As I understand it, you are looking for a block for modeling transmission lines defined by "Characteristic Impedance" and length parameters. Under the "Circuit Envelope" section of the "RF Blockset," you can find the "Transmission Line" block, which supports "Characteristic Impedance" and other physical length parameters. Here is the link to the documentation for the block:
Additionally, here is an example of a "Delay-Based Transmission Line" that demonstrates how to use the block:
Hope this helps!
