Question About Transmission Line Model

In network theory they are equivalent; i.e., if they were in a three port (line-in, line-out, ground), fixed frequency black box you would have no way to tell how the elements were interconnected internally, just what the impedance is at the exciting frequency. You could then use basic theory to determine the value of the elements in either model. As a practical matter try drawing two nominal T's in series, notice how you now have a nominal pi, lump the appropriate parameters and eliminate that node in the middle.