Antenna design for wearable devices

[Mike Wirth]

Folks,

Prototyping a wearable device using the nRF51822, so we're talking about Bluetooth LE and the 2.4 GHz band (although the protocol probably doesn't matter). Since it's wearable, size reduction is important. Currently using a chip antenna (with a discrete-part matching network, but going to a chip balun in the next rev).

Big issue is that the device will be worn next to the body, which is to first-order a bag of salt water for RF in this band. (That's why the band is available -- it's perfect for heating water in your microwave oven :-) So, rather than having a sufficient amount of free space around the antenna, as the app notes typically advise, it's essentially sitting on top of (effectively) a ground plane.

Surely this is not an unknown situation within this community. Any advice/app notes/design tools that deal with the situation? Chip (or compact PCB) antennas that are appropriate?

TIA,

Mike

PS: Thanks to Nordic for such a great part and software environment. I'm enjoying working with the ARM core and S110 stack :-) Very liberating.

[Janek Mann]

I have used the antenna Kristin mentioned in a previous project and it's a good one for wearable devices, it worked very well in our application. Perfect for very space-constrained projects where cost is no issue (it's quite expensive and there are no third-party drop-in replacements due to the unusual manufacturing process).

Otherwise I'm using PCB antennas which work ok so long as they are not too close to the body surface. If possible, you want to orientate your device so the PCB (and antenna) are as far away from the body surface as possible.