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WL1835MOD: WL1835MOD External Antenna and RF Trace specifications

Gaurav Bhatia
Prodigy 60 points
Part Number: WL1835MOD

I am designing my own custom board with WL18305MOD module in it. I have gone through many posts on TI forum and through datasheet of module. I would ideally won’t change much in the design to avoid cost of secondary evaluation or additional certificate from FCC. I am also comparing TI’s design with Beagle Bone Black Wireless (BBBW) module. My questions below are related to PCB design and external antenna selections.

 

Queries:

  1. TI recommends to have antennas with 2dB gain or less which is different from 3dB gain antenna from BBBW design. Is there any benefit of choosing for less gain antenna as I assume more gain is better for signal strength. I want to understand why TI is recommending 2dB gain antenna and over that asks to have additional 1 dB attenuation on RF trace to reuse the FCC certification while BBW has shorter traces and antenna of 3dB gain
  2. I also want to understand how antenna selection affects impedance balancing between two antenna traces affects. Why antenna 1 needs to be more away from module compared to antenna 2
  3. I am planning to have only external antennas in my design. I am planning to use what is recommended by TI but, do I still need to maintain certain distance between antenna or that only applies for on board chip antenna. Are there any specifications for designing external antenna
  4. If I use only external antenna with SMA connectors then would I still be able to reuse FCC certification

 

Your reply would be very helpful for me as I am not expert in RF and desperately need your help. I would really appreciate if you can reply me ASAP.

 

Thanks,

  • 0
    TI__Genius 12270 points
    Hi Gaurav,

    Please find below my responses to your questions:

    I would recommend that you compare your design to the WL1835MODCOM8B board. This was the board that TI used to certify the module.

    1) Our module is actually certified to be used with a max antenna gain of 3.2dBi (based on the pulse W3006 chip antenna). So you can use an antenna gain up to 3.2dBi in your design. What is important is not necessarily the peak gain, but the radiated pattern of the antenna. This comes down to your customer design requirements. For example a high gain (3.2dBi) antenna may be better for a product that is always fixed and you can adjust the orientation (even if you may have lower gains in some directions. On the other had maybe you requirements a more uniform antenna gain pattern and thus a 2dBi antenna gain may be better). The point in the certification is that we can accommodate up to a gain of 3.2dBi and still meet the regulatory requirements.

    The reason for the 1dB loss requirement is allowing our customer some freedom in the design. At a minimum you need to meet the trace distance from the output of our module to the U.FL connector. The statement of 1dB loss allows customers freedom on how to achieve this, but it is still required.

    2) The distance between antennas is for optimal/better MRC/MIMO performance. At a minimum you need half wavelength separation, thus the separation between antennas. It is not related to one antenna being closer to the module or not.

    3) If you are only using one antenna, there is no need to worry about the secondary antenna. You can leave this port open. For a single antenna design you need to ensure the 1dB loss to the antenna connection (PCB or External connector) is 1dB.

    4) yes, no issue here.

    I hope this helps.

    Thanks,
    Riz
  • 0 in reply to Rizwan Murji
    Prodigy 60 points

    Hello Riz,

    I have some follow up questions:

    1) What should be minimum distance between two connectors (J5 & J6) of external antenna. I might have just external antenna with two SMA or U.FL type connector

    2) Can I route the RF traces exactly like TI module board (screenshot attached) without on board antenna. Which means only U.FL connectors connected to the module. Will this be considered as significant change and I won't be able to reuse TI's FCC certification.

    Thanks,

  • 0 in reply to Gaurav Bhatia
    Prodigy 60 points
    Hello Riz,
    Could you please reply to my previous response.

    Thanks,
  • 0 in reply to Gaurav Bhatia
    TI__Genius 12270 points
    Hi Gaurav,

    My apologies for the delay. Please see my responses below:

    1) since you will be using external antennas connected to an SMA/U.FL, the distance is not critical as there will be inherent isolation between the 2 paths. What is critical is the distance between the radiating points of the antenna. What I would do is still try to separate the connectors on opposite sides of the Module.

    2) Yes you can. The method for our certification testing is called the radiated cabinet method. The key is that the distance and trace to the U.FL connector is maintained. If you do this, there should be no issue in reusing our certification.

    I hope this helps.

    Thanks,
    Riz