Ham Radio Extra Class Practice Test 2025 - Free Extra Class License Exam Questions and Study Guide

In a crossed-ellipse display used for Frequency Shift Keying (FSK) signals, the appearance of ellipses represents the mark and space frequencies. When one of these ellipses suddenly disappears, it typically indicates that selective fading has occurred, which affects the signal propagation conditions.

Selective fading can result from various environmental factors, such as multipath propagation, where the same signal arrives at the receiver via multiple paths causing interference. This can alter the amplitude and phase of the received signals selectively, leading to the disappearance of one of the frequency components (mark or space) from the display.

The other options describe different scenarios that do not specifically lead to the sudden disappearance of one of the ellipses. Filtration saturation would affect the overall quality of signal processing but not typically cause an ellipse to vanish outright. Frequency drift involves the receiver moving away from the desired frequency, which may cause distortion but wouldn't necessarily result in the disappearance of an ellipse. Inversion of the mark and space signals refers to a reversal of the original signaling sequence, which would instead manifest as overlapping ellipses rather than one disappearing entirely.