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They are not used very often anymore. People find them too expensive, and try to do the same thing other ways.
Lets say you were transmitting and receiving simultaneously with the same antenna. You could hook up the transmitter to circulator port 1, hook the antenna to circulator port 2, and the receiver to circulator port 3. Transmit power would go out the antenna, and power received would go into the receiver front end.
One, of course, has to make sure the receiver is not blown up by the transmitter, or that the broadband transmit noise does not kill the receiver noise figure.
A intuitive way to understand the function and name of the device that I find helpful is to imagine a disk with three ports. Signals entering at any port travel clockwise (or counter clockwise) to the next port. It is also why a good match at all ports is an issue. If a port is poorly matched (or connected to a poorly matched device) the reflection will appear at the next port compromising the isolation of the circulator. This is particularly important when a sensitive receiver and a powerful transmitter are both connected to an antenna.
Circulators are still used at shared signal sites with dozens or more transmitters within a small patch of ground. This greatly reduces IMD problems that would put spectral components at the frequency a receiver is tuned to.
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