Directional couplers are circuits where each of their four ports is separated from the input port and are passive reciprocal networks. All four of the couplers’ ports should be matched, and the circuit should not experience loss.
There are various types of couplers, including the following:
▪ Coupled-line couplers
▪ Bethe-hole couplers
▪ Multi-hole couplers
▪ Bi-directional couplers
▪ Dual-directional couplers
▪ Hybrid couplers
Directional couplers are used to sample a signal, sometimes both for the reflected and incident waves (an application that is referred to as a reflectometer, which is an essential component of a network analyzer). Directional couplers typically make use of microwave circuit distributed properties. The coupling process typically happens within one—quarter-wavelength or many quarter wavelength components of the device.
Within these distributed couplers, interaction takes place between the energy and fields of each distributed structure, and therefore couples signals from one to the other. Hybrid and lumped element couplers can be also be constructed. These couplers don’t rely on how waves and fields between transmission structures interact in order to generate performance that is similar to coupled line structures. Rather, they depend on networks that consist of un-coupled – or independent – circuit elements.
The term “directional” in directional couplers simply refers to the fact that the coupler has four ports, of which one is considered the input, one is considered the “through” port, one is considered to be the “coupled” port, and one is considered to be the “isolated” port, which is typically terminated. Certain commercial couplers have a fourth port that is terminated with a matched load.
When the signal is reversed and enters the “through” port, the majority of it will exit the “input” port. However, the coupled port would then be considered to be the port that was previously referred to as the “isolated” port.
The majority of waveguide couplers couple in a forward direction since they depend on many coupling holes. Stripline or microstrip couplers are considered backward wave couplers since they depend on coupled lines.
The coupled port on a stripline or microstrip directional coupler is nearest to the input port since it is considered backward wave. On waveguide broadwall directional couplers, the coupled ports are nearest to the output port since they are forward wave couplers.
An important point to consider is the fact that a forward coupler is considered in-phase, while a backward coupler couples in quadrature. A coupler that is made in stripline means it’s a backward wave coupler.
Hopefully, this elementary guide on directional couplers has been helpful!