What visual error is expected from the Night Effect Error on an ADF?

The Night Effect refers to the phenomenon where the performance of an Automatic Direction Finder (ADF) can be significantly affected during nighttime conditions. During the night, radio waves, especially those below 2 MHz, can reflect off the ionosphere and cause variances in signal reception. This can lead to erratic readings from the ADF, manifested as the bearing needle swinging unpredictably.

When the ADF experiences such reflections during nighttime, the signal may seem to come from multiple sources, resulting in the navigation display showing rapid changes in direction. This behavior is a consequence of interference and refraction in the ionosphere, creating instability in the signal being received.

In contrast to the other options, which suggest stable readings, consistent signal strength, or vertical needle oscillation, the erratic swings of the bearing needle capture the essence of how the Night Effect impacts ADF performance effectively, making it the correct answer.