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, but not to Port 2.
· Port 1 and 3 are decoupled as are Port 2 and Port 4.
Ideally, the output of Port 3 should be zero. However, practically, a small amount of power called back power is observed at Port 3. The following figure indicates the power flow in a directional coupler.
Where
· PiPi = Incident power at Port 1
· PrPr = Received power at Port 2
· PfPf = Forward coupled power at Port 4
· PbPb = Back power at Port 3
Following are the parameters used to define the performance of a directional coupler.
Coupling Factor (C)
The Coupling factor of a directional coupler is the ratio of incident power to the forward power, measured in dB.
Directivity (D)
The Directivity of a directional coupler is the ratio of forward power to the back power, measured in dB.
Isolation
It defines the directive properties of a directional coupler. It is the ratio of incident power to the back power, measured in dB.
