Description and Limitation

A. DESCRIPTION
One of the older types of radio navigation is the automatic direction finder (ADF) or non-directional beacon (NDB). The ADF receiver, a "backup" system for the VHF equipment, can be used when line-of-sight transmission becomes unreliable or when there is no VOR equipment on the ground or in the aircraft. It is used as a means of identifying positions, receiving low and medium frequency voice communications, homing, tracking, and for navigation on instrument approach procedures.
The low/medium frequency navigation stations used by ADF include non-directional beacons, ILS radio beacon locators, and commercial broadcast stations. Because commercial broadcast stations normally are not used in navigation, this section will deal only with the non-directional beacon and ILS radio beacon.
A non-directional radio beacon (NDB) is classed according to its power output and usage:

1. the L radio beacon has a power of less than 50 watts (W),
2. the M classification of radio, beacon has a power of 50 watts up to 2,000 W;
3. the H radio beacon has a power output of 2,000 W or more;
4. the ILS radio beacon is a beacon which is placed at the same position as the outer marker of an ILS system (or replaces the OM).

B. LIMITATIONS AND BENEFITS
Pilots using ADF should be aware of the following limitations:
Radio waves reflected by the ionosphere return to the earth 30 to 60 miles from the station and may cause the ADF pointer to fluctuate. The twilight effect is most pronounced during the period just before and after sunrise/sunset. Generally, the greater the distance from the station the greater the effect. The effect can be minimized by averaging the fluctuation, by flying at a higher altitude, or by selecting a station with a lower frequency (NDB transmissions on frequencies lower than 350 kHz have very little twilight effect).
Mountains or cliffs can reflect radio waves, producing a terrain effect. Furthermore, some of these slopes may have magnetic deposits that cause indefinite indications. Pilots flying near mountains should use only strong stations that give definite directional indications, and should not use stations obstructed by mountains.
Shorelines can refract or bend low frequency radio waves as they pass from land to water. Pilots flying over water should not use an NDB signal that crosses over the shoreline to the aircraft at an angle less than 30°. The shoreline has little or no effect on radio waves reaching the aircraft at angles greater than 30°.
When an electrical storm is nearby, the ADF needle points to the source of lightning rather than to the selected station because the lighting sends out radio waves. The pilot should note the flashes and not use the indications caused by them.
The ADF is subject to errors when the aircraft is banked. Bank error is present in all turns because the loop antenna which rotates to sense the direction of the incoming signal is mounted so that its axis is parallel to the normal axis of the aircraft. Bank error is a significant factor during NDB approaches.
While the ADF has drawbacks in special situations, the system does have some general advantages. Two of these benefits are the low cost of installation of NDBs and their relatively low degree of maintenance. Because of this, NDBs provide homing and navigational facilities in terminal areas and en route navigation on low-level airways and air routes without VOR coverage. Through the installation of NDBs many smaller airports are able to provide an instrument approach that otherwise would not be economically feasible.
The NDBs transmit in the frequency band of 200 to 415 kHz. The signal is not transmitted in a line of sight as VHF or UHF, but rather follows the curvature of the earth; this permits reception at low altitudes over great distances.
The ADF is used for primary navigation over long distances in remote areas of Canada.