Final answer:
ADF 'Night effect' error is more significant with lower frequencies during the twilight hours due to ionospheric changes affecting radio wave propagation.
Step-by-step explanation:
The 'Night effect' error in an Automatic Direction Finder (ADF) is most pronounced with lower frequencies and especially during twilight hours. This phenomenon occurs due to the changes in the ionospheric layers which affect radio wave propagation differently during night and day, with effects becoming more prominent during twilight when these layers are transitioning. At night, especially during twilight, the ionosphere can cause radio waves at lower frequencies to bend unpredictably, leading to navigation errors.