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I've written about non-directional beacons in the context of signal strength measurements. See http://www.cliftonlaboratories.com/signal_statistics.htm for the details.

"Non-directional beacons" or "NDB" as they are usually called, are the oldest radio aid to aircraft navigation still in operation. NDBs operate below the AM broadcast band in the 190-535 KHz range. (Some countries allow beacons to operate as high as 1750 KHz.) The NDB radiates an amplitude modulated signal with a Morse code ID, with the aircraft obtaining a bearing on the NDB with automatic direction finding equipment. The Morse is sent with a 1020 Hz tone in the majority of cases, although some use a 400 Hz tone.

Most NDBs operate with relatively low power, 25 watts or so being common, although some run as much as 2000 watts output. Antennas are generally either a base loaded vertical whip or a "T" antenna with the horizontal part of the T providing capacitive top hat loading.

Similar NDBs are used for marine navigation but with some operational differences. First, most marine beacons are in the 285 - 325 KHz  range. Second, unlike aviation beacons that operate continuously, marine beacons time share frequencies with sequential operation.  Many marine beacons in this frequency range have been replaced with DGPS transmitters. (DGPS stands for differential global positioning system and the DGPS transmitters send digital correction information that may be used to compensate for errors in GPS transmissions, thereby improving navigational accuracy.)

There's an active beacon DX'ing group, with their own Yahoo group http://groups.yahoo.com/group/ndblist/. A valuable resource for NDB DX'ing is the database at  http://www.classaxe.com/dx/ndb/rna/.

• Unusually high power, 2000 watts.
• It's located very close to the Atlantic ocean coast.
• It's the only beacon operating on 198 KHz

For these reasons, DIW can be heard throughout the United States and has been logged in dozens of other countries. (If you go to the NDB database, look for DIW's entry at 198 KHz. Then click on the "Heard In" listing for DIW and a map showing the locations at which DIW has been copied will appear.)

There's another interesting fact about DIW—it shares 198 KHz with the British Broadcasting Corporation's 500 KW long wave station at Droitwich. In addition to carrying domestic programming and, at night, the BBC's World Service, the Droitwich transmitter serves as a frequency standard with its signal held to a frequency accuracy of 1x10^-11, and it also carries (via slow speed phase modulation) power line load shedding control signals.

This prompted me to listen to 198 KHz and see if I could hear the BBC's Droitwich signal.