Explain the construction and working of a directional coupler used in microwaves. How does its function differ from an isolator?
A Directional coupler is a device that samples a small amount of Microwave power for measurement purposes. The power measurements include incident power, reflected power, VSWR values, etc.
Directional Coupler is a 4-port waveguide junction consisting of a primary main waveguide and a secondary auxiliary waveguide. The following figure shows the image of a directional coupler.
Directional coupler is used to couple the Microwave power which may be unidirectional or bi-directional.
Properties of Directional Couplers
The properties of an ideal directional coupler are as follows.
All the terminations are matched to the ports.
When the power travels from Port 1 to Port 2, some portion of it gets coupled to Port 4 but not to Port 3.
As it is also a bi-directional coupler, when the power travels from Port 2 to Port 1, some portion of it gets coupled to Port 3 but not to Port 4.
If the power is incident through Port 3, a portion of it is coupled to Port 2, but not to Port 1.
If the power is incident through Port 4, a portion of it is coupled to Port 1,
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