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Impedance matching with different Zo transmission lines

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DoYouLinux

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Hi all,

I am just wondering. If I want to transform an impedance using a transmission line, it means that I will traverse on a Smith chart clockwise respecting the lenght of the line.

In case that I use more transmission line with different characteristic impedances to do the matching, how do I traverse on the Smith chart ? If you can suggest me some documents explaining about this, would be great.

Thanks a lot,

DYL
 

There is a rule, cascading transmission lines always follow a clockwise rotation on the Smith Chart, and if the chart is normalized to the characteristic impedance of the transmission line, the rotation is along a concentric circle.
Read this, Chapter 5:
--Practical RF Circuit Design for Modern Wireless Systems, Vol. I – L. Besser, R. Gilmore--
 
Hi, vfone,

Thanks a lot for your explanation. Yes, I know this rule. However, will the trajectory of rotation in the Smith chart be the same if the characteristic impedance of the line changes from section to section ?

DYL
 

Smith chart is a normalized plot wrt characteristic imp. of the line. In order not to confuse,
1) Before you cascade a new line having different characteristic imp. denormalize the load seen by the new line.
2) Normalize it wrt to new characteristic imp.
3) Mark the new normalized impedance on the chart,
4) Then, you can go on safely.

See powerful smith chart tools on Transmission Lines Interactive Java Applets
 

The trajectory will follow the same clockwise rotation also if the characteristic impedance of the cascaded lines is different.
The best to understand this behavior is to get WinSmith 2.0 and play with it.
 

Hmmmm .... let me think ....

I really need to see some examples ...
 
Impedance matching by inserting a transmission line with different Zo is not much different compared to conventional stub matching.
A special case is for quarter wave length, were the inserted impedance can be seen as a mirroring point in a smith diagram as Zx= √(Zin•Zout)
General formula: Antenna Tutorials - Quarter Wave Transformer
 

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