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A directional coupler which splits power equally between the output and coupled ports (a {{nowrap|3 [[decibel|dB]]}} coupler) is called a ''hybrid''.<ref>Maloratsky, p. 117.</ref> Although "hybrid" originally referred to a [[hybrid transformer]] (a lumped device used in telephones), it now has a broader meaning. A widely-used distributed element hybrid which does not use coupled lines is the ''hybrid ring'' or [[rat-race coupler]]. Each of its four ports is connected to a ring of transmission line at a different point. Waves travel in opposite directions around the ring, setting up [[standing wave]]s. At some points on the ring, destructive [[wave interference|interference]] results in a null; no power will leave a port set at that point. At other points, constructive interference maximises the power transferred.<ref>Chang & Hsieh, pp. 197–198.</ref>
Another use for a hybrid coupler is to produce the sum and difference of two signals. In the illustration, two input signals are fed into the ports marked 1 and 2. The sum of the two signals appears at the port marked Σ, and the difference at the port marked Δ.<ref>Ghione & Pirola, pp. 172–173.</ref> In addition to their uses as couplers and power dividers, directional couplers can be used in [[balanced mixer]]s, [[frequency discriminator]]s, [[Attenuator (electronics)|attenuator]]s, [[phase shifter]]s, and [[antenna array]] [[antenna feed|feed]] networks.<ref>{{multiref|Chang & Hsieh, p. 227.|Maloratsky, p. 117.}}
=== Circulator ===
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