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A directional coupler is a four-port device which couples power flowing in one direction from one path to another. Two of the ports are the input and output ports of the main line. A portion of the power entering the input port is coupled to a third port, known as the ''coupled port''. None of the power entering the input port is coupled to the fourth port, usually known as the ''isolated port''. For power flowing in the reverse direction and entering the output port, a reciprocal situation occurs; some power is coupled to the isolated port, but none is coupled to the coupled port.<ref>Sisodia & Raghuvansh, p. 70</ref>
A power divider is often constructed as a directional coupler, with the isolated port permanently terminated in a matched load (making it effectively a three-port device). There is no essential difference between the two devices. The term ''directional coupler'' is usually used when the coupling factor (the proportion of power reaching the coupled port) is low, and ''power divider'' when the coupling factor is high. A power combiner is simply a power splitter used in reverse. In distributed-element implementations using coupled lines, indirectly coupled lines are more suitable for low-coupling directional couplers; directly
Distributed-element designs rely on an element length of one-quarter wavelength (or some other length); this will hold true at only one frequency. Simple designs, therefore, have a limited [[Bandwidth (signal processing)|bandwidth]] over which they will work successfully. Like impedance matching networks, a wide-band design requires multiple sections and the design begins to resemble a filter.<ref>Bhat & Khoul, pp. 622–627</ref>
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