Monitoring NDBs
Besides their use in aircraft navigation, NDBs are also popular with long-distance radio enthusiasts ("DXers"). Because NDBs are generally low-power (usually 25 watts, some can be up to 5 kW), they normally cannot be heard over long distances, but favorable conditions in the ionosphere 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. Also, since the band allocated to NDBs is free of broadcast stations and their associated interference, and because most NDBs do little more than transmit their Morse Code callsign, they are very easy to identify, making NDB monitoring an active niche within the DXing hobby.
In North America, the NDB band is from 190 to 435 kHz and from 510 to 530 kHz. In Europe, there is a longwave broadcasting band from 150 to 280 kHz, so the European NDB band is from 280 kHz to 530 kHz with a gap between 495 and 505 kHz because 500 kHz was the international maritime distress (emergency) frequency.
The beacons that are between 510 kHz and 530 kHz can sometimes be heard on AM radios that can tune below the beginning of the AM broadcast band. (For example, the "HEH" beacon in Newark, Ohio at 524 kHz is within the bandwidth of most AM radios, the "OS" beacon in Columbus, Ohio at 515 kHz and the "YWA" beacon in Petawawa, Ontario, Canada at 516 kHz can also be heard on some AM radios). Some beacons can also be heard on 530 kHz, although from the adjacent frequencies such as "LYQ" at 529 kHz in Manchester, Tennessee but for the most part, reception of NDBs requires a radio receiver that can receive frequencies below 530 kHz (the longwave band). A NDB in Miramichi, New Brunswick once operated at 530 kHz as "F9" but had later moved to 520 kHz. Most so-called "shortwave" radios also include mediumwave and longwave, and they can usually receive all frequencies from 150 kHz to 30 MHz, which makes them ideal for listening to NDBs. Whilst this type of receiver is adequate for reception of local beacons, specialized techniques (receiver preselectors, noise limiters and filters) are required for the reception of very weak signals from remote beacons.
The best time to hear NDBs that are very far away (i.e. that are "DX") is the last three hours before sunrise. Reception of NDBs is also usually best during the fall and winter because during the spring and summer, there is more atmospheric noise on the LF and MF bands.
Read more about this topic: Non-directional Beacon