Loop Antenna
A loop antenna is a single turn of wire whose dimensions are normally much smaller than a wavelength. When this condition exists, the current in it may all be considered in phase. This results in a magnetic field that is everywhere perpendicular to the loop. The resulting radiation pattern is sharply bidirectional, as indicated in Figure 1, and is effective over an extremely wide range of frequencies—those for which its diameter is about 
or less. The antenna is usually circular, but any shape is effective.
Because of its sharply defined pattern, small size, and broad-band characteristics, the loop antenna’s major application is in direction finding (DF) applications. The goal is to determine the direction of some particular radiation. Generally, readings from two different locations will be required due to the antenna’s bidirectional pattern. If the two locations are far enough apart, the distance and direction of the radiation source can be calculated using trigonometry. Since the signal falls to zero much more sharply than it peaks, the nulls are used in the DF applications.
While other antennas with directional characteristics could be used in DF, the loop’s small size seems to outweigh the gain advantages of larger directive antennas.
Ferrite Loop AntennaÂ
The familiar ferrite loop antenna found in most broadcast AM receivers is an extension of the basic loop antenna just discussed. The effect of using a large number of loops wound about a highly magnetic core (usual ferrite) serves to greatly increase the effective diameter of the loops. This forms a highly efficient receiving antenna, considering its small physical size compared to the hundreds of feet required to obtain a quarter-wavelength for the broadcast AM band. The directional characteristics of this antenna are verified by the fact that a portable AM receiver can usually be oriented to null out reception of a station. You should now be able to determine a line through which that broadcasting station exists when the null is detected.