Inside the world of designer antennas for wearables

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.

With the human body, there are different radio signal losses for bone, fat, and muscle that have to be taken into account in antenna design.

"It's a big mishmash of signal loss in a body," Venkat said. "If you implant something in a muscle layer, the signal reflects off the fat layer before it gets out and you lose a lot of that as heat. It's such a difficult process with so many different factors."

Venkat said he's absolutely confident that wearable devices aren't radiation hazards. "Bluetooth Low Energy is so weak that it's not going to impact health," he said.

Wearables, like smartphones, have to meet Specific Absorption Rate standards for the amount of radio frequency energy absorbed, as set by the Federal Communications Commission.

Venkat and Cambridge work with clients, some of them small startups, interested in designing wearable computers amid pressure to lower manufacturing costs and the final cost to consumers. With many smartwatches now selling for $150 to $200, there's plenty of pressure to cut costs to reach to more buyers.

Many times, to keep costs down, wearables are designed with low-cost off-the-shelf antennas without the flexibility or efficiency of a custom antenna that takes advantage of the materials and design of the wearable, he said.

Custom antennas potentially raise the materials price in a wearable by 10% to 15%, he estimated. Today, a small off-the-shelf antenna made of copper or ceramic could cost from 30 cents to 60 cents when purchased in bulk of 10,000 or more units.

Venkat estimated that 90% of wearables on the market today rely on the off-the-shelf, or reference design, antennas, mostly made by smaller companies. Companies like Samsung, which already produce smartwatches and smartbands, have the capacity to design custom antennas to fit the wearables being sold.

Some wearable antennas will be printed from copper on the inside of a wearable case to make the antenna larger and more efficient for wireless transmission, he predicted. For instance, a wearable shirt computer could have an antenna running the length of the shirt.

With better antennas, other benefits can result. A fitness buff wearing a smartwatch might not need to carry the smartphone while working out nearby, taking advantage of the maximum range of Bluetooth. And runners or bikers might even be able to communicate with Bluetooth Low Energy beacons arrayed along a path.

"The antenna will start to dominate how well that sensor information gets out," he said. "If you make more efficient use of the space inside a wearable, you can maximize performance."

Matt Hamblen covers mobile and wireless, smartphones and other handhelds, and wireless networking for Computerworld. Follow Matt on Twitter at @matthamblen or subscribe to Matt's RSS feed. His email address is

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