Impedance Matching Help

Discussion in 'Wireless & RF Design' started by crazyengineer, Feb 25, 2012.

  1. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Hello. So for a term paper, I'm building an RF transmitter. The impedance of my antenna driven at 1meg was 945j. Also, the output impedance of my output stage was 16.2-1.5j. The problem I'm having is trying to create a matching network. I thought I could use a smith chart, but I do not know how to find the normalized impedance since it's ZL/Z0.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    945j is a really crappy antenna. At 1 Mhz the wavelength is 300 meters so I guess you'll have to live with it. The Z0 you should be using is the characteristic impedance of the transmission line. You'll have to match the transmitter to the line and the line to the antenna.
     
  3. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    How does one transmit power via a purely reactive antenna (load)?
     
  4. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Yeah......I got it from Radioshack. But I'll just deal with it.

    The problem is that I have no idea what's the Z0 since everything will be done via breadboard. Should I use a short coaxial cable with the characteristic impedance already defined?
     
  5. crazyengineer

    Thread Starter Member

    Dec 29, 2010
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    Also, I forgot to mention the antenna has negative resistance according to my measurements (-76)
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Interesting ..... Time to review your measurement technique.
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    No need to assume that you will have a transmission line at all! Of course, without a transmission line, a Smith Chart won't do much for you. HOWEVER...the conjugate match theorem tells you that you can match any impedance to any other impedance with a simple L network. HOWEVER....you need to have some REAL component of your antenna impedance. -955j is only the reactive part....you need to know the radiation resistance as well...which may be only a few ohms.

    Eric
     
  8. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The OP measured a negative resistance of -76 ohms. It's going to be a challenge to do the matching!
     
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