Idea Transmission line in ADS: single ended, why ?

Hi,
Just started learning RFCD (and I will be doing PhD in this area, too !).
Books say that Transmission-line is a conductor pair. And this confuses me alot when I see
a single ended idea transmission line (TLIN) in agilent ADS. Kindly help me understand why and how they differ?
Thanks.

Search this question :Differential versus Single ended..
There are advantages by using differential circuits.You'll find the answers..

The model includes already the return path, it is an ideal ground connection! It is also possible to use circuit elements in ADS with return path! Consider the help function of ADS.

When you use Strip lines / TL you will have to put the substrate information on the MSUB. This defines an
assumed return path on the ground plane.
Just go make a 1Lamda Zo TL in the schematic then send it to Momentum(Layout) and see how they
defined that circuit/PCB. This should help.

Thanks All, especially to JohnJoe. Now, I undertand the point.