Directional Coupler Basics & Selection
Directional Couplers are passive four port devices which are used to couple part of the signal travelling in one transmission line to another port. They are typically used for sampling signals in a system. Directional couplers can be developed using a number of techniques – striplines, lumped element or discrete elements.
Directional Coupler Operation
A directional coupler has four ports –
The power that is passing from the input (port 1) to the output (port 2) is coupled with a coupled line and passes out through the coupled output port (port 3), which is isolated from the main output port (port 4). Any reflected power from the main output is coupled to the port that is terminated, resulting in minimal reflections within the device/system. The power level of the coupled signal is a parameter to be selected based on the requirement.
Directional Coupler Selection
When selecting a directional coupler it is important to know the parameters that you need to specify. Below are a list of the key parameters to look out for when selecting a directional coupler:
Coupling: This is the amount of power transferred to the coupled port (port 3).
Insertion Loss: This is the power lost in the main line of the coupler between port 1 and port 2 (taking into account the power transferred to port 3).
Directivity: Directivity is the measure of isiolation between the coupled (port 3) and isolated port (port 4). It is calculated by measuring the power level difference between port 3 and port 4. The higher the directivity the better the performance.
Isolation: This is the difference is power level between port 1 and port 4.