Magnetic Fields for Wireless Communication

An article Magnets attracting wireless attention on CNET News.Com succeeded in grabbing my attention. It reminded me of some experiments I did forty years ago broadcasting music around my parent's house using a large loop of wire fed by an audio amplifier and a modified transistor radio with a smaller loop driving the input of the radio's audio amplifier. I found I could transmit audio from floor to floor but the range was 10-15 feet at most. I've also seen magnetic loops wrapped around a studio used to transmit script dialogue to very small earpieces worn by actors in soap operas. The most common use of magnetic fields for communication today may be to transmit audio from a loop worn around the neck connected to an IFB receiver to a small earpiece. My magnetic field transmitter and, I suppose, the rest of these applications had their share of interference from the 60 Hz AC wiring.

The News.Com article discusses devices made by Aura Communications. Diving deep into the Aura Communications Web site, I found a datasheet that explains how Aura is getting around these problems. As suspected, the solution is a lot more complex than the simple inductive loops. The Aura Communications LibertyLink technology modulates the magnetic field with a 13.556 MHz (a frequency authorized for unlicensed ISM use) Gaussian Minimum Shift Keyed (GMSK) signal. Magnetic fields from a single coil have directionality similar to that of a dipole, so some sort of diversity is needed if the signal is to be received in any antenna position. Aura uses a tri-orthogonal diversity antenna array with "continuous three-dimensional spatial tracking" to eliminate nulls.

This magnet wireless system won't work for long range applications, which Aura Communications says is an advantage in a world of increasing RF congestion. The 2.4 GHz band used by Bluetooth, for example, shares spectrum with wireless LANs, cordless phones and nearby microwave ovens. The LibertyLink is designed for a range of 1.25 meters (ideal for those IFB to earphone links!) and provides a channel data rate of 204.8 kbps. Since magnetic fields are not propagated like RF waves and pass easily through the body, signal blockage or multipath fading is not an issue.

One of the practical applications for Aura's technology is a cord-free headset adapter. It will be available later this year from foneGEAR LLC at a suggested price of $59-$79, depending on the retailer. The News.Com article said the Department of Defense is using magnetic links to transmit images from a rifle-mounted camera to the soldier's helmet worn monitor. This allows them to poke the rifle around a corner or out of a foxhole and see what is there without exposing themselves to enemy fire.

Basic information on magnetic communication is available in the Aura Communications technical paper Near-Field Magnetic Communication Properties.