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PI-Network for antenna matching

Parcour
Intellectual 365 points

Hi,

many manufacturer of antennas recommend a pi network to build a matching network. What is the reason for that? From my point of view it should be possible to match the antenna to 50 ohm by using only two components (e.g. L + C). Do you have an idea?

Thank you very much!

fab

  • 0
    TI__Expert 3595 points

    More degrees of freedom and wider bandwidth.  Cost differential is not that much.

  • 0 in reply to Chris M
    Intellectual 365 points

    Ok, but does the BW depend on the types of the components. To match my antenna I used one shunt capacitor, one serial inductor and one shunt inductor:

    ----------L-------------------< antenna
      |                       |
     C                      L
      |                       |
    ___                 ___

  • 0 in reply to Parcour
    TI__Expert 3595 points

    Usually more to do with the difference in impedances.  I have to go back to the books to remember exact equations, but in general the bigger the impedance change the narrower the bandwidth.  And there is somewhere on the Smith chart that 2 components can't match to (again talking from a course that I took 10 years back).  Extra components also are good for knocking out the harmonics (like the inductor in yours, though you still need a DC block from the chip).

  • 0
    TI__Mastermind 33200 points

    HI Fab,

    With the pi-network, the antenna can always be matched to 50ohm. With only 2 components, there is a risk that the antenna can not be matched perfectly to 50hms. The extra component on the antenna match network gives complete freedom for matching compared to only 2 components. The majority of times, not all three components are needed but it is always good practice to allocate the foot prints on the pcb.

    Regards, Richard

  • 0 in reply to RGW
    Mastermind 8170 points

    The plots below are for the CC1110.  The actual device is modeled as a pair of pulse generators, as opposed to nice clean sine wave generators, to best emulate the CC1110 output.  The values are straight out of the Ti CC1110 data sheet and are fairly sensitive to adjustments of greater than 5%. Each component affect pass band loss, harmonic surpression and current consumption. The point is converting a harmonic rich 1GHz square into a FCC compliant output is much more complex than just a impedance matching problem.