The power transmitted in the main line is coupled to the secondary line through a variety of ways, and interferes with each other and only transmits in one direction in the secondary line.

Figure 1 shows three typical coupling structures of rectangular waveguide directional couplers. a is the two-hole coupling separated by 1/4 guide wavelength; b is the double series branch line coupling with the spacing and length equal to 1/4 guide wavelength; c is the continuous coupling of TE and TE propagation modes in the crack area. Take the two structures of a and b as examples. After the signal input from port ① is coupled to the secondary line in two ways, it is superimposed in phase due to the same stroke in the direction of port ④, and there is output; in the direction of ③, the stroke differs by 1/2 guide wavelength The reverse phase cancellation is isolated without output.

Figure 2 shows two typical coupling structures of the microstrip directional coupler. A is the dual parallel branch line coupling with the pitch and length equal to 1/4 of the lead wavelength, and b is the continuous coupling of the electric and magnetic fields in the parallel region. Taking the structure of b as an example, the signal input from port ① is coupled to the two ports of the secondary line by electric field to generate in-phase induced voltage, and magnetic field coupling generates reverse-phase induced voltage. The result is added at port ④ and there is output, and at ③, it is cancelled and there is no output in isolation.

Figure 1

Figure 2

Figure 3