Interesting radio frequency: ankle monitors on 314.2 MHz

Now that dongle-based SDRs are exploding in popularity, why not start exploring outside the usual frequency bands? With 64 to 1700+ MHz at your disposal there’s a lot of territory to cover. You’ll find that a number of unusual services occupy some of the lesser know bands not covered by conventional radio scanners.

One example are electronic home monitoring (EHM) devices worn by persons on probation and parole. These devices (such as the one pictured above from International Patent WO2007149391) operate predominantly on 314.2 MHz. Every half hour, the bracelet captures transdermal alcohol readings by taking samples of the insensible perspiration coming off the offender’s skin. The bracelet stores this data and, at pre-determined times, transmits it to the base station via the 314.2 MHz radio link. The base then relays data back to the supervising officer via telephone.

In addition to transmitting alcohol readings, these devices also “phone home” to their base station on a constant basis to show that the wearer is in range. It has been reported that these devices transmit a 7 mSec data burst once every 15-30 seconds and that it’s best to catch them by using AM modulation settings and the widest bandwidth possible. These are very low power devices, and it’s expected they will only be received when the wearer is in close proximity. Also, a number of automotive keyless entry fobs operate on this frequency as well; their signals should be distinguishable in that they won’t exhibit the repetitive transmission characteristics of the ankle monitors.

For those unfamiliar with the concept, these monitors are attached to the ankle and are worn by the subject wherever they go, as illustrated by Lindsay Lohan. So if you follow a 314.2 MHz signal, you never know who you’ll find!

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