Automatic Direction Finders: The grandfather of all radio navigation aids

Automatic Direction Finders

The grandfather of all radio navigation aids

By Frank Labue

July 1999


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.

The manufacturer will set one null as the "to" null. For example, if both the loop and sense antenna voltages are in phase and add to each other prior to the null position, the loop antenna is facing towards the transmitter. This will be set as the "to" null. The ADF needs both antennas to correctly point to the station.

This method of operation has been around since the 1930's and we still use it, virtually unchanged, today.

Operation and controls
ADF receivers have several operating modes that the pilot can select. In the ADF mode, both the loop and the sense antennas are used, the pointer is activated, and the ADF tries to point to the station. (This is the normal mode of operation.) Rotating the loop antenna control switch in either the left or right direction will cause the loop antenna and pointer to move left or right. Releasing the loop antenna control switch causes the loop antenna and pointer to rotate and point to the station again. The ADF indicator (RMI) consists of a compass card and pointer. In dual ADF systems, the RMI will have two pointers (for systems one and two). The pilot manually sets the compass card or, if the airplane has a flux valve, the compass card is slaved automatically to the aircraft's magnetic heading.

In the ANT mode, the loop antenna is disabled and all receiving is done through the sense antenna. The pointer should not move even if you turn the loop antenna control switch left or right. (This is the best mode for listening to your favorite ball game.) It is also good for identifying the station.

If the loop mode is selected, the sense antenna is disabled and all the receiving is done through the loop antenna. Rotating the loop antenna control switch in either direction will cause the loop antenna and pointer to rotate in the selected direction for as long as the switch is held. Releasing the control switch causes the loop antenna and pointer to stay where it is. You may also notice that the reception gets better when the pointer is 90 degrees away from the station and worse when the pointer is either pointed directly at or 180 degrees away from the station.

Some ADF control panels have a test position. Rotating the mode control switch to "test" will perform a self-test of the ADF system. In the self-test mode, the pointer will move to 45 degrees left of the lubber line to a heading of 315. The lubber line is a little arrow or line at the 12 o'clock position on the RMI.

The BFO switch
If you have a digital tuner, you can skip this paragraph.

Most ADF systems have a BFO switch mounted on the control panel. BFO stands for Beat Frequency Oscillator. With the BFO switch on, the ADF receiver generates an audio tone to help you tune in an NDB. On the control panel, select ADF mode and turn the BFO switch on; then rotate the tuning knob.

As you approach the station, you will begin to hear a high pitched tone. Continue rotating the knob and the tone will gradually decrease until you can no longer hear it. At this point, the ADF is tuned to the station and the signal strength meter should also be at its highest peak. You should be able to clearly hear the station's Morse code identifier. Rotate the knob in the same direction and you will begin to hear the tone again. Adjust the tuning knob so that you can not hear the tone.

The VBR filter switch
Transport category aircraft has a switch that is found on the audio select panel. These aircraft usually have multiple radios, nav aids, and audio integration systems and this switch has no effect on the ADF pointer.

Nevertheless, the VBR Filter switch is important if you want to identify a station. It is usually a rotary switch but some audio select panels have toggle switches.

Basically, it allows you to select either Voice or Range or Both audio filters in the ADF, VOR, and DME receivers. In the V position, the DME audio is grounded and voice audio can be heard on the ADF and VOR receivers. (This is another good choice for listening to that ball game.) In the B position, both Voice and Range filters are bypassed and all VOR, DME, and NDB audio can be heard. In the R position, DME audio is again grounded and the VOR and NDB Morse code identifier signals can be heard. If you are tuning in your local AM broadcast station, use the V position. If you are tuning in an NDB station, use the R position.

Troubleshooting tips
NDB radio frequencies are relatively low and range is usually limited. NDB radio waves can be reflected off the ionosphere. Shorelines, mountains, or cliffs can also affect them. Lightning and atmospheric static also emit radio frequencies that are close to NDB frequencies and they can affect ADF accuracy. These are all normal and natural phenomena.

A pilot may complain that he can receive the station but the pointer wanders back and forth. It may be possible that there is nothing wrong with the ADF system. If you have the opportunity to debrief him, ask if there were mountains, cliffs, shorelines, or lightning in the area.

One last thing . . . just kidding about that Y2K comment earlier — those GPS satellites will be around for some time.

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