A radio beacon is a transmitter at a known location, which transmits a continuous or periodic radio signal with limited information content (for example its identification or location), on a specified radio frequency. Occasionally the beacon function is combined with some other transmission, like telemetry data or meteorological information.
Radio beacons have many applications, including air and sea navigation, propagation research, robotic mapping and radio frequency identification (RFID).
A most basic aviation radio navigational aid is the NDB or Non-directional Beacon. NDBs are simple low frequency and medium frequency transmitters and they are used to locate airways intersections, airports and to conduct instrument approaches, with the use of a radio direction finder located on the aircraft. The aviation NDBs, especially the ones marking airways intersections, are gradually decommissioned, as they are replaced with other navigational aids based on newer technologies. Due to relatively low purchase, maintenance and calibration cost, they are still used to mark locations of smaller aerodromes and important helicopter landing sites.
There are also marine beacons, based on the same techonlogy and installed at coastal areas, for use by ships at sea.(1) Most of them, especially in the western world, are no longer in service, while some have been converted to telemetry transmitters for differential GPS. Chains of radio navigation beacons for marine use are still active around the Russian and Ukrainian coastline.(2)
NDBs typically operate in the frequency range from 190 kHz to 535 kHz (although they are allocated frequencies from 190 to 1750 kHz) and transmit a carrier modulated by either 400 or 1020 Hz. NDBs can also be colocated with DME stations and ILS marker beacons. NDB owners are mostly governmental agencies and airport authorities.
Antennas are of vertical polarization, either lattice towers with a "top hat" or T-like wire antennas supported between two masts. NDB antennas incorporate a segment that consists of an variable inductor or variometer and a variable capacitor in series, "tuned" to the particular frequency or frequencies assigned to that antenna. NDB's tuned segment is part of the antenna itself. There is often an electrical counterpoise, in the form of copper wires or wire mesh buried in the ground underneath the antenna.
Besides their use in aircraft navigation, aero NDBs and marine radio beacons are also popular with DXers. Because NDBs are generally low-power (usually 25 watts), they normally cannot be heard over long distances, but favorable conditions in the ionosphere can allow NDB signals to travel much farther than normal. Because of this, radio DXers interested in picking up distant signals enjoy listening to faraway NDBs. Also, since the band allocated to NDBs is free of broadcast stations and their associated interference, and because most NDBs do little more than transmit their Morse Code callsign, they are very easy to identify, making NDB monitoring a very entertaining niche within the DXing hobby.
In North America, the NDB band is from 190 to 435 kHz and from 510 to 530 kHz. In Europe, there is a longwave broadcasting band from 150 to 280 kHz, so the European NDB band is from 280 kHz to 530 kHz with a gap between 495 and 505 kHz because 500 kHz was the international maritime distress (emergency) frequency.
The beacons that are between 510 kHz and 530 kHz can sometimes be heard on AM radios that can tune below the beginning of the AM broadcast band. (For example, the "HEH" beacon in Newark, OH at 524 kHz is within the bandwidth of most AM radios, and the "OS" beacon in Columbus, OH at 515 kHz can also be heard on some AM radios). Some beacons can also be heard on 530 kHz, although from the adjacent frequencies such as "LYQ" at 529 kHz in Manchester, TN but for the most part, reception of NDBs requires a radio receiver that can receive frequencies below 530 kHz (the longwave band). Many so-called "shortwave" radios can receive all frequencies from 150 kHz to 30 MHz, which makes them ideal for listening to NDBs. Whilst this type of receiver is adequate for reception of local beacons, specialized techniques (receiver preselectors, noise limiters and filters) are required for the reception of very weak signals from remote beacons.(9)
The best time for NDB DXing is the last three hours before sunrise. Reception of NDBs is also usually best during the fall and winter because during the spring and summer, there is more atmospheric noise on the LF and MF bands.
ILS marker beacons
A marker beacon is a specialized beacon used in aviation in conjunction with an instrument landing system (ILS), to give pilots a means to determine distance to the runway. Marker beacons transmit on the dedicated frequency of 75 MHz. This type of beacon is slowly phased-out and most new ILS installations have no marker beacons.
Radio propagation beacons
A radio propagation beacon is specifically used to study the propagation of radio signals. Nearly all of them are part of the amateur radio service.
Single letter HF beacons
See: Letter beacon
A group of mysterious radio beacons with single-letter identifiers ("C", "D", "M", "S", "P", etc) transmitting in morse code have been regularly reported on various HF frequencies. There is no official information available about these transmitters and they are not registered with the ITU. Some investigators suggest that some of these beacons (the so called "cluster beacons") are actually radio propagation beacons for naval use.
Space and satellite radio beacons
Beacons are also used in both geostationary and inclined orbit satellites. Any satellite will emit one or more beacons (normally on fixed frequencies) whose purpose is twofold; as well as containing modulated houskeeping information (telemetry), the beacons are also used to locate the satellite (determine its azimuth and elevation) in the sky. Many satellites transmit two distinct beacons, the general beacon, which transmits limited information in CW or FSK and is used for locating and identifying the satellite and the engineering beacon, which transmits telemetry data in FSK or PSK.
