What waveguide component has a mechanical stop mounted on its base to limit movement to each extremity of travel?

The correct choice is a radio frequency (RF) switch. RF switches are designed to route signals between different paths and typically incorporate mechanical components to facilitate this function. The presence of a mechanical stop mounted on the base of an RF switch serves a crucial role in limiting the movement of the switch mechanism to the extremities of its travel. This ensures that the switch operates within a defined range, preventing damage or malfunction due to over-travel.

In contrast, the other components mentioned do not have this specific mechanical feature. A transmit-receive (TR) tube is primarily used in systems to switch between transmitting and receiving modes but does not feature a stop mechanism for travel. A linear polarizer is a passive device used to filter light waves and does not involve mechanical movement. A circulator, which is used to direct signals in one direction, also does not typically have a mechanical stop for movement. Thus, the characteristics unique to RF switches align perfectly with the question regarding the mechanical stop for limiting movement.