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Necessity of antenna feedline

Kenny Tay
Intellectual 970 points

I have an RF circuit built on top of the meandering monopole reference design (#6) in swrr070. However, based on product design concerns, I need to reduce the size of the PCB. I see a few ways of doing so:

  1. There is a 1.5 cm feedline to the antenna tuning circuit. I understand that this feedline is sized in such a way that it is close to 50 ohms (at least using the calculator at emclab.mst.edu/.../index.html) but I don't understand why the feedline is so long or if it even needs to exist in this circuit. For one, the impedance of the feedline is independent of the length of the feedline. In addition, I don't see why I'm not able to connect the SMA connector directly to the antenna matching circuit/antenna.
  2. I could possibly use the ceramic antennae which are significantly smaller in size. However, in #3, there is an additional narrower feedline from the antenna matching circuit to the ceramic antenna. Again, is this feedline avoidable? If so, why does it exist in the first place? Is it for better comparisons with the other antenna reference designs?

PS: Took some E&M back in college but I haven't used it in a while. Also, it seems like unlike software/firmware, RF related problems are hard to Google for....

  • 0
    TI__Mastermind 29570 points

    Hi,

    In the CC-Antenna-DK there is a calibration kit which contains an OPEN, LOAD and SHORT transmission line. This length of the transmission line is fixed in the kit so the characteristics of the antenna can be measured and the length of the feed line compensated. For the actual antenna design, the feed line length does not have to be followed exactly.

    For sub-1 GHz antennas, the antenna is normally too small compared to a quarter wavelength size. The GND plane size is normally fixed by the component area size. Ideally, the antenna should be a quarter wave length long and if this is not possible then the best condition is to make the antenna as effectively large as possible for the allocated area.

    Regards, Richard

  • 0
    Prodigy 40 points
    the feed line between the antenna and the tuning circuit acts a matching network to transfer the signal to and fro the circuit efficiently ( SWR=1:1 as possible).

    you cannot connect the sma directly as there is a difference in impedance between the sma and the circuit that will surely cause an impedance mismatch.

    I suggest you use an 0dB attenuator to minimize this length. if you are interested, i can forward to you the part number.
  • 0 in reply to Enrico Marcelo
    TI__Mastermind 29570 points
    The majority of our radio designs are configured to a 50 ohm interface via the SMA connector. The antenna kit is also designed with a 50 ohm interface to the antennas. The feed line on the antenna kit is kept at a certain distance just for the calibration kit that is also included on the board. Reducing the feed line on these 50 ohm antennas is possible without the need of any attenuators or matching.

    Previously we have designed folded dipoles(www.ti.com/.../swra118) that have a feed line that acts as a transmission line impedance match between the radio and the antenna. In the case of the folded dipole designs, then the feed line to the antenna cannot be changed since this is an impedance transformation. Matching the impedance from the radio to the antenna.

    For the 50 ohm feed line antenna ref designs, the feed line distance can be changed without any issues.