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A directional coupler is a four-port component with directional transmission characteristics that is contacted by a coupling device together with two pairs of transmission systems. Firstly, the performance index of the directional coupler is introduced, then the waveguide two-hole directional coupler, the double-branch directional coupler and the parallel coupled microstrip directional coupler are introduced.
1) directional coupler performance indicators
The directional coupler is a four-port network. Port "1" is the input, port "2" is the through output, port "3" is the coupled output, port "4" is the isolated terminal, and its scattering matrix is [S ]. The performance metrics describing the directional coupler are: coupling degree, isolation, orientation, input standing wave ratio, and operating bandwidth. The following are introduced separately.