NDB stands for Non Directional Beacon. It is a L/MF radio station which transmits a carrier wave modulated with an identifier that can be received by an ADF (Automatic Direction Finder) receiver and an indicator in the aircraft to show the direction the station is located. It can be used to determine your own position, as a marker for an ILS procedure, locator for an airway or as backup for the VOR.
If you own an ADF receiver with an extended frequency range you could also receive AM radio stations as a backup or just for your pleasure while flying. Likewise with the VOR page, we discuss operating this device and its technicalities without getting into deep detail.
The NDB station transmits its radio signals in the L/MF frequency band (300 - 3000 kHz). These frequencies have the ability to follow the curvature of the Earth thereby enabling reception (and thus navigation) beyond the horizon. These effects are more prominent in the evening and nighttime. The effect of the sun during daytime on the ionospheric layers in the atmosphere reduces the range of these frequencies.
You will notice that the effect the ionosphere has on reflecting radio signals depends for a large degree on the amount of solar spots on the surface of the sun. These spots will normally increase and decrease in about eleven (11) year cycles. Read more on that subject here.
The disadvantages of L/MF navigation are not limited to atmospheric conditions, which are most prevalent during sunset/rise (twilight effect). Selecting a station with a frequency below 350 kHz minimizes this effect. LF signals, especially from stations near the coast, refract when they cross the shoreline at the coast. This error can be up to 30 degrees from the normal, depending on which angle the signal crosses the land/sea border.
Mountains can act as a reflector for L/MF signals thus giving a complete false indication of where the station really is. Flying higher is the solution. Thunderstorm lightning is most noticeable in these low frequency bands and can give a short burst on the indicator, it momentarily points to the storm and indications are very erratic.
When listening to the ADF you can hear the storm approaching, it's a sort of poor mans (pilots) storm scope.
The ADF indicator does not have a flag to indicate to the pilot if reception is reliable. You will need to listen to the station to assess its usability.
ADF navigation uses instead of radials, bearings to describe your position to the NDB (or any other position). A bearing is measured clockwise from magnetic north and gives you the direction (or angle) between the aircraft and the NDB. The ADF indicator can have a fixed card or a manual (even slaved to the Directional Gyro is possible) card. The fixed card gives relative bearings to the NDB, measured from the airplanes nose (which is 0 degree).
Navigating with a fixed card ADF means adding your magnetic heading to the relative heading from the ADF to get the magnetic bearing to the station. With a movable card ADF you can set your magnetic heading on the card and read the magnetic bearing at the pointer and follow the needle to the station. This is method called homing.
This where you keep the nose and needle of the ADF pointed at the NDB. When there is no wind the magnetic heading will remain constant, but as there is almost always some wind it will push you off course and you must correct the magnetic heading to keep the aircraft nose pointed at the NDB. You will fly a sort of curved track to the station.
For this you will need to determine the reciprocal bearing from the NDB. Do this by adding the relative bearing to your magnetic bearing and subtract/add 180 degrees. Now you have the reciprocal bearing related to magnetic north. Now convert this magnetic bearing to true bearing by adding or subtracting variation (add easterly and subtract westerly).
Now plot this line with your plotter on the NDB and you have a line of position (LOP). Use a second NAVAID (NDB / VOR) to create another LOP and where they cross each other is your exact position.
Flying toward a NDB requires that you need to compensate for the wind. Fixed card ADF do not provide an automatic wind correction (the slaved types do). You will have to bracket to adjust for the wind and determine the WCA (Wind Correction Angle). When on course, the WCA is exactly the number of degrees the bearing pointer points to the left or right of the nose.
Station passage occurs when the ADF needle swings to at or near the reciprocal bearing. Tracking outbound from the station is done the same way as it is when flying inbound.