Directional Coupler Measurements

A directional coupler is a four-port network in which portions of the forward and reverse traveling waves on a line are separately coupled to two of the ports. Its properties may be summarized with the aid of the schematic diagram in Fig.-1 with matched terminations on the output ports.

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1. A portion of the wave traveling from port 1 (input) to port 4 (output) is coupled to port 3 (forward coupled), but not to port 2.
2. A portion of the reflected wave traveling from port 4 to port 1 is coupled to port 2, but not port 3.
3. A portion of the wave incident on port 2 to port 3 is coupled to port 4, but not port 1; and a portion of the wave incident on port 3 is coupled to port 1, but not port 4.

The performance of a directional coupler is usually described in terms of its coupling and directivity, which are defined in the following manner.

Coupling - coupling Factor or coupling is defined as the ratio of the power (incident) at input of the main line to the power coupled at the output in the auxiliary arm. Thus coupling is a measure of how much of the incident power is being sampled.
Directivity - Directivity of a directional coupler is defined as the ratio of the power output in the coupled auxiliary arm to the power flowing in uncoupled auxiliary arm. Theoretically directivity should be infinite because no power should couple in arm 4.

Main Parameter Measurement
Coupling Power
The ratio of the power that the coupled output port is sampled from the input port when all ports are terminated with reflection less terminations. The nominal coupling is specified as the average of the maximum and the minimum coupling within the frequency band.
  • Connect the coupler to the Network Analyzer as indicated in Fig 2.
  • Its measured b/w the input and coupled port of the coupler when the output port is terminated.
  • The Measurement Values can be read at S21.
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Insertion Loss
The net unrecoverable power in dB dissipated within the circuit at any frequency within the specified rang.
  • Connect the coupler to the Network Analyzer as indicated in Fig 3.
  • Its measured b/w the input and output port of the coupler when the coupled port is terminated.
  • The Measurement Values can be read at S21.
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Fig. 3 Setup for Insertion Loss Measurement
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VSWR
The SWR will remain constant for any orientation of the coupler in the transmission line. The standing wave ratio specified for either the main line path (input to output) and/or the coupled output port(s).
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Fig. 4 Setup forAuxiliary line VSWR Measurement Fig. 4.1 Setup for Main Line VSWR Measurement

Main Line SWR:- The main line SWR is the standing wave ratio measured looking into the main line input terminals when matched loads are placed at all the other terminals.
  • Connect the coupler to the Network Analyzer as indicated in Fig 4.
  • Its measured b/w the input and output port of the coupler when the coupled port is terminated.
  • The Measurement Values can be read at S11.
Auxiliary Line SWR: The auxiliary line SWR is the standing wave ratio measured in the auxiliary line looking into the output terminal to which the detector is normally connected when matched loads are placed at all the other ports or terminals.
  • Connect the coupler to the Network Analyzer as indicated in Fig 4.1.
  • Its measured b/w the input and coupled port of the coupler when the output port is terminated.
  • The Measurement Values can be read at S11.
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Isolation
Isolation is measured between the output port and coupled port of the directional coupler to ensure the quality of internal circuit and design perfection.
  • Connect the coupler to the Network Analyzer as indicated in Fig 5.
  • Its measured b/w the input and coupled port of the coupler when the input port is terminated.
  • The Measurement Values can be read at S21.
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