A circulator is a 3-port non-reciprocal microwave device which the ports can be accessed in such a way that when a RF signal is fed into any port it is transferred to the next port only. A 3-port circulator can be used as an isolator when one port is terminated with a matched load, since a signal can travel in only one direction between the remaining ports. The most common use of circulators are for single antenna two-way communcation systems such as radar, where the transmit output is connected to port 1 of the circulator, the antenna to port 2 and the receiver input is connected to port 3.