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How Would You Plot a Resistor on a Smith Chart? - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


mmaassel
Post subject: How would you plot a resistor on a Smith Chart? Posted: Wed Aug 08, 2007 10:32 am

Colonel


Joined: Mon Apr 10, 2006 12:43 pm
Posts: 25
Good day. I am having a brain freeze today.

How would you plot a resistor (an ideal 100 ohm resistor) on a Smith Chart?

Thank you for your time and help.

Michael


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IR
Post subject: Posted: Wed Aug 08, 2007 11:08 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Michael,

You should plot the resistance on the horizontal axis, at the point where the circle of normalized resistance R=2 intersects the horizontal axis. This gives the impedance of Zo=100+j0 - pure 100 ohm resistor.


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mmaassel
Post subject: How would you plot a resistor on a Smith Chart?Posted: Thu Aug 09, 2007 11:51 am

Colonel


Joined: Mon Apr 10, 2006 12:43 pm
Posts: 25
Good morning. IR - Thank you for your response. In reading your answer (and my question) I realized that I did not ask the correct question. So let me try again.

1) I have a network that gives me a complex impedance.
2) I now add a series (or parallel) resistor to the network
3) How does the series/parallel resistor move the complex impedance on the Smith Chart? Assume that the resistor is ideal.

Thank you for your time and help.

Michael


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IR
Post subject: Posted: Thu Aug 09, 2007 12:45 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Michael,

In general if you add a resistance you move along the arcs of the Smith Chart, since the reacetnace remains constanct. If you have a series resistor it will move your complex impedance along the corresponding arc towards infinity (If your increase your resistance), i.e. on the Smith Chart you will move towards the right side, vice versa if you decrease your series resistor value.

For Parallel resistance it is better to use the Smith Chart for admittances, which is a mirror view of the ''regular'' Smith Chart for Impedances. In this Smith Chart the infinity is located at the left side of the chart. Therefore, if you increase the parallel resistance value you will move to the left side of the Chart on the corresponding arc.



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