The bearing frame is a simple form of directional magnetic antenna . In the simplest case it consists of a wire, the end of which is bent into a ring. In principle, the antenna is a metal rod with a ring on top.
Such an antenna is most sensitive to signals that arrive parallel to the antenna plane. The antenna plane is the plane in which the ring lies. The antenna is the least sensitive to all signals that arrive perpendicular to the plane. When navigating , it is important to turn the antenna so that the signal is maximum. For example, if the north direction ( compass ) is known, there are two possible heading angles that differ by 180 °. Technically, the correct of the two angles can be determined by shielding one side of the antenna.
Monitoring frames were used in aircraft in particular in the early days of aviation.