Making RF connections for wearables

By Nick Wood, Sales & Marketing Director, Insight SiP

This is logical for an all-purpose design, but not for wearables, where the RF component is near the body, a strong absorber of microwave radiation, and space is at a premium. This article examines the challenges of RF wearable design and how to overcome them.

Use-case analysis

The first action is to analyse and define the use-case and the overall solution architecture:

• What will the wearable communicate with and when?
• What throughput/data rate is required?
• Is near continuous connection required, or in case of interrupt, is ‘store and forward’ or a simple break in data continuity acceptable?

From this, one can define the worst-case scenario to be supported – in terms of the position of the wearable and the device it connects to and any possible obstruction including the body of the wearer and the data rate necessary to achieve the usage objective. If one can assume direct line of sight between wearable and receiver, the
problem is much simpler than if not.

Wearable devices typically use Bluetooth to communicate, although they can include other radios. For this article we assume we are using a Bluetooth type radio. However, the RF issues are the same for other 2.4GHz protocols such as ANT+, Wi-Fi, or proprietary ones, and similar at 5Ghz.

The 2.4GHz frequency of Bluetooth is one easily absorbed by the human body, having the properties of water. Our studies show that a device held one side of the body will be attenuated by 60 to 80dB relative to a device on the other side, emphasising that the positioning of the wearable and receiving device is crucial.

Physical design choices

The key physical design issues for a device from a radio perspective are:

• Materials used in the housing
• Battery placement
• Placement of the radio component and PCB within the device
• Antenna choice/design

Technical considerations may conflict with aesthetic ones. For materials, the ideal is some RF transparent material such as polymeric plastic e.g. ABS, Perspex. Conversely conductive metal will be a highly negative choice. It is possible to use some metal, with an ‘RF window’ for radiation to escape, but performance will be degraded.

The battery is important for the RF design and is typically metal cased. Ideally, it would be placed to the side of the PCB containing the RF component, with the antenna separated as far as possible. As such the battery can form part of the ground plane for the RF solution.

Underneath the RF component is far less favourable and should be separated vertically by as much distance as possible.

The PCB placement is also critical due to the body’s absorption of RF radiation. Achieving the maximum distance between the body and the RF component, specifically the antenna, is crucial.

A separation of ¼ – which at 2.4GHz equates to 3 x 108 / 4 x 2.4 x 109 = ~ 3cm would be optimum. This might not be possible but nevertheless our extensive simulations show that at 2mm spacing from the body the antenna gain away from the body is close to -2dBi, increasing to 0dBi at 3 to 4mm.

Finally, antenna choice is vital. Antenna performance will be enhanced with a large antenna, whilst wearables aim to be small and convenient to wear.

There are three main options for the antenna, listed in increasing order of complexity:

• Use a module with integrated antenna
• Connect an antenna part to the RFcircuit
• Design a custom antenna as part of the wearable

The first option is the easiest, minimising design effort and risk. It also – for a certified module – removes the requirement to engage in lengthy and expensive certification efforts such as CE, FCC, and Bluetooth SiG.

We only recommend either of the other two options for experienced RF designers. An RF reference design may look relatively easy. In reality it is easy to get wrong. With wearables, where one is already battling a difficult environment, it may result in a poorly functioning device, or multiple development cycles, and problems with certification.

The possible upside of a custom antenna design is that it can be optimised with respect to the overall design of the device, provided the design can be truly optimised. This is not a process with straightforward rules and is best considered when performance is critical. It requires skilled RF designers working as a key part of the design
team and can be a time-consuming and costly exercise.

For most scenarios, there are miniature solutions available, small enough to work in most wearables and give good performance, difficult to improve on.

RF component placement

The problem is then reduced to placement of the RF component. Normally the module manufacturer will recommend a ‘keep out’ zone around the antenna portion. However, this might be hard to achieve in a space-constrained wearable. Again, there is a trade off between size and performance.

Typically, any small antenna will work best with a metal plane, called a ‘ground plane’, that has at least one dimension close to a quarter wavelength (3cm at 2.5GHz). This is often possible for devices operating in the 2.4GHz ISM band but can be difficult in the case of sub-Gigahertz communications. For the 868MHz ISM band (Zigbee, LoRa, or Sigfox) this would translate to a minimum size of 9cm, large for a wearable. Our simulations show that antenna gain and hence data throughput will decrease with decreasing ground plane dimension.

Conclusion

It is perfectly possible to design a radio-based wearable device that functions well. However, careful design is required to ensure that the performance is as good as it can be, although some degradation through proximity to the body is inevitable.

This article originally appeared in the January'25 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.