Adjust the depth of penetration of the probe into the slotted - line at both attenuators to obtain full - scale deflection (fsd).
Use the Slotted - line Detector to find two successive positions at which the signal reaches a sharp null. Note each position carefully against the scale on the waveguide.
These nulls arise because the signal from the microwave oscillator travels along the guide to the short - circuited end. Its energy cannot be absorbed by the short - circuit, so the signal is reflected back.