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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.

Directional Coupler Indicating Powers

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 CC

The Coupling factor of a directional coupler is the ratio of incident power to the forward power, measured in dB.

C=10log10PiPfdBC=10log10PiPfdB

Directivity DD

The Directivity of a directional coupler is the ratio of forward power to the back power, measured in dB.

D=10log10PfPbdBD=10log10PfPbdB

Isolation

It defines the directive properties of a directional coupler. It is the ratio of incident power to the back power, measured in dB.

I=10log10PiPbdBI=10log10PiPbdB

Isolation in dB = Coupling factor + Directivity

Two-Hole Directional Coupler

This is a directional coupler with same main and auxiliary waveguides, but with two small holes that are common between them. These holes are λg/4λg/4 distance apart where λg is the guide wavelength. The following figure shows the image of a two-hole directional coupler.

Two-hole Directional Coupler

A two-hole directional coupler is designed to meet the ideal requirement of directional coupler, which is to avoid back power. Some of the power while travelling between Port 1 and Port 2, escapes through the holes 1 and 2.

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