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A circulator is usually a three- or four-port device in which power entering one port is transferred to the next port in rotation, as if round a circle. Power can only flow in one direction around the circle (clockwise or anticlockwise), and no power is transferred to any of the other ports. Most distributed element circulators are based on [[Ferrite (magnet)|ferrite]] materials.<ref>{{multiref|Sharma, pp. 175–176|Linkhart, p. 29}}</ref> Uses of circulators include as an [[Isolator (microwave)|isolator]] to protect a transmitter (or other equipment) from damage due to reflections from the antenna, and as a [[duplexer]] connecting the antenna, transmitter and receiver of a radio system.<ref>{{multiref|Meikle, p. 91|Lacomme ''et al.'', pp. 6–7}}</ref>
An unusual application of a circulator is in a [[reflection amplifier]], where the [[negative resistance]] of a [[Gunn diode]] is used to reflect back more power than it received. The circulator is used to direct the input and output power flows to separate ports.<ref>Roer, pp. 255–256</ref>
Passive circuits, both lumped and distributed, are nearly always [[Reciprocity (network theory)|reciprocal]]; however, circulators are an exception. There are several equivalent ways to define or represent reciprocity. A convenient one for circuits at microwave frequencies (where distributed element circuits are used) is in terms of their [[S-parameters]]. A reciprocal circuit will have an S-parameter matrix, [''S''], which is [[Symmetric matrix|symmetric]]. From the definition of a circulator, it is clear that this will not be the case,
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