Tips for minimizing nRF52 PCB surface-area while retaining a decent range

[Ivan Herrera]

Hi! We're developing a BLE-sensor for tracking small animals. In the past we've used a different brand of BLE-module but we're looking to switch to the nRF52840. As we're working with small animals the size and weight of the total module have to be kept at an absolute minimum. We're trying to stay as close as possible to the reference designs provided by Nordic. A couple of questions we have:

• As we're keeping PCB-surface to a minimum, is there a suggested chip-antenna to be used in this application? We're currently looking at the 2450AT18B100 used in the Nordic Thingy 52. Range is rather important to us, as we're aiming to use the Long Range mode introduced in Bluetooth 5
• To power the module we're looking to connect either a LiPo or a small 3-4V coincell to the module. Are we allowed to increase the value of the capacitors on the voltage input lines (C6/C19 in the reference design) to account for the low current output by the coincell?
• Even though the documentation recommends against editing anything in the reference design, how detrimental would it be to add battery connectors to the bottom of the PCB on the side opposite of the antenna? That way we could keep the total size of the PCB to about 11mmx11mm (+ a couple extra mm for the antenna + tuning network)
• Is there a way to reduce the attenuation in signal strength caused by the body of the animal?

[Marjeris Romero]

Hi Ivan,

Ground plane size vs range is a general trade-off and you should be aware of that. I cannot say for sure if a such a small PCB size will be a good idea or not for your design, it will depend on what range is acceptable for your application and the enviroment you are in. My recommendations is to test you application using the nRF52840 DK with 0dBm output power or less and if the range is aceptable for you then you can downscale the PCB size. If you need to use +8dBm output power on the DK to optain good range then I doubt you will achieve good range using a PCB smaller than 20 mm x 20 mm (and preferably larger).

So keeping that in mind, I can try to answer the rest of your questions:

• The only recomendation we have for antennas is to use one that is design to be used in the same placement that you want (corner vs PCB side). The 2450AT18B100 has a small footprint and keepout area so it should be a good choice for small designs.
• Yes, you can change the value of these capacitors, but you should not decrease the total capacitance on the VDD line, see this page for more information.
• With "side opposite to the antenna" do you mean under the antenna? If so, this will not be a good idea, since whatever metal or ground plane you place under or right beside the antenna will reduce the antenna performance. So yes, it can be tricky to place the battery in such a small PCB without reducing the antenna performance, again it all comes down to what kind of range you are expecting and the enviroment.
• An animal body contains a lot of water, which will give dielectric loss and attenuation, the only way to reduce this is to place the antenna/PCB far away from the animal as possible.

So in conclusion, it will not be easy to optain both good range and a small footprint, you will need to sacrifice the one or the other... Again I recommend you first find out if you can scale down the PCB or not testing with the DK with lower output power. When the time comes you can also open a private ticket and attach your PCB files so we can review them before you send them to production to make sure everything looks right.

Best regards,

Marjeris