So, for fun, I entered the circuit for the bidirectional amplifier used in the BitX20, and did some basic simulation. You can see it in the image linked below. I was trying to figure out how to instrument the schematic so I could determine the input and output impedance (and eventually things like return loss as well). Still puzzling over stuff like this. But the circuit is pretty straightfoward. I may have to build this amplifier, just for amusement.

Addendum: If you don’t know what a BitX-20 is, you can find out lots by googling around, perhaps beginning with Farhan’s original page. If you check out his schematic, you’ll find three copies of this bidirectional amplifier.