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Practical impedance matching

  1. Jun 21, 2005 #1

    I have a few 50 ohm loop antennas tuned to resonate at 13.56 MHz. (inductive RFID antenna). I would like to test one antenna´s influence on the other, so I think one approach is to connect one antenna directly to a signal generator.
    Do I have to design any matching circuit between the antenna and the signal generator?

    any help is appreciated

  2. jcsd
  3. Jun 21, 2005 #2


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    Staff: Mentor

    How did you make a 50 Ohm loop antenna? My Stutzman and Theile book on Antennas says, "The input resistance for a one-wavelength perimeter [loop antenna] is about 100 Ohms. Other perimeter values give rather awkward input impedances." The only places where the input impedance of the loop antenna is real looks to be around 1.1 wavelength and 1.5 wavelength (for 100 Ohms and about 1.5kOhms respectively).

    Also, the loop antenna has a balanced input impedance (the two conductors at the input are symmetrical), so when you go from the unbalanced (single-ended) output of the signal generator into the loop antenna, you need to use a balun to do the conversion. So assuming that your loops are full-wave loops, you will need a balun to do the 50 Ohm single-ended to 100 Ohm differential conversion when going from your signal generator to the antenna. You'll also need the same type of balun when going from your 2nd loop antenna into your 50 Ohm spectrum analyzer or other 50 Ohm receiver.

    Hope that helps. You can look up baluns and how to make them in the ARRL Handbook or other antenna/RF books. -Mike-
  4. Jun 22, 2005 #3
    hi and thanks,

    Does it make a difference that I work with inductive loops only? (I´m only interested in the magnetic field produced by the loop)
    I didn´t make the loop myself...as far as I know, most commercially available inductive antennas for RFID are 50 ohm antennas. And I don´t know if the relation to the wavelength matters for those antennas either, as I work with antennas between 1 - 20 cm in diameter which is only a fraction of the wavelength of 3.53 m at 13.56 MHz.
    Do you still think I need to use a balun?

  5. Jun 22, 2005 #4


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    Staff: Mentor

    Well if they're listed as having a 50 Ohm input impedance at their operating frequency, then the manufacturers must be adding a balun network to the input of the "antennas". As long as you connect 50 Ohm sources (sig gen) and loads (spectrum analyzer), then you should be okay. When the loop isn't a full wave in diameter, it won't be very efficient for transmit or receive, but if the application is short range in nature, like some RFID applications, then that won't matter.
  6. Jun 25, 2005 #5
    I hope you don't mind me asking if you are trying to pump in a signal at the actual frequency (13.5Mhz) which seems unecessary, since it was designed to resonate at that frequency. i.e., why not pump in any lower frequency in which 13.5 Mhz would be a strong harmonic.
    The reason I'm asking is because the test leads and lines would seem to need to be extremely short and very low capacitance so as not to influence your experiment, and I am thinking this might not be a problem if you could use a lower frequency and get the same result (stimulation at the desired frequency).
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