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.
September 6, 1998
|About the Author ...
Tom Rogers is Avionics Editor for AVweb.
Avionics West, Inc. at Santa
Maria, California, one of the finest radio shops on the West Coast. Tom is an
instrument pilot, an FAA Designated Engineering Representative (DER) for
avionics, and has a Ph.D. in nuclear physics. (We're not sure why he got the
doctorate, but we call him "Dr. Tom," and he seems to like that.)
You can send Tom your avionics questions at
Tom's company, Avionics West,
is one of the worlds largest discounters of handheld and
panel-mount avionics for general aviation. They offer all leading brands of
aviation electronics at deep-discount prices. Every item sold by AWI is
covered by a 30-day no-questions-asked return privilege.
Avionics West features handheld GPS receivers from Garmin and Lowrance;
headsets from David Clark,
LightSPEED and Telex; and panel-mount GPS navigators from II Morrow. AWI
stocks these units and generally sells them at prices substantially below the
manufacturers' "Minimum Advertised Price" (MAP). Consequently, AWI is not
permitted to advertise these prices on its web site, but you can obtain them
by sending an email to the AWI auto-responder at
by telephoning AWI at 1-805-928-3601 (M-F, 8-5 Pacific Time).
|Ask Tom an Avionics Question
"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
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
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
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
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
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.