However, [[RF.Spice]] also offers a number of other transmission line models specifically intended for use in RF circuit analysis. These include the generic T-Line, the generic coupled T-lines, and a variety of physical transmission line models.
=== The Generic T-Line ===
[[File:tline.png|thumb|300px| The schematic symbol of the Generic T-Line device.]]
{{Note | Proper grounding of the transmission line devices is critical for a successful simulation.}}
[[File:tline2.png|thumb|350px400px| The schematic symbols of the Generic Open Stub (left) and Generic Short Stub (right) devices.]]
=== Open and Short Stubs ===
The Generic Open Stub and Generic Short Stub are two one-port devices based on the Generic T-Line device. They represent terminated generic transmission line segments. In the open stub case, the termination load is Z<sub>L</sub> → ∞, and in the short stub case, the termination load is Z<sub>L</sub> = 0. The [[parameters]] of both open and short stub devices are the same as those of the generic T-line device.
[[File:tline3.png|thumb|250px300px| The schematic symbol of the Generic Coupled T-Lines device.]]
=== The Generic Coupled T-Lines ===
Many passive RF devices such as directional couplers, hybrids and some filter designs involve segments of parallel coupled [[Transmission Lines|transmission lines]]. According to the coupled mode theory, one can define even and odd mode impedances for such [[Transmission Lines|transmission lines]]. The resulting RF structure can be modeled as a four-port network device as shown in the opposite figure. Note that the four-port device has eight pins. Ports 1 and 2 correspond to the input and output of the first transmission line segment, while Ports 3 and 4 correspond to the input and output of the second (coupled) line segment. It is very important to connect and ground the negative pins at the input and output of the two transmission line segments.