ADF is still one of the most difficult pieces of avionics equipment to install and maintain even in today's GPS world. First off, ADF stands for Automatic Direction Finder. Years back, believe it or not, ADF was not automatic, the pilot had to tune the ADF via what we used to call a "coffee grinder" until he/she heard the ADF audio and noticed the ADF needle moving. The only purpose of the ADF is to point to the non-directional beacon or show passage there of. This system is one of the most complicated systems in our aircraft. I’ll start by trying to describe what the antenna sees. Yes, in the avionics world, antennas do see and hear. A radio wave from a beacon consists of two electromagnetic field components; an electric field we will call the "E" field and a magnetic field we will call the "H" field. You may ask, "why is the magnetic field not called the "M" field? Nobody seems to know the answer, even yours truly, and I’m the "avionics expert"? Their fields are perpendicular in space, and their amplitudes vary sinusoidally with time. All of the ADF stations transmit a vertically polarized wave, which means the "E" field is vertical and the "H" field is horizontal. The "H" field induces a voltage into the loop windings of the ADF antenna. The loop antenna consists of two perpendicular windings on a square ferrite core. By measuring the lag between these two fields, the ADF is able to determine the direction of the beacon. We could go into the "Sine and Cosine" theory, but who really cares as long as it points to the proper direction. We discussed a little bit about the loop antenna, but another antenna in all ADF systems is the sense antenna. The main purpose of this antenna is to maintain a reference phase. It is 90 degrees out of phase with the loop antenna. It's very important during the ADF installation to tell the ADF system where the antennas are located on the aircraft so the system will not point 180 degrees off.

Let’s discuss some of the functions and parts of the ADF system. All ADF systems have two antennas. With the older ADF systems you have two separate antennas. The loop antenna is usually located on the bottom of the aircraft and is a flat antenna. The sense antenna is the long wire that runs normally from top of the tail to the top of the cabin. This antenna is sometimes located on the bottom of the aircraft, especially on larger aircraft. The modern ADF system has what we call a combined loop, sense antenna. It works far better than the old system, plus has less drag and does not pick up ice like the long wire antenna. 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 that will get ground away. By the way, those antennas sell for around $900.00, so go easy on them when cleaning or moving. Most ADF receivers have several positions 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. The indicator needle should park at the 90-degree position. Normally you would use this mode to improve station identification or listen to the ball game. In the "ADF" mode, the pointer is activated and the ADF tries to find the beacon. Some ADF systems have a "BFO" position. This is seldom used in the United States. What it does is inject a one thousand hertz tone into the audio line. This feature allows audio identification of interrupted carrier beacon stations that are used in various parts of the world. If your ADF is equipped with a "TEST" button this should cause the needle to go to the 90-degree position whenever the button is pressed and held. The tuning knobs are for selecting the frequency that you desire. The indicator is normally a compass card and a needle that always points to the station that is selected. Some of the more elaborate systems have a slaved compass card that really makes it a lot easier during an ADF approach. Some ADF systems such as the King KR-87 can be displayed on and EFIS or Sandel EHSI system.

Non-directional beacons broadcast between 200 and 415 kHz. Non directional beacons have a three-letter identifier, and the audio frequency is 1020Hz. At times there could be voice on the ADF, such as weather forecast. When the ADF is used in conjunction with a localizer it is called a compass locator. It’s the same thing; it just has a different name. This is the band the we see the most problems with due to the fact some aircraft systems such as the alternator and magneto systems often create havac in this range not to forget most aviation ADF fixes are very weak in the sense of transmitter power. Just about any ADF will pick up the AM broadcast band due to the massive power these stations put out.

Most ADF stations are quite weak and range is limited, but in some cases such as broadcast stations, distances of over 500 miles have been obtained. Things such as crossing shorelines and reflections off the ionosphere affect ADF. Another thing that will degrade an ADF is static build up and thunderstorms. The reason is that static noise and thunderstorms are in the same frequency range on which the ADF receives. Some people call the ADF a "poor man’s Stormscope" because if lightning is going on the ADF will in some cases point to it.

Aircraft ADF’s are getting and expensive. With GPS approaches poping up everywhere along with the high minimums associated with ADF approaches, one wonders just how much should we spend on ADF repairs. Many pilots remove the ADF for GPS installations. I haven’t heard one yet say the wish they had left the ADF in the aircraft. Think of the last time you need your ADF, in most cases it was used only during your instrument training. I normally recommend that unless the ADF can be repaired easily and economically it should be put to rest unless you really have a requirement for the ADF. Purchasing a new ADF isn’t wise in most cases. The ADF, antennas and installation often run around $5,000.00. While this amount will normally not put you into a good IFR GPS it will put you within striking range.