An S-band radio beacon was left on the moon by the Apollo 17 mission, transmitting FSK telemetry on 2276.0 MHz.(3)
Driftnet buoy radio beacons
Driftnet radio buoys are extensively used by fishing boats operating in open seas and oceans.(4) They are useful for collecting long fishing lines or fishing nets, with the assistance of a radio direction finder. According to product information released by manufacturer Kato Electronics Co, Ltd., these buoys transmit on 1600–2850 kHz with a power of 4-15 W.
Some types of driftnet buoys, called "SelCall buoys", answer only when they are called by their own ships. Using this technique the buoy prevents nets and fishing gears from being carried away by other ships, while the battery power consumption remains low.(5)
Distress radiobeacons, also collectively known as distress beacons, emergency beacons, or simply, beacons, are those tracking transmitters that operate as part of the international Cospas-Sarsat Search and Rescue satellite system. When activated, these beacons send out a distress signal that, when detected by geostationary|non-geostationary satellites, can be located by triangulation. In the case of 406 MHz beacons which transmit digital signals, the beacons can be uniquely identified almost instantly (via GEOSAR). Furthermore, a GPS position can be encoded into the signal, thus providing both instantaneous identification & position. Distress signals from the beacons are homed by Search and Rescue (SAR) aircraft and ground search parties who can in turn come to the aid of the concerned boat, aircraft, and/or persons.
There are three kinds of distress radiobeacons:
- EPIRB (Emergency Position Indicating Radio Beacons) used for maritime distress,
- ELT (Emergency Locator Transmitters) signal aircraft distress
- PLB (Personal Locator Beacons) are for personal use and are intended to indicate a person in distress who is away from normal emergency response capabilities (i.e. 911)
The basic purpose of distress radiobeacons is to get people rescued within the the first 24 hours following an accident, when the majority of survivors can still be saved .
IEEE 802.11 Wi-Fi beacons
In the field of Wi-Fi (wireless local area networks using the IEEE 802.11b and 802.11g specification), the term beacon signifies a specific data transmission from the wireless access point (AP), which carries the SSID, the channel number and security protocols such as WEP (Wired Equivalent Protection) or WPA (Wi-Fi Protected Access). This transmission does not contain the link layer address of another Wi-Fi device, therefore it can be received by any LAN client. (6)
AX.25 packet radio beacons
Stations participating in packet radio networks based on the AX.25 link layer protocol also use beacon transmissions to identify themselves and broadcast brief information about operational status. The beacon transmissions use special UI or Unnumbered Information frames, which are not part of a connection and can be displayed by any station. (7) (8) Beacons in traditional AX.25 amateur packet radio networks contain free format information text, readable by human operators.
This mode of AX.25 operation, using a formal machine-readable beacon text specification developed by Bob Bruninga, WB4APR, became the basis of the Automatic Packet Reporting System (APRS) networks.
- Appleyard, S.F., Linford, R.S. and Yarwood, P.J.: "Marine Electronic Navigation", 2nd edition, Routledge & Kegan Paul, ISBN 0-7102-1271-2, pp. 68–69, 1988.
- Connolly, R.: "Navigation Beacons", Radio & Communications Monitoring Monthly, 3(4), ISSN 179-7809, p. 58, April 2008.
- Jessop, G.R., G6JP: "VHF-UHF manual" (4th ed), Radio Society of Great Britain, ISBN 0-900612-63-0, page 2.19, 1983.
- Pelagic Fishing Methods in the Pacific, Western Pacific Regional Fishery Management Council.
- Robert Connolly: "Greyline DXing, Fishing NDBs and NOTAM software", Radio User, 5(6), pages 35–36, ISSN 1748-8117, June 2010.
- IEEE: "Local and metropolitan area networks — Specific requirements — Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band (IEEE Std 802.11b-1999)", Institute of Electrical and Electronic Engineers, p. 2, 2003.
- AX.25 Link Access Protocol for Amateur Packet Radio
- Terry L. Fox, WB4JFI: "AX.25 Amateur Packet-Radio Link-Layer Protocol, Version 2.0", The American Radio Relay League, Newington, CT, ISBN 0-87259-011-9, p. 18, 1984.
- Remington, S., KH6SR: "On the Art of NDB DXing", The Longwave Club of America, 1987-1989
- Klawitter, G.: "Funk-Baken und Indikatorstationen" (in German), Siebel Verlag, ISBN 3-89632-055-6 (2001)
- NDB list website by Alan Gale, G4TMV
- Godfrey Manning: "Sky High: ADF and NDBs", Radio User, 2(12), pp. 25, ISSN 1748-8117, December 2007
- Godfrey Manning: "Sky High: NDB/ADF", Radio User, 3(1), PW Publishing Ltd, ISSN 1748-8117, pages=24–25, January 2008.
- The WiFi Alliance: WPA deployment for public access, 2004.
- Five steps to creating a Wireless Network
- Sheldon Remington, NI6E (ex KH6SR): "On the Art of NDB DXing", The Lowdown, 1987-1988.
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