5 things you need to know about antennas for wearable devices
18-12-2015 | By Marco Enge
The body is the enemy of wearable devices
Wearable devices such as smart watches, smart glasses and fitness monitors present a specific set of challenges for antennas. Firstly, they are by definition held very close to the body at all times, which poses a problem. Human tissue is very lossy, which changes the frequency response of the antenna and degrades its radiation efficiency.
A wearable antenna therefore has to be immune to the near field effects of the body in order to avoid detuning. Maximising the distance between the antenna and the body can help, as can including a ground plane to isolate the antenna from the body.
As well as the body’s effects on the antenna, the antenna’s effects on the body should be considered. The FCC limits on radiated energy for partial-body exposure should be taken into account; this is currently a specific absorption rate of 1.6 Watts per kilogram of body tissue.
Antenna performance can drastically affect battery life
Battery life is important for wearable devices, but how can antenna design help? The energy consumed during RF transmission can use up a high proportion of the device’s power budget. A well-designed antenna subsystem can save power by not being sensitive to the body’s near field, by decreasing the number of retransmissions and by having high gain or pattern diversity.
Building in a level of reconfigurability may be an additional solution to fine tune the antenna’s performance for a given situation, since the optimum radiation pattern changes based on how the device is used.
Wireless modules are the easy option. Or are they?
For miniature designs like wearables, it is often quicker to use pre-certified wireless modules for connectivity; the design work is done for you and you know you have a reliable wireless subsystem for your design that’s already been optimised for size and power efficiency. Using modules also means you have more time to focus on differentiating your product by concentrating on the other features. Typically these modules will come with a chip antenna already integrated into the design.
On the other hand, the main concern with using existing modules is the antenna performance can change depending on how the module is positioned in the wearable device. For best results check with the manufacturer what the optimum position of the antenna is, but it’s usually safest to place it as close to the edge of the system as possible, such as in a corner, and as far from the body as possible. If it’s buried in the centre of the design, the performance will be negatively affected since proximity to PCB traces, other components, or even plastic (which can act as a dielectric) can de-tune the antenna to lower frequencies.
Custom antennas can help differentiate – but only if done right!
Rather than use standard chip antennas, some manufacturers are working on custom antennas to tackle the challenges wearable devices pose, such as body proximity, form factor, and metal cases. There are pros and cons to this approach; it means that the design of the whole device isn’t centred around the performance of the standard chip antenna, and it can result in better, more reliable connectivity which make for a better overall user experience (if done right!). On the other hand, it can add a lot of design time and cost to a device in a market where time to market and price point are both crucial.
Use a mobile phone’s flexible antenna for really tiny spaces
One solution that’s widely used in the mobile space which could solve problems in wearables is flexible antenna technology. Borrowing the flexible plastic substrate materials already used in flexible PCBs, a metal trace can be printed in various patterns to create the different antenna topologies. This flexible antenna is typically stuck to the inside of the case with glue or double sided tape, occupying a very minimal amount of space, and their designs can be completely customised, even for multi-band antennas. In the future, printing antenna patterns directly onto the inside of the case may also be possible.
Other emerging technologies include conductive textiles, currently a hot topic in the wearable antenna industry. Textile antenna ideas have traditionally faced several challenges; firstly, maintaining output stability when twisted or folded, secondly, surviving the washing machine, and thirdly, meeting consumers’ aesthetic expectations. These challenges are not insignificant. Although many suppliers are researching the area, it may be a while before we see commercial products.
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