13-09-2017, 10:17 AM
Directional couplers are four-port circuits where a port is isolated from the input port. Directional couplers are passive reciprocal networks, on which you can read more about our basic network theory. The four ports are (ideally) even, and the circuit is (ideally) lossless. The directional couplers can be made in microstrip, stripline, coaxial and waveguide. They are used to sample 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 of wavelength or multiple 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 built as well. These couplers do not rely on the interaction of fields and waves between transmission structures to create performance similar to that of coupled line structures, but are based on networks composed of independent (unbound) circuit elements.