A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio frequency power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix for an ideal three-port circulator is
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.
There are circulators for LF, 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 waves propagating in a magnetised material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines 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 behaviour from 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 between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function.
Radio frequency circulators are composed of magnetised 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 are based on electronics rather than passive materials. However, the power handling capability and linearity and signal to noise ratio of transistors is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies.
- Bosma, H. (January 1964), "On Stripline Y-Circulation at UHF", IEEE Trans. Microwave Theory & Techniques 12 (1): 61–72, doi:10.1109/TMTT.1964.1125753, ISSN 0018-9480, http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=1125753
- Chait, H. N.; Curry, T. R. (1959), "Y-Circulator", J. Appl. Phys., Suppl. to 30 (4): 1525 and 1535, doi:10.1063/1.2185863, ISSN 0021-8979, http://link.aip.org/link/?JAPIAU/30/S152/1
- Hogan, C. L. (1953), "The Ferromagnetic Faraday Effect at Microwave Frequencies and its Applications", Rev. Mod. Phys. (The American Physical Society) 25 (1): 253–262, doi:10.1103/RevModPhys.25.253, http://prola.aps.org/abstract/RMP/v25/i1/p253_1
- US 3935549, Jachowski, Ronald E., "Ferrite Circulator", issued January 27, 1976
- Ohm, E. A. (1956), "A Broadband Microwave Circulator", IRE Trans. on Microwave Theory and Techniques MTT-4: 210–217, http://ieeexplore.ieee.org/iel6/22/24866/01125064.pdf
- Wenzel, Charles (1991), "Low Frequency Circulator/Isolator Uses No Ferrite or Magnet", RF Design Magazine, http://www.wenzel.com/pdffiles1/pdfs/RFDesign3.pdf
- Circulators and Isolators 
- Federal Standard 1037C
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