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

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.

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.

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

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.