Poor RF performance on custom PCB using the reference design

[Adam Varga]

Hello people, 

I had to make a tiny, 4 layer PCB based on an nRF52832 chip (WLCSP) and I've chosen to use a PCB antenna.
I've copied the reference design (nRF52832-CIAA Reference Layout 1_0) as much as possible to reach a good RF performance, but that didn't end up happening, as I'm seeing the max BT range about 1meter. Can you help me point out where I was wrong?

The dimensions are similar between the reference board (11.44 x 9.56mm) on the left and my pcb (14.1 x 8.2mm) on the right of this picture:

My design compared to the reference:

1. Antenna width, high and position in relation to the chip is the same
2. L1 & C3 are the same value (3.3nH, 1pF, same position, but smaller footprint (0201)
3. Lots of GND area and vias around the antenna (inner layers are also GNDs with keepout zones around the antenna)
4. Antenna is further away from the board edge because of manufacturing limitations (0.279mm vs 0.409mm)
5. I've checked several online calculators to get as close to 50Ω as possible. The parameters that I've used:
   1. 4 layer FR4, dielectric constant of 4.2 (provided by the manufacturer)
   2. 0.676mm thickness between the outer copper layers (provided by the manufacturer)
   3. The suggested gap ended up being 0.137mm

https://chemandy.com/calculators/coplanar-waveguide-with-ground-calculator.htm

My guesses so far:

• Maybe the copper filling with those pointy edges causes disturbance.
• I've used the wrong calculator.
• The end of the antenna is further away from the board edge.

Any help is much appreciated!

Thanks, 

Adam

[ketiljo]

The reference design doesn't have ant antenna, this comes in addition to what's shown here. A typical PCB antenna can be the one on the nRF52-DK: 

It's basically just a track. Try to solder a 25 mm long wire to the PCB.

[Adam Varga]

Thanks ketiljo, 

I'm afraid I don't understand it correctly. Are you saying that the reference layout that's on the nRF52832's website is not showcasing a usable and valid bluetooth PCB antenna?

[ketiljo]

Yep, none of the reference layouts don't have antennas. The type of antenna varies with the application so it's up the designer to handle that. 

[Adam Varga]

I see, it makes sense now.
It was really misleading then, because the schematic and pcb design made me believe that it's a working board, hence the name "reference design".

[ketiljo]

The term "reference design" is often used for just the parts needed to make the device work as intended without being a full design. A building block more than anything else.