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A 'circulator' is a passive electronic component with three or more ports in which the ports can be accessed in such an order that when a signal is fed into any port it is transferred to the next port (only), the first port being counted as following the last in numeric order. When one port of a three port circulator is terminated in a matched load, it can be used as an 'isolator', since a signal can travel in only one direction between the remaining ports.^[1]
There are circulators for VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of propagating waves in a magnetized material, and 3-port "turnstile" or "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetized material. Waveguide circulators may be of either type, while more compact devices based on stripline cables are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behavior of a true 4-port circulator.
In radar, circulators are used to route outgoing and incoming signals between the antenna, the transmitter and the receiver. In a simple system, this function could be performed by a switch that alternates connecting the antenna to the transmitter and the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulse, however, requiring a circulator for this function.
Radio frequency circulators are composed of magnetized ferrite materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.
There have also been investigations into making "active circulators" which use transistors instead of ferrites. However, the power handling capability and linearity and signal to noise ratio of the latter is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies. Because both sender and receiver of a radar or any communication system consist of transistors (or vacuum tubes) the transistor based circulator merely means, that the sent signal is subtracted from the received signal. Nevertheless most communication systems are simplex and duplex systems are composed of two simplex systems by means of frequency division multiplexing or time division multiplexing.

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References

Notes

1. For a description of a circulator, see US patent for circulator. This US Patent and Trademark Office site sometimes fails to serve-up images. If the link does not work, please try again later

References

★ C. L. Hogan, "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications" Rev. Mod. Phys. 25, 253–262 (1953)

★ E. A. Ohm, "A Broadband Microwave Circulator", IRE Trans. on Microwave Theory and Techniques, MTT-4 210-217 (1956)

★ H. N. Chait & T. R. Curry, "Y-Circulator," J. Appl. Phys., Suppl. to 30, 1525 and 1535 (1959)

★ H. Bosma, "On Stripline Y-Circulation at UHF", IEEE Trans. Microwave Theory & Techniques V12 N1 61-72 (1964)

★ Federal Standard 1037C and MIL-STD-188