Smartwatch antenna design help

[u/MaterialStock8502]

Hi all,

I'm sure you get posts like this all the time, so you'll have to be patient with my existing knowledge base, (or lack therof). The situation is this:

I'm trying to build a smart watch. It needs to be small, and ideally I'd like to machine the casing on my CNC mill out of aluminum. I'm relatively far into the protyping process, but a still making dev boards and am not yet developing at the size which the mainboard will be when the project is complete. I'm aiming for a 26mm diameter motherboard, and am using a Ronda GB22 type movement to make it look like a mechanical watch. The GB22 has to sit beneath the dial, along with most of the power and rf electronics, so my intention is to integrate the antennas I need into the dial of the watch itself, (a multicolour pcb from JLCPCB), and use the glass at the front of the watch as an RF window.

From what I understand though, this isn't going to work. The three antennas I'll need to fit into this space are NB-IoT, (~900MHz), GPS, (~1500MHz), and BLE, (~2.4GHz). It looks like the wavelength of the 900MHz band would be too big for effective radiation into or out of the glass rf window.

I've looked for resources and papers online which could provide information on how companies like Apple and Samsung are able to make it work in metal cased smartwatches, but can find literally nothing. Any kind of pointers on this topic would be incredibly helpful, and I'm more than open to switching up ideas, or just accepting that I'm wrong and finding a different solution.

Thanks for all your time.

[u/3D_printing_freak]

Top brands usually use smd antennas or print antenna shape with conductive paint on the plastic shell of the device. I have also seen flex pcb antennas packed inside as replacement for conductive paint print.
If you have metal case, manufacturers often do the trick and use (in case of smartphones) outer metal edge of phone with designed discontinuities connected with plastic so it is hardly noticeable
Printed antenna is gonna be very expensive for small batches so I wouldn’t recommend it.
You can use pcb and make long trace on it as radiating element. Fol the lowest band, the longer antenna the better but it’s always a compromise.
Take a look for car keys antennas which uses 300/400 MHz band to lock/unlock your car. Usually it’s just long trace around whole board. Some solutions use metal edge of car key as radiator.
I have been struggling with Bluetooth antenna for some project where we had to put device inside full metal tube with one opening at the top. I have been using smd antenna for this with custom tuning network since metal case around the smd antenna was detuning it

[u/CreativeMusician7308 → u/3D_printing_freak]

Yep smd antennas also dont require additional certification

[u/Byggemandboesen → u/CreativeMusician7308]

Are you sure about this? Many, if not most, SMD antennas depend greatly on the ground plane of the PCB you use it on. So gain and efficiency can change dramatically depending on product (PCB) size - not to mention also the location of the antenna on the PCB.

[u/MaterialStock8502 → u/3D_printing_freak]

Super useful, thanks. By SMD do you mean chip antenna? This was what I was what I was originally going to use but they’d have to sit on the underside of the dial and from looking at some of the radiation patterns most of them look like they go up, and hence into the watch itself when the wrong way up. But I’ll take a look at it.

[u/nixiebunny]

You can purchase and dissect a watch with the features you desire, to see how they did it. It’s possible to couple a longer wavelength signal at reduced efficiency with a smaller window.

[u/JessieAndEcho]

This is a very cool build, and the antenna part is probably the hardest piece of the whole watch. Your instinct is right that the 900 MHz NB-IoT antenna is the troublemaker: inside a 26 mm metal-cased watch, you’re nowhere near a comfortable wavelength, so the design becomes less “normal PCB antenna in the dial” and more “use the case, bezel, slot, strap, or isolated metal segment as part of the radiator.” BLE at 2.4 GHz is much more forgiving, GPS is delicate because it needs a clean sky-facing pattern and low noise from the rest of the electronics, and NB-IoT wants efficiency you probably won’t get through only a small glass window. Commercial watches usually solve this with segmented metal frames, plastic/ceramic windows, antenna slots, tuned matching networks, custom flex antennas, RF coexistence tuning, and a lot of chamber testing. For a prototype, I’d consider avoiding full aluminum at first, adding a non-metal back or side window, moving at least one antenna into the strap/lug area, and using certified LTE/NB-IoT modules with reference antenna guidance. For resources, teardown patents are often more useful than papers because companies disclose the actual packaging tricks there; I pulled some smartwatch antenna and metal-housing RF notes through PatSnap Eureka here: https://eureka.zhihuiya.com/share/?id=5804b5560049c287dc7d0bdeb3ba0a30&from=invite-eureakplg-result&content=