Because NDBs are long-wave signals, they have the advantage that they follow the curvature of the earth for long distances. They are ground wave signals as opposed to sky wave signals.
This allows the signal to be received at much greater distances at lower altitudes as compared to VHF signals. This makes them handy for low flying airplanes, balloonists, boats, and people on the ground.
Besides being useful in learning CW, NDBs are useful navigational aids, which is, of course, their primary purpose. If a person picks up a NDB signal, he knows he will end up at an airport of some kind if he simply follows the signal back to its origin. This requires that the person receiver have some capability to determine the NDBs signal direction. Figure 1 shows this basic use.
If a person can hear two NDBs on his or her receiver, can determine with some accuracy the signal direction of each, has a list of the NDB locations, and has a map, the listener can determine or ”his own location by triangulation.
This is done by locating each NDB given location on a map, and drawing an extended azimuth from that location. Where the two azimuths intersect is the position of the listener. Figure 2 shows how this is done.
Using the simple method shown, a person can determine his or her position and then using the scale on the map, figure out how far the listener is from his destination. If three NDBs can be heard and plotted, the ”uncertainty can be further reduced.
By the way, this method works for repeaters, AM broadcast radio stations, and FM broadcast radio stations, as well. As long as the location of the two repeaters, the two AM broadcast radio stations, or the two FM broadcast radio stations are known and a person has a map on which they can be located, the position of the listener can be similarly fixed by triangulation. Several hand-held antennas that are commercially sold for foxhunts are ready made for this application.
There are some interesting stories about aviators who used AM broadcast stations in a fog or storm to fix their positions when their regular navigation equipment failed. Some of the older and cheaper transistor radios contained bar antennas with enough reception directionality —which was usually considered a pain in the neck —that they, too, can be used to fix positions from AM broadcast stations.
In listening to NDBs, be aware that the CW tone is a little different than that typically used in CW operations. The modulated tone is either 400 Hz or 1,020 Hz. In determining the frequency of the NDB, the difference in tone from that usually used for CW by amateurs may cause a person to be a little off-frequency in zero-beating the signal using a pre-set, internally generated tone. Some of the NDBs may periodically broadcast weather data and pertinent airfield information, as well, in addition to simply repeating the callsign over and over.