Advice for wearable bluetooth antenna

[tonyh]

Hello,

We are currently developing a second version of a wearable BLE device and would like some advice about the antenna design. Our device is meant to be worn in the middle of the chest and this gives a lot of challenges for connectivity. We have experienced a lot of connection issues from the human body absorbing a lot of the radio waves and would like advice on how we should design our antenna. For a ballpark estimate we are hoping for around 200Kbps at 20 feet in all directions (especially when the human body is between our device and the phone). Previously we have used a trace antenna but since our board needs to be directly contacting the skin we weren't able to get a good connection with it. We are not sure if we should go with a chip antenna or put a coax port and attach a FPC antenna (we can place it max 15mm away from the skin in our enclosure). We are also looking into adding a nRF21540 which we are hoping will also increase the range. Any advice or feedback on if this is realistic will be much appreciated. 

Thank you for your help.

[ketiljo]

The body is obviously going to shield the signal pretty good and there's no way around that. The signal will reflect and bounce off the ground, walls and objects near by so it will find it's way around the body. But outdoors you usually don't have other reflections than from the ground. 

For the antenna, you need an antenna that's either far enough from the body (>10 mm) so it's not affected or an antenna that's designed to be placed on the ground plane so that the ground plane will shield the antenna from the body, something like this: https://www.johansontechnology.com/datasheets/2450AT42E0100/2450AT42E0100.pdf

Using a FEM like the nRF21540 will have impact on the battery lifetime.

[tonyh]

In your reference designs you use the 2450AT18D0100 from Johanson and when I asked for advice from them they also suggested this chip. Do you think an antenna on top of a ground plane will have better performance for our device's unique location on the body?

[ketiljo]

With this antenna, I belive you still need to have some distance to the body. Did they say anything about that? The efficiency is lower with the on-ground antenna so that may be why they suggested another one. I'd ask them for advice.