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

[AmbystomaLabs]

You can use anything with a tighter tolerance (ie, 50 or 30ppm) as long as the load capacitance and ESR are the same as what Nordic specifies.  Since I notice that this design is rather old and Nordic specified 12pF which is no longer shipped you may want to switch to a different packaged part. There are plenty of 4-SMD crystals in 12pF with an ESR of 30 to 50ohms which is similar to the original crystal.  The rerouting of the crystal is pretty low risk and any of us can guide you on getting the grounding right. Since it would be 4-SMD you can also just mimic the routing for the crystal on a nRF52 reference design.

One question, you know the nRF24L01 is slated to be discontinued, right? You sure you want to put so much effort into this.  You might be able to find an nRF51/52 module that is footprint compatible with your old module and then you can just port your code over.  There is softdevice support on 51/52 for ESB, ANT, BLE, etc. 

[JoE1205]

Hey its me again.
I was able to recreate the layout by using the pick and place file. But im stuggling with the antenna positioning and dimensions. Is there a way to get the exact dimensions?

[AmbystomaLabs]

Not sure whose antenna you are referring to since a few were mentioned.  On the Nordic ones you can just measure in the design files. If they are Altium files you can open the library model and it will give the dimensions.  If you are using the gerbers then any gerber viewer lets you measure distances.

With the TI mifa they give the dimensions in the pdf.  See enclosed.swra117d.pdf

[ketiljo]

There are no plans to discontinue the nRF24L01+

[AmbystomaLabs]

The Nordic website currently shows the nRF24 series as "not recommended for new designs" which often implies that it will soon be discontinued. But perhaps not in this case as you state.