In WRspice, the transmission line element represents a general lossless or lossy transmission line. There are actually three historical models unified in the WRspice model: the SPICE3 lossless transmission line, the SPICE3 lossy (LTRA) transmission line convolution approach of Roychowdhury and Pederson , and the Pade approximation lossy line approach of Lin and Kuh .
The device line is keyed by the letters `t' and `o' equivalently, as above. In SPICE3, `o' calls the lossy convolution model, but this is not necessarily the case in WRspice. One can enforce use of the convolution model by using ``level=2'' in the device or model line, the default (``level=1'') is the Pade approximation model. In the lossless case, the level parameter has no effect.
Above, n1 and n2 are the nodes at port 1, n3 and n4 are the nodes at port 2. Note that this element models only one propagating mode. If all four nodes are distinct in the actual circuit, then two modes may be excited. To simulate such a situation, two transmission line elements are required.
There is a fairly lengthly list of parameters which can be applied in the device line, or in a model. If a model is referenced in the element line, the element defaults to the parameters specified in the model, though any of these parameters can be overridden for the element if given new values in the element line.