Ever wonder how an ADF knows which way the NDB is? Why it needs both a loop antenna and a sense antenna? Or what that "BFO" mode is good for? AVweb's avionics editor takes you through the basics of one of the the most complicated avionics systems in your airplane, the lowly ADF.
"ADF" stands for Automatic Direction Finder. "Direction finder" because it's job is to point in the direction of whatever station you tune in. "Automatic" because many years ago airplanes used manual direction finders (or "DFs") which required manually rotating a loop antenna looking for a null on a signal-strength meter.
The purpose of the ADF is to point to a non-directional beacon and that is the only purpose. Yet believe it or not, the ADF is one of the most complicated avionics systems in our aircraft.
Theory of operation
It's easier for me to explain to my wife why I was seen out with a redhead than to explain how the ADF works but I'll give it a shot. The radio waves from an NDB consist of two electromagnetic field components: an electric field called the E-field and a magnetic field called the H-field. (Why isn't the magnetic field called the M-field? Nobody seems to know.)
These fields are perpendicular in space, and their amplitudes vary sinusoidally with time. NDBs transmit a vertically polarized wave which means the E-field is vertical and the H-field is horizontal. (You're following this, right?)
The H-field induces a voltage into the windings of the ADF loop antenna. The loop antenna consists of two perpendicular windings on a square ferrite core. By measuring the phase difference between these two windings the ADF is able to determine the direction of the beacon. (We could go into the sine and cosine theory here, but who really cares as long as it points to the proper direction?)
In addition to the loop antenna, all ADFs have another antenna called the sense antenna. The ADF can tell from the loop antenna signals that the NDB is one of two possible directions, 180 degrees apart, and the sense antenna lets it figure out which of the two is correct.
All ADF systems have loop and sense antennas. With the older ADFs, they are two separate antennas. The loop antenna is a flat antenna usually located on the bottom of the aircraft, while the sense antenna is a long wire that often runs from top of the tail to the top of the of the cabin. (On larger aircraft, the sense antenna may be located on the bottom of the aircraft.)
More recent ADFs have a combined loop/sense antenna. It works far better than the old systems, plus it has less drag and is much less vulnerable to icing. The combined loop/sense antenna usually goes on the bottom because it's easier to install there, and if you forget to lower your gear, it will be the first thing to get ground away. By the way, these antennas typically cost $800.00 so go easy on them when cleaning or moving.
Most ADF receivers have several modes that the pilot can select. If the "ANT" mode is selected, the loop antenna is disabled and all receiving is done through the sense antenna. This mode provides the clearest audio reception, so you'd normally use it to identify a station or listen to Rush Limbaugh. On King ADFs, the needle should park in the 90-degree position when the receiver is in "ANT" mode; other brands may work differently.
In the "ADF" mode, the pointer is activated and the ADF tries to point to the station.
Some ADF systems have a "BFO" position. "BFO" stands for "beat frequency oscillator" and what it does is to generate an audio tone to let you identify beacons that identify themselves using interrupted-carrier keying. This feature is is seldom used in the United States except for a few marine beacons, but can be very useful when flying in other parts of the world.
If your ADF is has a "TEST" button, this should cause the needle to slew to the 90-degree position whenever the button is pressed and held. If it doesn't, then this function is usually activated by switching to "ANT" mode.
The ADF indicator consists of a needle and a compass card. The needle points to the stations when the receive is in "ADF" mode. The compass card may be fixed, manually rotatable with a knob, or (in more expensive RMI systems) slaved automatically to the aircraft heading. A slaved compass card is expensive, but sure makes it a lot easier to fly an ADF approach.
The tuning knobs let you select the desired NDB frequency. Nondirectional beacons for aviation use broadcast between 200-415 Khz. Most ADFs also receive the standard AM broadcast band at 550-1600 Khz. Aviation NDBs have a two- or three-letter identifier broadcast in morse code at an audio frequency of 1020Hz. Some NDBs also broadcast voice, often transcribed weather.
When a nondirectional beacon is co-located with the outer marker of an ILS system, it's called an "outer compass locator" or LOM. Same thing as an NDB, just a different name. When an ILS is equipped with a LOM, the ADF can be a big help in visualizing your position while maneuvering for the approach.
Most NDB stations are quite weak and range is limited, but in some cases such as broadcast stations, distances of over 500 miles have been obtained. ADF accuracy is affected by such things as crossing shore lines and reflections off of the ionosphere. Another thing that will degrade and ADF is precipitation static and thunderstorms. The reason is because P-static and lightning discharges are in the same low frequency range that the ADF receives on. Some people call the ADF a "poor mans radar" because if lightning is going on the ADF will in some cases point to it. But I wouldn't count on it.
Do you really need an ADF?
Outside of instrument training and picking up ball scores or listening to oldies-but-goodies, I really question the need for an ADF. I know there are still some airports that are only served by an ADF approach, but all of these are getting GPS overlays. In my 425 instrument hours, I've never had to do an actual ADF approach outside of training. Usually ADF approach minimums are higher than other approaches to the same airport if that airport has more than one approach.
ADFs can be tricky troubleshoot. Antenna installation and upkeep must be done perfectly or the ADF will never work properly. Repair cost usually isn't too expensive unless that $800 combined loop/sense antenna takes a dive. If that happens just forget about that vacation you were going to take.
If my ADF died, I might be sorely tempted to apply the repair money toward the purchase of a GPS instead. And if you really miss listening to the oldies, the ball scores or Rush, a Sony Walkman sounds a lot better anyway.