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A Non-Directional (radio) Beacon, or NDB, is a radio transmitter at a known location close to the airport, used for aircraft navigation.
Besides their use in navigation, NDBs are also popular with long-distance radio enthusiasts ("DXers").
Because NDBs are generally low-power, they normally cannot be heard over long distances, but sometimes, especially during the night, ionosphere conditions can allow NDB signals to travel much farther than normal.
Because of this, radio DXers interested in picking up distant signals enjoy listening to faraway NDBs as a DXing hobby.

In Europe, the NDB band is between longwave AM broadcasting band and Medium wave AM broadcast band, from 255 to 525 kHz. This can be seen on this picture:


NDB`s uses amplitude modulation (AM) and can be received with AM-mode, but it is better to hear them with SSB (LSB or USB) depending of the particular situation to avoid possible interference signals which are very common on this band. Using SSB demodulation, the receiving bandwidth is smaller than AM, so the Signal to noise ratio will be better. Some NDB`s Hunters uses CW demodulation with very narrow CW filters in order to get even better SNR. In that case, you have to tune your receiver exactly to frequency of the identification signals which is about ±1020 Hz from the AM Carrier, but there are NDB`s with 400 Hz offset from the AM Carrier also.

There are thousands NDB`s around the world. Many of them are on the same frequency. Because of this, during the night, with good propagations, sometimes, two or more NDB`s can be heard on the same frequency...

There is interesting YouTube Video in which I am presenting some Non Directional Beacons received at my home in Macedonia with ColibriNANO SDR receiver and Mini-Whip active antenna. ColibriNANO SDR is high dynamic range receiver which helps in receiving weak NDB-signals, especially because NDB-band is between LW and MW AM Broadcast band with very strong signals which can easy overload some wideband SDR-receivers. You can watch the video on this link.



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