What frequency type is associated with pronounced ADF 'Night Effect' error?

The pronounced ADF 'Night Effect' error is primarily associated with lower frequencies. This phenomenon occurs because at night, the ionosphere becomes more conductive, which affects the propagation of radio waves. Lower frequency signals (such as those in the LF range) tend to reflect off the ionosphere more than higher frequencies, leading to increased interference and errors in navigation.

At night, ADF receivers picking up these lower frequencies may experience navigation errors due to the combination of ground reflection and sky-wave propagation, which can significantly distort the bearing information received by the aircraft.

Higher frequencies, medium frequencies, and ultra-high frequencies typically do not exhibit the same level of distortion or error due to night propagation characteristics since their interaction with the ionosphere and ground tends to differ from that of lower frequencies. Thus, the correct answer effectively identifies the frequency type prone to these specific nighttime errors.