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Queries related Measurement of antenna S11 parameter, VSWR, smith Chart

Shubhendra Singh
Intellectual 885 points

Hi All,

          I have a Spring helical Antenna of 433 MHz, when I measure it's S11 while connecting directly on port of Network Analyzer at 433 MHz i get 

          S11 value   : -19 dbm.

          VSWR        : 1.2

          and a complex value of 32 -j10 on smith chart.

Ques : Now my first query is After placing my antenna on Board how should i measure it ? 

The way i tried is, I used an coaxial cable of 70 cm and calibrate it to end. then i solder it on the starting point of matching network. 

But the antenna parameter values got changed.

Ques: Is this the correct way of doing it?

Ques: Do i have to make antenna 50 Ohm using smith chart  and matching circuit? Or just good S11 and VSWR are sufficient?

Ques: How should i approach the antenna tuning on my board and verify it whether its correct or not?

I am totally new to all this and i would appreciate any guidance towards right approach

Thanks

  • 0
    TI__Mastermind 46531 points

    sasuke,

    No, a 70cm cable is too long to calibrate and account for its parasitics and phase change.

    The correct way would be to have an SMA or UFL connector on your board's RF trace and connect it to directly to your network analyzer.

    If your board is already designed and built w/o one of these connectors, another option is to mount a short (5~6cm) semi-rigid cable. See section 6.2.1 in our "Antenna Selection Guide".

    If you are using this method:

    1) Connect the open cable to your network analyzer's port by itself. Your network analyzer should have a "open port extension calibration" tool. Ensure you do this before soldering the cable to your board. 

    2) When you are soldering the cable to your board, make sure that your cable's outer GND shield has a solid connection to the PCB's GND, or else the inner conductor can pull your RF trace off the board if the cable comes off. 

    Hope this helps,

    Seong

  • 0 in reply to Seong Kim
    Intellectual 885 points

    hello Seong,
    Thanks for your quick reply. Can i use a long LMR cable + 5 cm semi rigid cable.

    Thanks

  • 0 in reply to Shubhendra Singh
    TI__Mastermind 46531 points
    sasuke,

    Using a long LMR for S11 measurements will not yield accurate results, even if you use port extension cal. Port extension should only be used with short cables that are 6cm max.

    Try this experiment:
    1) Connect your cable to port 1 only
    2) Use the port extension calibration for Port 1 "OPEN"
    3) Look at the smith chart on your network analyzer
    4) If the calibration was done successfully, you should only see dots on the right side of the smith chart.
    5) Repeat with a shorter 5cm cable and see how it compares

    BR,
    Seong