What is an NDB?
NDB stands for 'Non-Directional Beacon'; essentially they are (usually) low-power transmitters operating between 190kHz and 530kHz, bleating out nothing more than a repeated callsign in slow morse code. Once used by both aviators and sailors as a location-fixing aid; the ones we will describe and discuss here are those intended for aircraft navigation, the maritime systems having been turned off some time ago. Their slice of the NDB spectrum, around 300kHz, now contains DGPS (Differential Global Positioning System) stations often using the same sites as the old maritime beacons. For the most part it's aero NDBs that you'll hear, tuning around the LF band.
NDBs were one of the first radio aids to navigation as the technology involved in setting them up and more importantly using them was simple. It is still possible to use them as a sole navigation means today, indeed learning how to is still mandatory (and wise) for piloting, but few do since more technologically sophisticated systems are easier to use and more accurate. It can, and is, argued that they are an anachronism and obsolete; noises keep being made about turning the whole system of NDBs off but they still exist and indeed will probably continue to do so for several disparate reasons:
All that said, NDBs - at least in the developed world - are inessential, and their owners although unlikely to turn a functioning beacon off may well reconsider the continued operating maintenance costs and the immediate costs of fixing one that has died. Many have fallen off the air in such sad fashion, perhaps most notably of late 'TUK' 194kHz on Nantucket Island, a former 'powerhouse' coastal beacon. Another scenario is the encroachment of suburbia on many NDB sites - what was formerly a cheap bit of rural property starts to look like a valuable plot 'ripe for development'. Military beacons come and (mostly) go with the fortunes of the facilities with which they're associated; BRAC (the base-closure committee) makes sure this trend is negative. In short, the net trend is down - beacons are going dark far faster than other ones are being established.
Types of NDB
There are two main types of aero NDB. Relatively loud ones dot the country, located usually at or near airfields. They use typically 25 - 50 Watts transmitter power. Anywhere in North America or Europe a sufficient number of NDBs will be receivable to provide a reasonable 'fix' on your location by 'triangulation'. These NDBs typically have (unique, at least within a given country) three letter callsigns, sometimes apropos of their location, sometimes just alphabet soup. This callsign is transmitted in slow Morse Code, repeated every few seconds, at about 1kHz (nominally 1020Hz, but good luck) by US beacons, sometimes 400Hz (particularly in Canada) in straightforward AM (Amplitude Modulation - just like AM broadcasters). Sometimes (especially in Canada, France and elsewhere) a long 'dash' fills the space between the Morse callsign (called DAID - Dash After ID), and in some countries the signal is sent as plain CW (no AM-like carrier. But in North America, NDBs are AM or AM derivative.
Originally, it was incumbent on the pilot or his navigator to tune in to the NDB, listen to the morse to make sure it was the right one, then physically rotate a loop antenna on the outside of the fuselage until the signal from the NDB dropped out; loop antennas have very sharp 'nulls' or deaf spots in their receiving patterns. This null indicated the direction of the NDB in relation to the plane. More recently the pilot has a unit called an ADF (Auto-Direction Finder) in his cockpit, which when tuned to an NDB automatically points the direction compass-like on a display. Much easier. (Incidentally, the former method is what you'll use if you're sufficiently sick as to want to try and track down some NDBs for yourself. Aircraft optional.)
The second type is usually less powerful (<25 Watts) and (in the US) has a two letter callsign, usually the first two letters of the airport code of the airport with which it is associated eg 'RD' for 'RDG' Reading, 'CX' for 'CXY' Harrisburg. These are at a site a few miles away from the airport, directly in line with the main runway at a point where 'planes are beginning their final approach for landing; they are co-sited with the 'Outer Marker' beacon of the airport's ILS (Instrument Landing System). The Outer Marker is a 75MHz transmitter with antennas that beam straight up to illuminate an oval-shaped patch of sky directly overhead; if the 'plane hears this Outer Marker beacon and is at approximately the right height, its ILS radios will also be able to receive the narrow 'locator' (left/right) and 'glide-slope' (up/down) beams from the airport itself, which will guide the pilot down to touch-down even in zero visibility. The purpose of the NDB is to guide the 'plane to the Outer Marker to begin with, if need be; their official name is 'Compass Locator' or COMLO. Much less commonly, meaning I haven't come across one yet, the COMLO can be at the ILS' 'Middle Marker' site, which is part way down the ILS approach toward the airport. Pilots are trained (but again nowadays rarely choose) to use a COMLO beacon to make a so-called 'NDB approach'; the idea is they line themselves up roughly with the end of the runway by finding the beacon and turning in on a given bearing. These approaches usually start at a much greater height and so are of steeper descent than ILS approaches since they don't have the safety advantage of the 'too high/low' information inherent to ILS.
Incidentally, ILS is another radio-navigation aid that is slated for the chop, but is never really likely to be for largely the same reasons that NDBs will probably remain immortal.
A third kind of NDB is quite rare, but are very loud and have huge coverage. These, typified by 'CLB', Carolina Beach NC, 'GLS' Galveston TX, 'DIW' Dixon NC, and 'TUK' on Nantucket MA, are coastal stations running hundreds or thousands of Watts. Their intent is obviously for long-range over-water navigation.
Antennas and sites
Most NDBs truly are unimposing, unassuming, low-key purely utilitarian things.
Generally all that there is to them is a small patch of ground about the size of a city lot, a hut containing as simple a transmitter as it gets, and a simple antenna. The antenna is most often a wire antenna supported by 'phone poles; the visually rudimentary approach jives with the simplicity of the technology.
Rarely are NDB antennas any higher than 60 feet; indeed 40 feet
could be considered 'big-time', many being merely 20-25ft. high.
All the antenna types are vertical in nature and vertically polarised, fed against a (usually woefully small) ground system underneath the antenna. The horizontal top section of a 'T' antenna acts merely as top-loading to increase the effective height (length) of the attached vertical section.
Vertical antennas need a good ground plane - usually arranged to be in the form of buried radial wires starting from under the antenna feedpoint away in all directions and ideally at least 1/4 wavelength in length; being that could be at NDB frequencies about a quarter of a mile, that is obviously impractical. A minimum 'rule of thumb' is that the radials should extend as far out as the antenna is high; even this would be impossible at many NDB sites I've visited. So, the long and (mostly) short of it is that most NDBs have horrible ground systems, seriously affecting their radiating capability. That to do their alloted task actually means that they don't have to radiate particularly well is perhaps the saving grace.
Another major factor in how well a vertical antenna radiates is the nature of the ground (dirt) itself and what's on it surrounding the antenna for a significant distance, say, miles. If it's sea-water well, that's as good as it gets. Built up city; well that's about as bad as it gets. This is probably the predominant factor that separates the audible-for- thousands-of-miles NDB from the I-know-it's-down-this- street-somewhere kind.
Lowfers (amateur radio low-frequency radio enthusiasts, who transmit with under a watt between 160kHz and 190kHz ie immediately adjacent to the NDB frequency band) should be gratified to know that for the most part, their antenna and ground system installations exceed those of most NDBs.