The coupler is a quadrature hybrid coupler with a 90 ° phase difference between the output ports (port 2 and port 3). Design of microstrip circuit of coupler. In order to achieve strong coupling, four connected coupling lines are used here. The coupling line is easy to realize strong coupling, and the bandwidth is more than an octave. The difficulty lies in the fact that it is more difficult to handle, mainly because the line is relatively narrow and the gap is small, and it is difficult to realize the cross line connection. Therefore, the original coupler has been expanded, as shown in the figure below.

 

The design of the deployable coupler is slightly simpler, but the principle is the same as that of the prototype. The expanded coupler can be simulated by equivalent circuit. It consists of four wires, all of which have the same line width and gap. If we assume that each conductor is only coupled with adjacent conductors, while ignoring the coupling of far conductors, we can expand its equivalent circuit to two parallel coupled conductors. Then, you can use the coupling line analysis method to analyze the coupler. The directional coupling characteristics of all directional couplers are generated by superposition of two or more waves or wave components at the coupling port and cancellation at the isolated port. For the branch directional coupler shown in the above figure, the length and spacing of the branch lines are quarter wavelength.

For greater bandwidth, multiple λ/  4 Coupling section. The design of the coupler is roughly the same as that of the distributed element filter. Each part of the coupler is considered as part of the filter. By adjusting the coupling coefficient of each part, the coupling port can have any classical filter response, such as a ripple (elliptic function) filter such as the maximum flatness (Butterworth filter) or a specified ripple (Chebyshev filter) response. Ripple filter is the maximum change of coupling port output in passband, and the nominal coupling factor is usually used as positive or negative dB value.

There are many types of couplers, but two common couplers are branch line type and coupling line type. There is also a Lange coupler, a microstrip directional coupler for tight coupling.