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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:

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

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    Joe, pretty much everything is wrong on your design.  (sorry)

    On the antenna it is unlikely you can follow the guidelines and come up with something that works.  You need to model it. Standard FR4 while normally 4.6 Er at 10MHz is closer to 4.3 at 2.4GHz. Rarely do the pcb vendors spec it correctly and often just include the 10 or 100MHz value.

    To model it you can use the free version of Sonnet.  It can normally handle a mifa.  In fact I think the demo comes with a mifa for cell band.

    But even easier is to just copy someone elses mifa.  TI publishes one in their BLE docs that is nice.  It shows up on all their dev boards. Nordic uses a similar one on their BLE usb stick.  But not sure if they publish the artwork.

    After that your routing for the RF off the chip is just a disaster.  It will never work.  You routed it like it was DC. Since you don't know RF you should copy the routing from a reference design.  Everything off of the RF needs to be characterized.

    While you are doing that you should spend some time looking over the reference designs so that you get the chip epad grounding right with the correct vias and also get all the DC caps hooked up correctly with the correct grounding. Right now all that stuff on your design is just a train wreck. And, your chip parts are way too big.  They appear to be 0603 or 0805. You need to do the RF with 0402 or 0201 and if you copy someone elses routing you need to use the same size components.  At 2.4GHz there is a big phase difference across the length of a 60 or 80 mil long part so it is much easier to plan and use 20 or 40 mil long parts. I always tell people at 2.4GHz 1mm is almost 10 degrees so 2mm for an 0805 part is 20 degrees which is a lot.  On the smith chart it can easily change a shunt solution to a series solution or a capacitive solution to an inductor solution.

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  • 0

    Joe, pretty much everything is wrong on your design.  (sorry)

    On the antenna it is unlikely you can follow the guidelines and come up with something that works.  You need to model it. Standard FR4 while normally 4.6 Er at 10MHz is closer to 4.3 at 2.4GHz. Rarely do the pcb vendors spec it correctly and often just include the 10 or 100MHz value.

    To model it you can use the free version of Sonnet.  It can normally handle a mifa.  In fact I think the demo comes with a mifa for cell band.

    But even easier is to just copy someone elses mifa.  TI publishes one in their BLE docs that is nice.  It shows up on all their dev boards. Nordic uses a similar one on their BLE usb stick.  But not sure if they publish the artwork.

    After that your routing for the RF off the chip is just a disaster.  It will never work.  You routed it like it was DC. Since you don't know RF you should copy the routing from a reference design.  Everything off of the RF needs to be characterized.

    While you are doing that you should spend some time looking over the reference designs so that you get the chip epad grounding right with the correct vias and also get all the DC caps hooked up correctly with the correct grounding. Right now all that stuff on your design is just a train wreck. And, your chip parts are way too big.  They appear to be 0603 or 0805. You need to do the RF with 0402 or 0201 and if you copy someone elses routing you need to use the same size components.  At 2.4GHz there is a big phase difference across the length of a 60 or 80 mil long part so it is much easier to plan and use 20 or 40 mil long parts. I always tell people at 2.4GHz 1mm is almost 10 degrees so 2mm for an 0805 part is 20 degrees which is a lot.  On the smith chart it can easily change a shunt solution to a series solution or a capacitive solution to an inductor solution.

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