when first entering the search mode, but if you press the scan ("?") button it will scan ±500 Hz for several seconds. If it finds a beacon, it will lock onto it while flashing "500" in the display. When using the DSP, you also must remember that if you do not receive a signal initially, you should instigate a rescan using the scan button (doubtful on a powder day when you buddy is dying). This feature was added to the DSP in version 2.8 of their software (if you have an older DSP, you can get the software updated). Without the upgrade, the DSP does a poor job of locating out-of-frequency beacons; in word-of-mouth conversations, the old DSP software was worse than the Barryvox shown in the above table.

Both the Tracker and Pieps DSP engineers have gone to great lengths to receive signals from out-of-frequency analog beacons. The problem is not with these newer beacons, but with the off-frequency transmissions of the older beacons. It was extremely difficult for me to accept that my faithful Ortovox F1 was transmitting in an unsafe range, but I eventually accepted this fact and retired "old blue."

Pieps Frequency Tester

Version 3.1 of the Pieps software can measure the frequency of other beacons. The illustration to the right shows the DSP measuring the frequency of a Tracker. At BeaconReviews.com, we performed more than 100 tests on 51 different beacons at various temperatures and compared the results to a $20,000 spectrum analyzer--the DSP's frequency tester did an excellent job.

The DSP will then display the amount that the transmitting beacon is off. For example, if the screen displays "F020" and the arrow points to the left, it means the transmitting beacon is transmitting 20 Hz below the standard 457,000 Hz, or at 456,980 Hz. In the illustration to the right you can see that this Tracker is transmitting at exactly 457,000 Hz. As long as the number displayed is equal-to-or-less-than 080, the beacon is within specifications.

Additional Reading

Bruce Edgerly & John Hereford of Backcountry Access, Inc. (the company that makes the Tracker) wrote an excellent article on this topic. Read it here.

Somewhat related to frequency drift, the "continuous carrier" signal can confuse digital transceivers. Read about it here.

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