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How to connect a network analyser to the miniature PCB helical antenna of design note DN038 to calculate antenna matching values?

Ron Segal1
Prodigy 150 points

What is the best practical method of connecting a network analyser to the brilliant, miniature PCB helical antenna design of design note DN038 to calculate antenna matching network parts values?

We would like to calculate the values of the matching network parts to match the antenna to a board that will be transceiving in the 915MHz band, by using a 2 port VNA to output a Smith Chart. Then using a smith chart matching app (www.rfmentor.com/content/smith-chart-matching-app) to calculate the values.

Although a 2 port VNA is available we have minimal experience in its use, so would appreciate some advice on how best to physically connect it to a board under test that implements the helical antenna PCB design.

Any other advice to match the antenna to the board would be greatly appreciated. This size of this antenna design would be perfect for the limited space on the board, so we would really like to make good use of it rather than one of the other meandering etc types.  

Many thanks, Ron.

  • 0
    TI__Mastermind 33190 points

    Hi Ron,

    You only need a single port for antenna measurements so a 2 port VNA will be fine.

    1. Remove the DC blocking cap to disconnect the radio
    2. Solder semi-rigid cable shielding as close to the DC blocking cap pad position as possible; both for RF reasons and mechanical. Make sure the cable is within the GND plane for the section of the semi-rigid cable on the PCB.
    3. Calibrate VNA and de-embbed the semi-rigid cable
    4. Solder semi-rigid cable 50 ohm core to the DC blocking cap pad that is connected to the antenna pi-network.
    5. ANT1, ANT3 : NC; ANT2: 0 ohm
    6. Measure impedance at 915 MHz; this impedance value can then be entered into a matching program. 

    When you measure the antenna on your prototype board, make sure the GND plane is roughly similar size as the GND plane on the final design. Otherwise, there will be a delta in the measurements.

    Regards,

       Richard

  • 0 in reply to RGW
    Prodigy 150 points

    Thanks Richard, really appreciate your step by step instructions. We'll be moving forward with that approach. Cheers Ron.