Non Directional Beacons (NDBs) transmit in the Low Frequency (LF) and Medium Frequency (MF) bands, operating from 190 to 1750 KHz vertically polarized radio signals continuously and in all directions. The radio signal propagates as a surface wave does, which provides quite a big amount of errors on the ADF which may sometimes cause the instrument indication not to be reliable.

The Automatic Direction Finding (ADF) equipment on-board of aircraft is used together with the Non Directional Beacon (NDB) transmitters installed on the ground.

At night, the range of NDBs in the medium frequency (MF) range is greater due to changes in the ionosphere, a layer of the Earth's atmosphere that reflects and refracts radio waves. During the daytime, the ionosphere is heavily ionized by the sun's radiation, which can absorb and scatter MF radio waves, reducing their effective range.

However, at night, the levels of ionization decrease significantly. This reduction allows the ionosphere to better reflect MF radio waves over longer distances, thus increasing the range of NDBs. This phenomenon is known as "skywave propagation," where the radio waves can travel much farther due to bouncing between the Earth's surface and the ionosphere.