Interesting article from Mini-Circuits Blog:
Directional couplers are an important type of signal processing device. Their basic function is to sample RF signals at a predetermined degree of coupling, with high isolation between the signal ports and the sampled ports — which supports analysis, measurement and processing for many applications. Since they are passive devices, they also operate in the reverse direction, with signals injected into the main path according to the devices’ directionality and degree of coupling. There are a few variations in the configuration of directional couplers, as we shall see below.
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