A directional coupler is a passive device which couples part of the transmission power by a known amount out through another port, often by using two transmission lines set close enough together such that energy passing through one is coupled to the other. Most directional couplers have four ports: input, transmitted, coupled, and isolated. The term "main line" refers to the section between the input and transmitted ports. On some directional couplers, the main line is designed for high power operation (large connectors), while the coupled port may use a small SMA connector. Often the isolated port is terminated with an internal or external matched load (typically 50 ohms). It should be noted that a directional coupler is a linear device. Any port can be the input, which will result in the directly connected port being the transmitted port, adjacent port being the coupled port, and the diagonal port being the isolated port.
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