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NRF52811 using chip antenna

Juniorprogrammer

Hi,

I did recently start with my pcb design for the nRF52811, but I have some questions about the antenna. I want to use a 2450at18a100 chip antenna of Johanson Tech. 

1) In the datasheet of the chip antenna there are 2 options. One with matching circuit and one without. Do I need this extra? Because the L1 en C3 are included in the reference design of the nrf52811.

2) Also did I read about a pi-network (this is used for antenna tuning I guess) do I need to implement that in my circuit?

3) Are there any other points I should keep in mind? This is my first time with RF, I don't want to miss anything.

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  • 0

    Hi,

    you need a separate matching network for the radio and one for the antenna. If you need one component, a T- or a Pi-network depends on the antenna.

    For the one you have listed here a T-network i recommended, and I strongly recommend you to add the T-network and go for option b) from the antenna vendor. 

    Illustration, here is the antenna matching network illustrated as a pi-network, bit a T-network is also fine! 

     Antenna selection in general: 

    Antenna choice is complex and depends on many factors including technical and commercial considerations. Here are some alternatives:

    Monopole, printed PCB antenna: This is easy to make and easy to tune, you also only need one impedance matching component, so it’s cost effective. Here the spacing is the issues, you need to make it about 23 mm long needs a minimum of 5 mm clearance to the ground plane. High bandwidth, making it fairly resistant to detuning. Link to our whitepaper: https://infocenter.nordicsemi.com/pdf/nwp_008.pdf?cp=12_18


    Meander antenna, printed PCB antenna, ex. our dongle antenna design: Requires a smaller area than the monopole antenna, but usually requires a pi-network for tuning in addition to length. Lower bandwidth than a quarter wave monopole antenna. Here is a link to our nRF52840 Dongal design files as an example of this: https://www.nordicsemi.com/Software-and-Tools/Development-Kits/nRF52840-Dongle/Download#infotabs

     

    Chip antenna: Higher BOM, but the antenna is small. The downside is that it usually has less gain. It requires a matching network, based on the vendors recommendations. It has a lower bandwidth than a quarter wave monopole antenna so it can be sensitive to detuning.

    Considering your target of a very small form factor a chip Antenna makes sense but reducing antenna size most often results in reduced performance. Some of the parameters that suffer are:

    • Reduced efficiency (or gain)
    • Shorter range
    • Smaller bandwidth
    • Distorted radiation pattern
    • More critical tuning
    • Increased sensitivity to component and PCB spread
    • Increased sensitivity to external factors (“body” effect, ground plane etc.)

    It is often better not to reduce antenna size too much, if you can avoid it.


    As a rule we do not recommend specific chip antennas when customers choose to use them. The antenna vendors can assist on choosing the right antenna for a specific design, for example Johanson has a useful tool that helps with this selection: https://www.johansontechnology.com/chip-antenna-selection

    When designing a product, start with the placement of the antenna, and then the placement of the nRF52... and then place the rest!

    Useful links:
    General pcb design guidelines for nRF52-series

    pcb guidelines for nRF52811

    Download reference design QFN

    Download reference design CSP

    And we offer to do HW reviews, so when you have a schematic and layout ready, feel free to upload to a case on DevZone, so that we can check and give you feedback on your design. 

    Best regards,
    Kaja

     



  • 0 in reply to Kaja

    Hi Kaja,

    Thank you very much for your great response. Firstly I wanted to use a PCB antenna, but I was afraid of the antenna tuning. But you suggest to use a PCB antenna am I right?

  • 0 in reply to Juniorprogrammer

    As I wrote in the last response, choosing antenna type is up to you. 

    But we can do the antenna tuning for you, RF measurements and antenna tuning is a free service we offer to all our costumers. 

    Best regards,
    Kaja

  • 0 in reply to Kaja

    My first choice was a pcb antenna, so I will pick that up again. I didn’t know there was a service like that.

    I will finish my schematics and then upload my design. 

    Is there any guide how to trace a meander antenna or can I copy the one from the dongle?

    Kind regards,

    JP

  • 0 in reply to Juniorprogrammer

    Hi,

    we don't have a guide for a meander, but we have this one for a quarte wave monopole, but you can copy the one from the dongle, and just add some extra length for tuning.

    Make sure you also copy the distance form the antenna to ground plane, and add a pi-network for matching.

    Best regards,
    Kaja

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