Main applications of RF circulators

　　The transmitter output is connected to port 1, the antenna is connected to port 2, and the receiver is connected to port 3. Therefore, the transmitter power will circulate to the antenna rather than the receiver, and the signal received by the antenna will circulate to the receiver. In this way, the receiver is isolated from the transmitter, but the antenna has power from the transmitter and transmits the received signal to the receiver without any mechanical switching.

　　RF isolator: RF circulators can be used as RF isolators. This is very useful for protecting amplifiers that must operate at high VSWR levels. If directly connected to the antenna in these situations, the power amplifier may be damaged due to high voltage or current levels.

　　Usually, transmitters need to operate over a wide bandwidth, and in these cases, it is impossible to maintain good impedance matching across the entire bandwidth, and destructive levels of VSWR may be observed. To overcome this issue, a loop protector can be used to protect the power amplifier PA from the influence of reflected power.

　　For this, the transmitter is connected to port 1, the antenna is connected to port 2, and port 3 is typically connected to a 50 Ω load - this is necessary. Because the isolation level depends on good impedance matching presented on different ports. If there is poor matching or open circuit, the isolation performance will be compromised.

　　The transmitted power is transferred from port 1 to port 2 and transmitted to the antenna. Any reflected power will return along the feeder and be transmitted from port 2 to port 3, where it can be absorbed in the load. In this way, the RF power amplifier will be able to operate with high line VSWR, but protected by a loopback device acting as an RF isolator.

　　The load on port 3 needs to provide matching impedance: the isolator needs to match impedance on the port to maintain isolation level from that port.

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