nRF52811 clarification on PCB antenna design

[gian]

Hello,

I am using the nRF52811 for my project. I am in the layout design phase...almost completed and I need some clarifications before I launch the first Proto.

I don't have experience with RF design so, I read a lot and I am using the nRF52840 Dongle as a reference for the PCB antenna.

• Looking at the gerbers provided for the dongle, I did not see any ground plane below the antenna area, can you please confirmed that it is correct?
• For the specific PCB antenna in the dongle...is there some calculation available to understand that design? Shape...W,L...
• I put the 3.9nH and 0.8pF for the filtering and matching (50ohms) and I have also added a Pi circuit just before the antenna for the final tuning. Is there a suggested way to measure the antenna impedance? Should I add a coaxial connector, such as an MM8130-2600 to measure the ANT impedance with a VNA?
• If I use an MM8130-2600, should I put it just before the Pi circuit or just after?

Thanks for your support 

Gian

[ketiljo]

1. Yes, it's correct that there are no ground plane under the antenna.

2. The dimensions of the dongle are nor critical. But as all radio designs, the larger the ground plane you have, the better antenna gain you will have. To a certain point of course. 

3. The best is usually to cut the track at the 50 ohm point and solder a thin coaxial cable to the board. Calibrate the VNA with the cable so that the reference point is at the end of the cable. 

4. Place the connection at the 50 ohm point, so between the matching network (C3 / L1 in the ref schematic) and the pi-filter.