NDB

An Non Directional Beacon is comprised of an omni-directional Vertically polarised antenna at a known stationary location coupled to a Medium to Low Frequency Transmitter that has an amplitude modulated identification of one, two or three-letter Morse Code callsign. In Australia they use the three-letter Airport Code when located at an airport.

The NDB is described in ICAO Annex 10 that specifies that NDBs operate between 190 kHz and 1750 Khz.

There are a few types of NBs:

En route used to mark airways for homing etc
Approach NDBs that have a hold and approach associated with them
Localiser beacons for guidance to an ILS
Locator beacon for guidance to an ILD