@ret the keyword probably is circulator. It's a device with three equal ports, that forwards RF signals only to one of the neighboring ports, around the circle.
A signal going in port A comes out on B, going into B out of C, and into C comes out in A.

Can be used to join a receiver, transmitter, and antenna, so that the signal from the transmitter goes to the antenna only, the signal from the antenna goes to the receiver only (and the receiver doesn't output a signal, so nothing goes into the transmitter).
Nothing smart inside there, apart from the genius engineering itself. It doesn't produce a signal in itself, and is not particularly "dangerous" to import.

[Based on the pic in the thread, link also doesn't work for me]