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50 ohm Network Analyzer for 75 ohm HDTV antenna

 
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rtso
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Joined: 03 Apr 2011
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Location: Los Angeles

PostPosted: Mon Apr 04, 2011 4:37 am    Post subject: 50 ohm Network Analyzer for 75 ohm HDTV antenna Reply with quote

If I have a TV antenna that is designed for 75 ohms, what do I have to do in order to get a proper measurement of S11? My Network analyzer is 50 ohms.
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Schubert
Antenna Wizard


Joined: 08 Apr 2009
Posts: 161

PostPosted: Mon Apr 04, 2011 5:46 pm    Post subject: Reply with quote

There might be a setting in your network analyzer.

However, it shouldn't matter much. You'll be able to see the frequency response which will be pretty accurate, and you can tell if the antenna is tuned correctly or radiating.

If you really want to see the difference, make sure you have a vector network analyzer (magnitude and phase). Note that you are measuring the complex reflection coefficient gamma, which is equal to:

Gamma = (Zant - 50)/(Zant + 50)

So the impedance of your antenna is:
Zant = (Gamma + 50)/(Gamma - 50)

Then the new s11=gamma75 is:

Gamma75 = (Zant - 75)/(Zant + 75)

and substitute it all in and solve it.

If you have one of the older scalar network analyzers, I don't think you can get the answer directly. But I don't think you need that kind of accuracy in s11.
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rtso
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PostPosted: Fri Apr 08, 2011 4:21 am    Post subject: Reply with quote

Schubert - That was a great answer. Thank you very much!!
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