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T or Pi equivalent model of Transmission Line, wrong!?

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stevenjian

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pi model transmission lines

Recently , I found an interesting question that the T or PI model (Derived from ABCD parameter) of A "Single" Transmission Line shows that there are transmission zeros when the electrical length goes to: say.. 180 degree or so (ex: arms of T MODEL is Z11-Z12=jZoTan(Θ/2) -> ∞ like "open"), BUT HOW? IS there anything wrong about the model?

Thanks for advance
 

pi and t model transmission

These models are only accurate for frequencies low enough that there is only a few degrees of phase shift through the line. To make the model accurate at higher frequencies the number of pi or T sections must be increased.
 

transmission line pi model

In the limit the sections become very small (infenitesimally small) and the number of sections becomes infinite, so that one T/PI section always is small compared to the wavelength it represents. This model is derived from the telegrapher equations. Refer to microwave engineering -pozar.
 
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    hilmi

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t and pi equivalents

Uh......I understand that the transmission line is a distributed element.The point is :If each element goes to "short" or "open" , why the whole circuit does't ? ^^a
 

transmission line+t+model

This is clearly because you started from Z or Y parameters, which by definition have a open/short at in/output. With a TL length of 180 degrees, this open/short will be converted to another open/short (refer to smith chart). Nothing unusual happening.
 

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