Posted on March 6, 2019 linda strong aircraft parts
Automatic direction systems serve a fairly simple purpose— locate, and point, in the direction of an NDB signal. Non-directional beacons (NDBs) are used for anything from Morse code broadcasts to voice interfaced communication to inform pilots about weather conditions. They communicate important information directly to the automatic direction finder (ADF). Simple enough, right? Let’s take a look at how an ADF accomplishes this functionality using two types of antennas: loop and sensing.
An NDB emits two electromagnetic field components at a time. One, is referred to as the E field, or electrical field. The other, is known as the H field, or magnetic field. Both fields run perpendicular to one another. The H field runs on the x-axis or plane, and the E field runs on the y axis or in a polarized direction.
A loop antenna is a structure that utilizes two perpendicular windings on a closed loop, and a ferrite core. Due to the closed loop functionality, H voltage is split and becomes null at two locations on the loop, these “null points” allow the ADF to assume that one point is facing in the direction of an NDB.
A sensing antenna receives the E field voltage. By measuring this voltage, and that of the two windings on the loop antenna, the ADF is able to calculate the location of the NDB signal. Both structures are typically located on the underside of an airframe.
ADFs are not as common in newer aircraft. More efficient models using GPS technology and VOR/ILS components have replaced many of the ADF units that were once utilized in locating NDB stations and signals. These advanced versions are less vulnerable to icing and water damage. In addition, technological advancement has enabled the antennas to be combined into one unit, reducing drag.
Pilots that are well acquainted with the functionality of an ADF might still prefer this mechanism. Interestingly enough, ADF components were once referred to as the “poor man’s lightning detector” as they operate on the same low frequency of lightning discharge and will sometimes detect spots of hazardous weather. In the early beginnings of fighter jet design, this capability was utilized quite frequently. From the founding of non-directional beacons to modern systems, ADF’s have acted as the springboard for the VOR and GPS technologies we see today.
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