Some items to note when listening for NDBs:
Sometimes there is a effect.” This occurs when radio waves are reflected back to the ground from the ionosphere and interfere with the ground waves. This typically occurs between 50 to 100 km, or about 31 to 62 miles from the transmitter and is generally noticed just before sunrise and just after sunset on frequencies above 0.350 MHz.
There are also effects due to geography. Mountains and cliffs can reflect the ground waves to either cause interference or incorrect azimuth readings. If a listener is near an ocean coast, there can also be a shoreline effect. This is when the ground wave bends near a shoreline, especially if the ground wave is traveling parallel to the shoreline.
The NDB Gap
Earlier I indicated that NDBs are found between 0.190 to 0.530 MHz. There actually is a small gap in the NDB allocation at around 0.495 to 0.505 MHz. This is because the frequency 0.500 MHz has traditionally been used as the international maritime distress frequency and there are still international maritime allocations around that frequency. Just below 0.190 MHz, at 0.136 MHz, there is a new amateur allocation available in Canada and Europe.
Radio Amateur Operation
In the 2007 World Radiocommunication Conference, WRC-07, the frequency 0.136 MHz was allocated to amateurs after nearly a century of having been banned. It is available on a secondary basis and transmission is limited to a maximum radiated power of 1-watt effective isotropic radiated power. However, the 0.136-MHz allocation does not generally apply to U.S. amateurs except for a handful who have special experimenters’ licenses from the FCC.
However, U.S. amateurs may take some heart in the fact that beginning sometime in 2013, there may be a new long-wave allocation at 0.472 to 0.479 MHz approved by the FCC. The allocation was recently approved at the 2012 World Radiocommunication Conference, WRC-12. The power limit will likely be 1-watt effective isotropic radiated power and it will be a secondary allocation. The FCC has yet, however, to post when this might be approved.