Body contact bluetooth antenna for childrens wearable project - Page 1
Forum section: electronics / rf-microwave

author: Day101
author_rank: Contributor
timestamp: October 19, 2019, 02:24:58 pm
content: I've spent the last few months working on a small wearable thats designed for children. It's in almost direct body contact and normally located underneath the child while they sleep. It's a small PCB encapsulated in silicone rubber. I wasn't aware that the human body absorbs bluetooth signals so much. I've made a prototype with an NRF52840 dongle but when the device is laid upon the received signal on my samsung galaxy s5 is almost -100db even within the same room. I'm not an RF engineer so excuse my clumsy use of terminology and general idiocy. I've been trying to decide if I made a mistake by using bluetooth if another RF frequency might be more suited to indoor, body contact situations (433Mhz, etc?)My first thought is to research body contact antennas that have better transmission range. Are there any I should look at? Has to be cheap (a few dollars max) and reasonably compact.I'm also slightly nervous that if the body absorbs bluetooth, then it's not great from a marketing stand-point.Does anyone have any advice relating to this? Are there any good compact cheap bluetooth antennas that are good for body contact situations that I should investigate?I'd prefer if the device talks direct to a phone with bluetooth since it's so ubiquitous and cheap. I was already intending to extend the range to everywhere by updating a cloud SQL database so that phones and web can see the data, even when nowhere near the child. So the user would leave an old android near the child and it relays the data by wifi or mobile data signal to the cloud SQL database.But an alternative I could try are the cheap 433mhz transmitters that I have in my drawer. (thesehttps://www.ebay.co.uk/itm/5x-433Mhz-RF-transmitter-and-receiver-kit-Module-Arduino-ARM-WL-MCU-Raspberry-VS/303275080545) But I'm not sure if those are universally approved from a regulatory perspective, and I'm not sure if they work better than bluetooth. I've got extremely low bandwidth requirements. It only really needs to send ~10 bytes every minute. Obviously adding a 433mhz relay device also increases my TODO list significantly.The primary environment is the child is lying on the device. The electronics are in a few millimetres of silicone rubber. They're in the house or maybe outdoors. The worst case is when they're lying on the device so that's what I'm focusing on.I feel completely stupid to not realise that I would have such range problems. I didn't realise bluetooth absorption wasTHISbad. My previous prototypes had the transmitter on a cord that sat outside the body area so I was getting the signals from the other end of the house. But putting an antenna on a cord isn't easy to incorporate with the other requirements of the sensors involved.Any help or advice is appreciated.TIA-Jason

author: Grandchuck
author_rank: Super Contributor
timestamp: October 19, 2019, 08:27:23 pm
content: https://sites.tufts.edu/eeseniordesignhandbook/files/2018/05/Nguyen-EJ-edit.pdfThe above is what I found that seems relevant.

author: E Kafeman
author_rank: Regular Contributor
timestamp: October 19, 2019, 08:49:03 pm
content: QuoteNo. There are no prefabricated antennas that are both good and cheap, especilay if worn close to body.For reasonable good antenna efficiency for your product is it a kind of antenna that is designed for each product.Cost is for the design job. The antenna is in similar cases mostly a part of PCB structure => low production cost.QuoteAny 2.4 GHz, when pressed close to body will have reduced functionality. Indoor is that a minor problem and outdoor can it in worst case be almost no wireless range at all.Laying on antenna in a indoor bed is a less problem due to that if antenna is correct designed will radiate 50% of its power below bed without problem and multipath due to room reflections does then still create a decent room coverage.Different types of small patch-antennas are well known and often used in similar cases but selected antenna type is much what size area that can be allowed in your design.If checking existing bed-wetting alarms you will find that PIFA is also a relative common antenna. It is a simple antenna to design but it require a more controlled antenna placement to work well under different conditions. A common problem is, if available space is small can it can be problem with keeping antenna shape correctly tuned while still flexible if built in soft material.An antenna must have some height above body to be able to radiate well. For antenna patch or IFA can height be as low as 2-3 mm, molded inside silicone. Type of silicone affect antenna by its dielectric constant so that is also a part of antenna design as well that antenna often must be stable even when it is a bit flexible.You can also search for antennas for body worn textiles. What types of antennas that works well with low height above body is very similar ti your problem.If you need an antenna that is optimized relative your product space limitations can I provide commercial contacts in Sweden or Norway, with previous experience with this kind of products and using by me designed body worn antennas. Can not offer any direct help as I am too booked already.433 MHz is a less practical frequency as antenna size and height is 5 times as big for same antenna efficiency and with much less room-multipath=> harder to get even room coverage. Body absorption is still very high at this frequency as well. Advantage is that outdoor coverage can be expected longer for same TX power as if 2.4 GHz.433 MHz is else a good selection in Europe for this kind of transmission of short messages but for simpler worldwide compatibility is often 868 MHz a better alternative as similar or same antenna and same radio often can be used in US (902 MHz to 928 MHz) by minor software changes.For these frequencies must you then also provide customer an dedicated receiver instead of an common available cellphone and a software app.It is a bit similar situation for many wireless baby-sitters. A few of them does also provide a dedicated receiver, which then allows them to use DECT and long coverage distance but most of them are using 2.4GHz and are assuming that an cellphone can be used as receiver.QuoteBody will absorb around 50% of all transmitted power for a small antenna of type PIFA. A bit depending on situation but basically is 50% of antenna nearfield radiation sphere covered by body as controlled radiation direction not is possible for a small antenna. Some patch-antennas can have a very small back loob but have other drawbacks, besides need of a relative big antenna area.Absorbed amount of heat energy if BLE is however very low. Can be compared with a lit candle 20 meters away in same room when transmitting and if not transmitting can it be compared with same candle, if not lit.

author: Day101
author_rank: Contributor
timestamp: October 20, 2019, 06:21:06 pm
content: Thank you, you've saved me a lot of wasted effort!I spent a few hours reading papers on wearable antennas and I grasped the basics, enough to know it's not an easy thing for me to tackle. I'd have to start spending a lot of money to get the right help. I think my best path right now is to accept the range limitations and do the code for the cloud SQL database. That should give an easy(ish) solution that adds some extra features.For the hardware, I'll stick with my plan to use an off-the-shelf compact module (https://www.insightsip.com/products/bluetooth-smart-modules/isp1807). I can't do everything in version 1 or I will never get anything finished.If I find the range is complete garbage then I'll attempt to use some PIFA antenna or similar. And I'll come to you for your professional assistance, or maybe you can direct me to someone that can help me. Luckily my design has one side that always faces away from the body, so that should help a lot.Thank you again,-Jason