microwave101，干货比较多
传送门：https://www.microwaves101.com/encyclopedias/directional-couplers
Directional couplers are four-port circuits where one port is isolated from the input port. Directional couplers are passive reciprocal networks, which you can read more about on our page on basic network theory. All four ports are (ideally) matched, and the circuit is (ideally) lossless. Directional couplers can be realized in microstrip, stripline, coax and waveguide. They are used for sampling a signal, sometimes both the incident and reflected waves (this application is called a reflectometer, which is an important part of a network analyzer). Directional couplers generally use the distributed properties of microwave circuits. The coupling process generally occurs within a quarter-wavelength or multiple quarter-wavelength portions of the device. In these distributed couplers the energy and fields of one structure interact with the energy and fields of another distributed structure, coupling signals from one to another. Lumped element and hybrid couplers can be constructed as well. These couplers do not rely on the interaction of the fields and waves between transmission structures to create performance similar to that of coupled line structures but instead rely on networks composed of independent (un-coupled) circuit elements.
What do we mean by "directional"? A directional coupler has four ports, where one is regarded as the input, one is regarded as the "through" port (where most of the incident signal exits), one is regarded as the "coupled" port (where a fixed fraction of the input signal appears, usually expressed in dB), and one is regarded as the "isolated" port, which is usually terminated. On some commercial couplers the fourth (isolated) port is terminated internally or exterminally with a matched load - making a four port device seem like a three port device. If the signal is reversed so that it enter the "through" port, most of it exits the "input" port, but the coupled port is now the port that was previously regarded as the "isolated port". The coupled port is a function of which port is the incident port.
定向耦合器 ，盗图
wiki：https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers
wiki上是将功分器与定向耦合器放在一起介绍了，为啥？