NDB’s (and Marine Beacons)
My main area of interest with regard to beacons is logging airport/airfield NDB’s (Non Directional Beacons) which are used by mainly light aircraft to determine their current location and direction. There are still hundreds of NDB’s active, but they are slowly closing and being replaced with satellite/GPS based systems. See below for more information on NDB, Marine and various ham radio beacons.
Maritime beacons are used by shipping for navigation and their callsigns usually consist of 1 or 2 letters sent in Morse Code at a speed of around 8wpm (words per minute). There are fewer of these maritime beacons around now as navigation is being dealt with by GPS and other satellite technologies but you can still hear some.
Non Directional Beacons (NDB’s) are used by aircraft for navigation purposes. They, like the maritime beacons, mostly inhabit the part of the spectrum between Long Wave and Medium Wave (i.e. 270-500kHz approximately). NDB’s identify by sending their call letters in Morse code and usually consist of 2 or 3 letters (which quite often bear a relationship to the actual location of the beacon). Again, these beacons send their callsign at 8wpm or so. NDBs transmit a continuous carrier on their designated frequency with the callsign/ident spaced at either 400Hz or 1020Hz either side of the carrier. NDBs are allocated frequencies based on what other beacons in the same geographical area. Carrier frequencies allocated will end in in either a whole or half kHz (0.5 or 1kHz). One thing to note though, is that most NDBs transmit a few Hz from their nominal allocated frequency. This could be due to the age of the transmitter (the crystals that are used to set the frequency can change characteristics over time, which will change the frequency by some amount. Also extreme weather can affect frequency accuracy (ice build up on the antenna can change its resonant frequency which will affect the radiation pattern of the antenna, etc. Various lists on the internet list the NDB callsign together with its allocated frequency and observed USB and LSB offsets. Occasionally an NDB will transmit something other than its callsign, such as “TST” which means ‘Testing’, sometimes you may hear the ID but with an extra character, usually an ‘E’, indicating the beacon is running on emergency power - due to a transmitter fault, or possibly a loss in mains voltage and running on emergency backup power (batteries).
Although these beacons are only designed to have an operational radius of just a few tens of nautical miles, they can often be received MUCH further afield. It is not uncommon for UK based listeners to hear NDB’s located in North America, which is about 3000 miles away — slightly outside the ND’s designed operational range!
NDBs use physically short antennas that are usually top loaded. a full size quarter wave at 300kHz is a rather impractical 250m tall. There is an advantage to using a shortened vertical antenna, which is that the radiation angle is quite high and it will have a limited coverage range. Usually, the antenna has a capacity hat, which looks like a horizontal ‘X’ on top of the vertical radiating element. There may, or may not be wires attached to the capacity hat. The antennas on NDBs are around 5-10m tall and ground mounted adjacent to the transmitter housing.
There are many excellent sites devoted to NDB’s, These sites will give you a wealth of information about these beacons and reception techniques. Because of the transmission mode that most NDB’s use, there are tricks to ensuring you have the best chance of hearing and identifying the beacons. The two sites in the links give a good overview of what NDBs do and how they do it: https://www.cfinotebook.net/notebook/avionics-and-instruments/non-directional-radio-beacon and https://en.wikipedia.org/wiki/Non-directional_beacon. Both are well worth a visit. Below are some images of NDBs showing antennas, transmitters, etc. Please note, these are not my images but credit to the origin is given for each one.