nrf24l01+, pcb mifa antenna design

[JoE1205]

Hello guys,

I tried to design a pcb antenna for a nrf24l01+, since my company doesnt allow to buy cheap smd breakout board version from china anymore

The pcb stats of the producer:

• 2-layer FR4 1.55mm (61.023mil)  thickness, εr = 4.6
• Copper thickness, 35μm (1.378mil)

What I used:

• Guideline for pcb mifa antenna design:https://www.cypress.com/file/136236/download :
  Antenna basic dimensions: Page 10
  Cutting antenna length to ~115mil  as told on page 14
• nrf24l01+ schematic pictured in the datasheet page 66 (rev1.0)
• https://www.eeweb.com/tools/microstrip-impedance for calculation resulting into a feedingline width of 2.82mm (111.023mil), this is double the value of the recommended value in the cypress file. Not sure whats right.

This results into following desgin block:

The antenna area is clear of any signals aswell as ground on top or bottom layer.

Since I have absolutly zero experience with RF antenna design I would like to have some opinions regarding my design.

Thanks so far,

Joe

[Øyvind]

No, the nRF24 series will not be discontinued and is still available through our distributors as long as we have customers buying these. However, the nRF24 series have not been updated/further developed in the last 10 years, this is why we do not recommend these for new designs.

Kind regards,
Øyvind

[JoE1205]

Hey guys, Im back

the current state of me trying to copy the design is the following. I designed it as a design block in EAGLE in order to import it into my project later. Thats why GND-plane is not implemented yet.

Had to find an other crystal which fits the requirements mentioned by AmbystomaLabs.
So far the GND-plane is planned for the dotted area, whereas the antenna is planed to be free of GND or any other signals on top and bottom layer.  Anyway Im unhappy with this huge empty space on the left. I guess its okay to reduce the size, but Im unsure about the holes. Is it okay to remove them completely and basicly cut everything whats not needed on the left? By the way: Whats the intention behind placing all the holes? Whats their use? I cant find a clear pattern so for me it looks very random. Do I need some more holes to seperate the antenna from the rest of the pcb, which is not shown in the picture? Still learning about the whole RF design topic and I very appreciate all your advices.

Thank you :)

[Øyvind]

Hello, and welcome back!

This is starting to look good. 

JoE1205 said:

Anyway Im unhappy with this huge empty space on the left. I guess its okay to reduce the size, but Im unsure about the holes. Is it okay to remove them completely and basicly cut everything whats not needed on the left?

The open area on the left should be used to add pads and/or GPIO pins for physical connection to flash/program/debug. Ref. the reference design I referred to in this comment, the image shows that there is a connector placed in the left side of the board. The holes which you mention are via-holes and are used to connect and ensure that the GND layers are connected. 

I recommend that you remove the vias and finish your layout with components, GND-plane, and traces. Then send the layout to us, and we can help with adding vias. Have a look at this tutorial on Eagle from Sparkfun, and our tutorial on RF design.
Kind regards,
Øyvind

[JoE1205]

Hi,

i implemented everything in my current project, but I had to do some change since the size of the antenna is quite big related to the rest . Actually Im not sure if this layout is still fine.
So what I did:
-Antenna is on bottom layer
-all components on top layer
-connection of antenna to components via via-in-pad of closest component
-changed the place of the crystal

GND-Plane is not implemented yet

[AmbystomaLabs]

That 10 pin SMT part and its associated components should really be behind the ground plane (ie, picture a vertical line running through the feedpoint of the antenna.  Everything to the right of that line will be ground or ground and circuitry to the left should be antenna and nothing else). The SMT part moves the effective ground plane closer to the antenna and will change the antenna radiation characteristics, passband and impedance significantly. Similarly, that hole next to the antenna for a mounting screw is a big concern.  If you mount to plastic and use a plastic screw or plastic snap then it will be ok as long as there is no copper on that hole. But you can't have any metal there. Even the plastic will change the tuning a bit.