Lesson 3: Transceiver Interference

AvSAR responders carry a wide variety of metallic and electronic devices that can interfere with an avalanche transceiver. Rescuers must be aware of the potential for transceiver interferences, and may have to take steps to overcome its effects in a rescue situation. These may include reducing search-strip widths in the coarse-search phase, and turning off all electronic devices while conducting the transceiver search.

Interference Effects

Metal parts near the transmitter may block the signal or detune antennas, and active electronic devices may cause background noise and/or random signal readings. The problems caused by interference depend on the type, mass, and size of metal parts, and the amplitude and frequency spectrum of electronic devices. Many interference effects increase exponentially the closer the transceiver is to the interfering device or object. For practical purposes, this means that most interference effects can be minimized by keeping electronic devices and metallic objects at a specified minimum distance from the transceiver.

The effects of interference on a transceiver may include:

Reduced transmit range (up to 30% when in less than 20 cm to the transceiver).
Reduced receiver range.
Increased power consumption of the receiver.
Misleading distance and direction readings (“ghost signals”).
Detuning of antennas.
Permanent magnetization of antennas; some transceivers have magnetic ON/OFF switches that may be affected by the magnetic field of other devices.
Increased background noise, making it harder to distinguish between multiple signals.

Managing Interference

Recommendations in Transmit Mode.

The transceiver should be kept at a minimum distance of 20 cm from each operating electronic unit.
Position the transceiver on the opposite side of the body from other devices.
Video cameras should be worn on the helmet, not on the chest, and any wireless controls must be in a pocket at least 20 cm from the transceiver.

Recommendations in Receive Mode.

All electronic and metal devices should be kept at a minimum distance of 50cm from the transceiver, and all electronic devices should be turned completely OFF.
In a two-rescuer companion-rescue scenario, one rescuer should call for help while the other searches with his electronics turned off before the caller comes to assist.
In a one-rescuer scenario, the lone rescuer should complete a quick transceiver search with communication electronics off, and then call for help once the transceiver search has been completed.
If it is not practical to turn off digital radios, then radio transmissions should be kept to a minimum while the transceiver search phase is ongoing.
Analog-only radios may be left on, but transmissions should be kept to a minimum and the speaker should not point it directly at the transceiver.
If the source of interference cannot be turned off (powerlines, antenna towers, buildings), it may help to switch to analog mode with a manual volume control, and use a narrower search-strip width.