The power transmission in the main line is coupled to the secondary line in several ways, and interferes with each other and transmits only in one direction in the secondary line.
Fig. 1 is the three typical coupling structures of a rectangular waveguide directional coupler.
A is a two-hole coupling with a 1/4-guide wavelength, and B is a two-series branch-line coupling with the spacing and length equal to the 1/4-guide wavelength; c is in the crack area Te
and Te Continuous coupling of two propagation modes.
Take A and B two structure as an example, from the port ① input signals are coupled to the secondary line, the direction of the port ④ due to the stroke is equal to the same phase superposition, there is output, in the direction of the ③ is the stroke of 1/2-guide wavelength and reverse cancellation, is isolated without output. Fig. 2 is two typical coupling structures of the microstrip directional coupler. A is a two-parallel branch-line coupling between the spacing and length equal to the 1/4-guide wavelength, and B is the continuous coupling of the electric field and the magnetic field in the parallel region. Taking the structure of B as an example, the signal input from the port ① is coupled with the electric field to generate the same phase induction voltage on two ports of the auxiliary line, and the magnetic field coupling produces the reverse phase induction voltage.
The results are added and output at the port ④, and the ③ is offset and isolated without output. In addition, the directional coupler of other transmission lines can be formed (Fig 3).