Measurement Couplers

The powers of waves travelling in a waveguide in each direction can be sampled by means of directional couplers. From the coupled powers (measured, e.g., by a standard power meter), quantities of interest can be computed. These are, in particular:

• Powers carried by the waves incident on and reflected from the load.

• Net power absorbed in the load.

• Magnitude of the load reflection coefficient.

An ideal coupler:

• Responds only to a wave travelling in one direction.

• Does not affect the sampled field (does not cause additional reflections).

If a coupler is not ideal, it introduces systematic errors in the computed results. The main factor affecting the accuracy with which a load reflection coefficient can be measured is the coupler’s directivity, usually expressed in dB. An ideal coupler has infinite directivity. On the other extreme, when a “coupler” responds equally to waves travelling in both directions, its directivity is 0 dB.

Two couplers are needed to simultaneously sample the waves travelling in both directions. For the purpose of this program, these two couplers are supposed to be:

• Identical.

• Causing no reflections.

• Characterized only by their directivity.

The Measurement Couplers group box enables the user to select the coupler directivity or “make” the coupler ideal. The coupler settings affect only the results displayed in the Power Meas and Load Meas pages of the bottom tabbed container.

Ideal

Check this box to consider the coupler to be ideal. Uncheck it to consider the coupler having a finite directivity.

Directivity

Enter a nonideal coupler’s directivity in dB (or step it by the attached up-down control). The directivity controls are disabled for an ideal coupler.