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Antenna matching 433MHz

Discussion in 'Electronic Design' started by Andreas, Sep 4, 2004.

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  1. Andreas

    Andreas Guest

    I have problems to match an pcb antenna of a small embedded µC
    controlled RF-system. The frequency is 433MHz, matching target is 50
    Ohm, antenna is very small, about lambda/10. Therefore the antenna
    radiation resistance is very low, about 1 Ohm, and it behaves
    capacitive. Let's say we have a complex Z=1-j60 Ohm. Now I can calculate
    or use a smith chart to get a matching network. Simulation with RFSIM
    showed that calculation is correct, but if I measure the pcb with
    antenna and matching network with an HP spectrum analyzer, I don't get
    50 Ohm. What goes wrong? Do I have to consider a more complex physical
    model of the used inductors and capacitances with its resonant
    frequencies, Q etc.? Any hints?
    Thanks!

    Regards,
    Andreas
     
  2. What are you expecting to see with a spectrum analyser??
    This sounds like a job for those knowledgeable folks at
    rec.radio.amateur.antenna (added herewith).
     
  3. It's quite normal.
    There are probably just one or two 'stray' components that are causing
    enough difference to be noticed. What impedance do you measure with your
    SA? And how do you measure it? An SA is not the best instrument,
    normally, for that job.

    You can tune out any residual reactance by tweaking the inductor value
    in your matching network. If the resistive part is wrong, you need to
    re-scale the network values. This is usually a lot easier than trying to
    track down strays, which you often have to live with anyway.

    What bandwidth do you need? Antennas and matching networks are often
    made broader in bandwidth than the signal requires, because they may be
    detuned by nearby objects.
     
  4. Andrey

    Andrey Guest

    And how do you measure it? An SA is not the best instrument,
    With Spectrum Analyzer one can measure return loss, if there is a sweep
    generator output in it. This method requires also directional coupler of
    some sort, sometimes known as bridge. This method does not resolve the
    impedance, only allows to make a judgement if the antenna is tuned
    propoerly.

    Regards,

    Andrey
     
  5. Kevin Kilzer

    Kevin Kilzer Guest

    It would help if we knew the answer you found. Is it close? How did
    you couple the analyzer? Where is the analyzer's calibration plane?

    Can you measure the SWR using the intended driver (I assume it is a
    CMOS gate driving the matching network)? Is the radiated power what
    you expect?
    You need to consider a model as complex as the problem. We cannot
    judge that without seeing the test rig.

    Kevin
     
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