Impedance Matching Help

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

Dec 29, 2010
156
2
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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1,850
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)?

Dec 29, 2010
156
2
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?

Dec 29, 2010
156
2
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!