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Inside the world of designer antennas for wearables

Matt Hamblen | May 14, 2014
In some cases, custom antenna developers must account for how fat and skin affects a Bluetooth signal.

Sixty years after bulky "rabbit ears" TV antennas, engineers are designing tiny custom ones that can fit inside of wearable devices.

Some of the wearables will be implanted inside the human body for medical purposes, posing challenges for antennas that carry a Bluetooth wireless signal through skin, muscle and bone to reach out to a smartphone or other device. Other antennas will run in smartwatches or even through entire sets of clothing, allowing the them to stretch over a greater distance.

"When wearables started appearing three years ago, we started exploring how we can cram the radios inside to make them user friendly," said Arun Venkat, principal antenna engineer at Cambridge Consultants in Boston. The company, based in Cambridge, England, outsources research and development.

Venkat holds doctorate in electrical engineering and has helped design 18 custom antennas for wearables over the past eight years. The list of products includes some wearable tags to help the U.S. Army track its soldiers.

"The human body affects radios, which is key to making a successful device," he said in an interview. "It's important to know what the body does to the radio signal and how much range will result to have a good device."

In that sense, Venkat is as much a biomedical engineer as an antenna engineer.

If a person wears a smartwatch on his left wrist that transmits heart rate data to a smartphone attached to his right arm, designers have to take the size of the user into account, along with other factors, such as how perspiration will interfere with a radio signal.

In a recent demonstration, Cambridge showed how it will be possible to implant a device in a person's back to stimulate nerves for back pain at a greater depth than before.

When a device is implanted within skin at greater than 4 centimeters, the Bluetooth signal traveling over a 2.4 GHz pathway can fall off, making a connection to a smartphone difficult. The demonstration by Cambridge showed that a custom designed antenna design could allow signal to travel up to 2 meters when implanted at 6 centimeters under the skin. Bluetooth Low Energy signals normally travel 10 to 15 meters in an open space.

"Skin has electrical properties, both electric and magnetic, and it's important to balance the two to get a good signal out of the body," Venkat said. "It matters if the skin is wet or dry."

Through 5 mm of skin, a Bluetooth signal will lose 3 decibels on average. That compares to the loss of 6 decibels for a Bluetooth signal going through a standard concrete wall with no rebar inside, Venkat said, reciting such data as easily as his ABC's.


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