I am designing a device with two stacked parallel PCBs of which the top one serves as a front panel (with touch pads), while the bottom PCB holds all the components. I need to add a Bluetooth LE module to the bottom PCB and since the top PCB is mostly covered in copper, and the enclosure will likely be metal, I need to find a way for Bluetooth to radiate.
I see two options here:
1. Choose a Bluetooth module with U.FL connector. Mount it together with all the components on the bottom side of bottom PCB. Mount another U.FL connector on the bottom side of front panel. From there I could draw a PCB antenna on the front panel either on the top side (and make a via to U.FL) or on the bottom side.
2. Choose a Bluetooth module with chip/PCB antenna and mount it on the top side of bottom PCB. Make a opening in the copper layers of the front panel large enough for bluetooth to work.
The second approach will be much cheaper than the first (U.FL connector and cable add up to $7.50 on Mouser) but I'm worried it will make my bluetooth radiation pattern too directional, and I can't imagine what is the reasonable size of the opening in copper layer? I can't afford to spend much front panel area on that feature. OTOH, if the opening area required would be about the same as area required for a PCB antenna + keepout area (approach 1) then I can do it.
Best Answer
I've dealt with RF for about 10-12 years now, and my best recommendation is to spring for the extra $$ and use the antenna mounted to the top PCB. Putting the radiating element under another layer of copper, silicone, and other materials will attenuate the signal by (very exact answer here) a lot. On the other hand, if you plan on using the Bluetooth interface in very close proximity (less than a meter), it may not matter much. You could always build a prototype of each one and use a spectrum analyzer to measure the performance before putting your design into production. Also, if the whole thing is going to be encased in metal, the BT antenna mounted to the top PCB will probably be severely attenuated as well. You may want to consider a material with better RF transparency for the case. Plastic, maybe?