Automatic Direction Finders
The grandfather of all radio navigation aids
By Frank Labue
Ok, I know. Why are we talking about ADFs when there are VOR/ILS and GPS navigation systems around? I know the newer systems are more modern and more accurate. Still, the ADF was guiding pilots long before Y2K was a problem and will be available long after Y2K wipes out all those fancy GPS satellites.
Yes, the trusty old ADF can trace its roots back to the late 1920's. It is the grandfather of all radio navigation aids. In fact, it's been around so long that I bet many of you have forgotten how an ADF system works or, why it needs both a loop antenna and a sense antenna. And, what the heck is that BFO mode good for anyway?
The ADF is an Automatic Direction Finder. It will do exactly what its name says. The ADF automatically points in the direction of the NDB (Non-Directional Beacon) you tune in — roughly the same way a kid will continually to point to his favorite toy store while you try to drive past it.
Theory of operation
A ground station is shown on the charts as Non-Directional Beacon (NDB). They are called non-directional because they don't contain any directional information. The NDBs transmit equally in all directions, like waves caused by a pebble that has been thrown into a pond. Radio waves from an NDB create an electromagnetic field. The electric field is called the E-field and the magnetic field is called the H-field. The E and H fields are perpendicular in space, and their amplitudes vary like a sine wave. If you're not confused yet, keep reading.
Non-Directional Beacon frequencies
Aeronautical non-directional beacons broadcast on relatively low frequencies (200 to 415 kHz). The ADF will also receive the standard AM broadcast band at 550 to 1600 kHz. Land-based aeronautical navigation aids such as VORs and NDBs have a two- or three-letter identifier broadcast in Morse code. Some NDBs also broadcast audio, usually weather information. Here's a tip — Get a local VFR sectional chart. It will tell you the frequency, Morse code identifier, and direction to the NDB stations nearest to you.
Two antennas are better than one
Now let's see what happens on the aircraft. All ADF systems have both loop and sense antennas. The loop antenna, is usually a flat plate antenna located on the bottom of the aircraft, while the sense antenna is usually a simple wire or foil type antenna imbedded in a fairing. The loop antenna consists of two perpendicular windings on a square ferrite core. The H-field induces a voltage into the two windings of the ADF loop antenna. Because the windings are on a closed loop, the phase angle of the voltages vary as the antenna is rotated. Rotating the loop antenna, you will find there are two points where the voltages exactly cancel each other out. These points are called nulls. Only one of these nulls points to the NDB. The other null is 180 degrees away from the NDB. If we use only a loop antenna we could be heading in the opposite direction. This is not good. The sense antenna determines which null is correct.
The sense antenna simply receives the electric portion of the electromagnetic field and produces a voltage that is always in phase with the transmitter. By measuring the combined voltage of the two windings in the loop antenna and comparing that to the voltage received by the sense antenna, the ADF is able to determine the direction to the beacon.
The receiver will "electronically" rotate the loop antenna (and pointer on the RMI) to achieve a minimum voltage output or null. Prior to the null, the ADF receiver compares the loop antenna voltage to the sense antenna voltage. If both loop and sense antenna signals are in phase prior to the null, they will add to each other. If both signals are out of phase prior to the null, they will subtract from each other. By adding and subtracting the two signals, the ADF can tell the difference between the two nulls.
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