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Impedance Matching Chip Antenna for CC2650

Johnny Lewis33
Prodigy 215 points
Other Parts Discussed in Thread: CC2650

Hello,

We are designing a custom PCB for our system using the CC26504XS as the MCU. For the RF front end, we copied the design given by TI for a single-ended output using the reference documents so we have the same capacitor and inductor values placed as close to the MCU as possible. It's my understanding that the antenna must be matched to 50 ohms to have perfect matching with this balun. We are using a chip antenna by Taiyo Yuden (AH316M245001-T) as our overall system has very small dimensions. The antenna followed the design given by the manufacturer's documents also. 

We placed an SMA connection that can be connected to by altering the T-junction like the evaluation boards by TI for the CC2650. We are measuring the antenna impedance using a network analyzer that's connected to the board from this SMA. The network analyzer is calibrated using a SOL calibration kit as described in the technical manual. Is this the best method for determining if the antenna impedance is close to 50 ohms? From TI's document, Antenna Selection Guide, the antenna performance was evaluated this way (measuring impedance with Smith Chart and S11 for return loss). 

We are asking this as our antenna measurements from the network analyzer is giving results that indicate quite poor performance (S11 = -2.10 dB @ 2.45 GHz, impedance of 16.9 ohm @ -69 degrees). However, the SimpleLink iOS app can still connect to the PCB when we load SimpleBLEPeripheral on the MCU on the PCB. We've also tried soldering discrete components (pi matching network) to adjust the impedance of the antenna to 50 ohms however it seems to have no affect on the return loss or the Smith Chart. 

We are in contact with the antenna manufacturer also but was wondering if one of your engineers has insight on doing antenna measurements as well.

Thanks,

Johnny

AH316M245001-T
  • 0
    Mastermind 9125 points
    I am not an antenna expert. But if you want to measure the S-parameter of antenna, the balun circuits should be disconnected form the antenna, right?
  • 0
    TI__Guru** 106705 points
    Hi Johnny,

    It sounds like you are doing everything correctly. You should just continue working with the antenna manufacturer for input on how to best tune their antenna.

    Even with a very poor match the radio will still work. The range will be quite poor though.

    Cheers,
    Fredrik
  • 0 in reply to Robert Chen
    Prodigy 215 points
    Hi Robert,

    Yes that's correct, that's what I was saying with the T-junction and SMA. The balun is disconnected from the antenna and then a 0-ohm jumper is placed that connects the SMA to the antenna feed point. We've checked the connection multiple times and that was working properly at least.

    Johnny Lewis
  • 0 in reply to Fredrik K
    Prodigy 215 points
    Hi Fredrik,

    Yes, hopefully the antenna manufacturer will know more as I'm not an RF expert. We did a quick range test with SmartRF Studio by using an evaluation board as a constant transmitter and our PCB as a constant receiver. We reached up to 30 cm with 0 CRC error but I believe BLE range should be quite larger in air/open room.

    Thanks,
    Johnny