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CC1310: Dipole Antenna Matched Directly to Source Impedance

Samir Nooshabadi
Prodigy 95 points
Part Number: CC1310
Other Parts Discussed in Thread: CC2500

Hi,

I have design constraints on the actual size of my PCB, but not on an off-board antenna that can be fed through my PCB enclosure. Initially I had a chip antenna for the CC1310 in 915MHz mode, and was using a balun to match the CC1310 to the antenna. Unfortunately, my ground plane is not able to provide adequate enough performance for a chip antenna, and I would like to use a dipole antenna instead.

The required load impedance for the CC1310 seems to be 44 + j15. Am I able to simply design a dipole antenna with that impedance and attach it directly to the RF_N and RF_P pins, using the appropriate caps for FCC regulations as well? Or is there something else that I need to know about the CC1310's RF pins.

Thanks, 

  • 0
    Expert 1240 points
    Samir,

    A dipole in air presents a nominal ~73 Ohm impedance so your radiation efficiency will drop some without a matching network on the PCB.

    However, being external, the net gain in operational range will probably well exceed this loss.

    To avoid FCC and other issues, I highly recommend employing the TI 50 Ohm Reference design for 915MHz .
  • 0 in reply to Zebryk
    Prodigy 95 points

    Thanks Zebryk,

    But I'm still a little confused. Can't an antenna be considered as a matching element between the source and the load, in this case the source being the CC1310 and the load being free-space? If the CC1310 wants to see an impedance of 44+j15, can't I design a dipole (not a half-wave dipole with 73Ohm impedance you mentioned) that will give me that impedance when combined with the air's impedance of 377Ohm?

    TI has released an app note for another chip, the CC25xx series, doing exactly this, but with a trace dipole antenna. 

    Can I do the same on the CC1310?

    Thanks,

  • 0 in reply to Samir Nooshabadi
    Expert 1240 points

    Samir,

    Sorry, but from your E-Mail I thought you wanted an external dipole antenna?

    Generally, these are center-fed coaxial half-wave antennas like what TI supplies in their development kits.

    They are adjusted to present a 50 Ohms and do not require a ground plane.

    The example you refer to is a folded dipole which has a nominal impedance of ~300 Ohms.

    If you wanted to install one outside your enclosure, which can be done, a challenge is the interconnection between the PCB and the 300 Ohm.

    (Almost all RF connectors are 50 Ohms today. When TV antennas were common, 300 Ohm "twin-lead" cables and connectors were readily available.)

    If however, your real objective is better operational  range without ground plane, please see the below as a compromise.

    It has a number of proven antenna designs which will avoid complications with ETSI and the FCC.

    www.ti.com/.../cc-antenna-dk2

  • 0 in reply to Zebryk
    Prodigy 95 points
    Hi Jay,

    Thanks again! I'm looking at the app note right now. I was planning on using a custom two-wire dipole antenna, and soldering the antenna straight to the board, this way I can have the freedom of tailoring the antenna impedance to my source.
  • 0 in reply to Samir Nooshabadi
    Prodigy 95 points
    I forgot to add that the CC2500 that is used in that app note wants to see an impedance of 80 + j74 Ohm from the RF pins. The dipole they designed satisfied that. I want to do the same but for the CC1310, and I can calculate the dipole length needed. Can I apply the same approach to CC1310?
  • 0 in reply to Samir Nooshabadi
    Expert 1240 points

    Samir,

    "On the Texas Instruments CC1310EM-7XD-7793 reference design, the optimal differential impedance seen from the RF pins into the balun and filter and antenna is 44 + j15."

    Your reasoning is sound but I would be concerned about spurious and harmonic radiation.

    "The CC11xx required differential filtering in order to pass ETSI & FCC regulations." (Richard Wallace)