EMI Hands-and-Wrists Check:: A Worthy Addition to Your Departure Check.
A trailhead departure check is an essential step at the beginning of every backcountry outing. The goal is to make sure everyone has the gear they need, that gear is working, and that everyone in the group is on the same page about the tour plan. The transceiver check is the most important part of this process. With the explosion of electronic gadgets being used in the backcountry, BCA suggests adding a small step to your departure check to ensure that no transceivers in the group will be affected by electromagnetic interference (EMI): Add a hands-and-wrists check to identify potential sources of active EMI.
There’s a big difference between active and passive interference. Active EMI comes from electronic devices that are turned on, creating an electromagnetic field that can cause “false” distance and direction indications or a loss of receive range when the user is searching. Active interference is a bigger concern than passive interference because active interference is more likely to disrupt a search, and it is more difficult to control. Active EMI sources can include cell phones, headlamps, GPS units, cameras, two-way radios, “first-generation” electronic airbags, and the batteries in heated garments.
Passive interference can be caused by metallic objects that are on top of someone’s beacon in transmit mode, temporarily “detuning” the transmit antenna, possibly decreasing the distance at which it will be captured. It can also cause your transmit beacon to use more battery power (we’ll spare you the physics lecture). For these things to occur, the metallic object needs to be extremely close, i.e. “nested” or “spooning” with the transmit beacon. Therefore, it is easy to manage by wearing your transceiver in its harness or putting it in a different pocket than anything metallic. Passive sources can include cell phones (turned on or off), foil-wrapped items, and aluminum crystal cards.
For a passive interference source to affect your transmit signal, it basically must be “spooning” with the transmit beacon. Just a millimeter or two makes a huge difference. Wearing your transceiver in a harness—or putting it in a different pocket--should eliminate the problem.
The current rule-of-thumb for mitigating EMI is the “20/50 rule:” Make sure any metal objects are kept at least 20cm (8in) away from your beacon when in transmit mode, and that any active electronics are kept at least 50cm (20in) away from a beacon in search mode. Basically, search with your receiving beacon at arm’s length. This is good technique anyway, as it ensures you’ll be looking forward while searching, enabling you to spot surface clues. However, there are certain items that are almost always kept within 50cm of your hand, where your transceiver would be held during a search. The most common are heated gloves, smartwatches, and Oura Rings. Chest-mounted items can also be suspect, such as POV cameras and 5-watt UHF/VHF radios (2-watt FRS radios have not proven to produce significant EMI).
Active electronics should be identified and moved away from the hands and wrists during the departure check. In this photo of a heated glove, the batteries have been removed from their zippered pocket and placed on top in the corresponding position. Note how close the batteries are to the searching transceiver. Heated gloves must be turned off during a search. Smartwatches and Oura Rings should be placed on the non-searching hand. POV cameras should be removed from the chest or shoulder and placed on the helmet, if used at all.
Snowmobiles and “environmental” noise sources such as power lines, are exceptions to this rule. Turn off your snowmobile before going into search mode or get at least ten meters away. In developed areas with potentially high environmental noise, shorten up your search strips during the signal search, for example, from 40 meters to 20 meters.
The best time to incorporate a hands-and-wrists check into your departure check is during the “E” part of the well-recognized “DBEAST” process:
D = Display
B = Batteries
E = Electronics
A = Airbag and Radio
S = Search
T = Transmit
For a full run-through on how to do a departure check, including a hands-and-wrists check—see our video on “Trailhead Checks and EMI” in Season 2 of BCA’s [EDGE]ucation series, featuring BCA co-founder Bruce “Edge” Edgerly, Sales Manager Andy Wenberg, and Snowmobile Ambassador Mike Duffy.