Longwave
Longwave
Main page
2257646

Longwave

logo
Community Hub0 subscribers
Read side by side
Longwave
View on Wikipedia
from Wikipedia

Tuning dial on 1946 Dynatron Merlin T.69 console radio receiver, showing LW wavelengths between 800 and 2000 metres (375–150 kHz)

In radio, longwave (also spelled long wave or long-wave[1] and commonly abbreviated LW[2]) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dating from the early 20th century, when the radio spectrum was considered to consist of LW, MW, and short-wave (SW) radio bands. Most modern radio systems and devices use wavelengths which would then have been considered 'ultra-short' (i.e. VHF, UHF, and microwave).

In contemporary usage, the term longwave is not defined precisely, and its intended meaning varies. It may be used for radio wavelengths longer than 1,000 m[2] i.e. frequencies[note 1] smaller than 300 kilohertz (kHz),[3][4] including the International Telecommunication Union (ITU) low frequency (LF, 30–300 kHz) and very low frequency (VLF, 3–30 kHz) bands. Sometimes the upper limit is taken to be higher than 300 kHz, but not above the start of the medium wave broadcast band at 520 kHz.[5]

In Europe, Africa, and large parts of Asia (International Telecommunication Union Region 1), where a range of frequencies between 148.5 and 283.5 kHz is used for AM broadcasting[6] in addition to the MW band, the term longwave usually refers specifically to this broadcasting band, which falls wholly within the LF band of the radio spectrum (30–300 kHz). The "Longwave Club of America" (United States) is interested in "frequencies below the AM broadcast band"[5] (i.e., all frequencies below 520 kHz).

Propagation

[edit]

Because of their long wavelength, radio waves in this frequency range can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called ground wave, is the main mode in the longwave band.[7] The attenuation of signal strength with distance by absorption in the ground is lower than at higher frequencies, and falls with frequency. Low frequency ground waves can be received up to 2,000 kilometres (1,200 mi) from the transmitting antenna. Very low frequency waves below 30 kHz can be used to communicate at transcontinental distances, can penetrate saltwater to depths of hundreds of feet, and are used by the military to communicate with submerged submarines.

Low frequency waves can also occasionally travel long distances by reflecting from the ionosphere (the actual mechanism is one of refraction), although this method, called skywave or "skip" propagation, is not as common as at higher frequencies. Reflection occurs at the ionospheric E layer or F layers. Skywave signals can be detected at distances exceeding 300 kilometres (190 mi) from the transmitting antenna.[8]

Non-broadcast use

[edit]

Non-directional beacons

[edit]
Main article: Non-directional beacon

Non-directional beacons transmit continuously for the benefit of radio direction finders in marine and aeronautical navigation. They identify themselves by a callsign in Morse code. They can occupy any frequency in the range 190–1750 kHz. In North America, they occupy 190–535 kHz. In ITU Region 1 the lower limit is 280 kHz.

Time signals

[edit]

There are institutional broadcast stations in the range that transmit coded time signals to radio clocks. For example:

Radio-controlled clocks receive their time calibration signals with built-in long-wave receivers. They use long-wave, rather than short-wave or medium-wave, because long-wave signals from the transmitter to the receiver always travel along the same direct path across the surface of the Earth, so the time delay correction for the signal travel time from the transmitting station to the receiver is always the same for any one receiving location.

Longwaves travel by groundwaves that hug the surface of the Earth, unlike mediumwaves and shortwaves. Those higher-frequency signals do not follow the surface of the Earth beyond a few kilometers, but can travel as skywaves, 'bouncing' off different layers of the ionosphere at different times of day. These different propagation paths can make the time lag different for every signal received. The delay between when the long-wave signal was sent from the transmitter (when the coded time was correct) and when the signal is received by the clock (when the coded time is slightly late) depends on the overland distance between the clock and the transmitter and the speed of light through the air, which is also very nearly constant. Since the time lag is essentially the same, a single constant shift forward from the time coded in the signal can compensate for all long-wave signals received at any one location from the same time signal station.

Submarine communication

[edit]

The militaries of the United Kingdom, Russian Federation, United States, Germany, India and Sweden use frequencies below 50 kHz to communicate with submerged submarines.

Amateur radio

[edit]

In the ITU Radio Regulations the band 135.7–137.8 kHz is allocated (on a secondary basis) to Amateur radio worldwide, subject to a power limit of 1 watt EIRP. Many countries' regulators license amateurs to use it.

LowFER

[edit]

In North America during the 1970s, the frequencies 167, 179, and 191 kHz were assigned to the short-lived Public Emergency Radio of the United States.

Nowadays, in the United States, Part 15 of FCC regulations allow unlicensed use of the 160–190 kHz band a transmitter / amplifier output power to the antenna of at most 1 watt, with an antenna at most 15 meters (49 feet) high; this is called Low Frequency Experimental Radio (LowFER). The 190–435 kHz band is used for navigational beacons.

Frequencies from 472–479 kHz are available to licensed amateurs as the new 630 m band, part of the now-defunct maritime band, but this is often considered a medium wave sub-band.

Historic

[edit]

Swedish station SAQ, located at the Varberg Radio Station facility in Grimeton, is the last remaining operational Alexanderson alternator long-wave transmitter. Although the station ended regular service in 1996, it has been maintained as a World Heritage Site, and makes at least two demonstration transmissions yearly, on 17.2 kHz.[9]

Broadcasting

[edit]

Longwave is used for broadcasting only within ITU Region 1. The long-wave broadcasters are located in Europe, North Africa and Mongolia.

Typically, a larger geographic area can be covered by a long-wave broadcast transmitter compared to a medium-wave one. This is because ground-wave propagation suffers less attenuation due to ground conductivity at lower frequencies.[10]

Many countries have stopped using LW for broadcasting because of low audience figures, a lack of LW on new consumer receivers, increasing interference levels, the energy inefficiency of AM and high electricity costs at transmitters.

Between 2014 and 2015 Russia closed all of its LW broadcast transmitters.[11]

As of 2024 more than half of LW frequencies are unoccupied and some of the remaining services are scheduled for closure. BBC Radio 4 (UK) announced that it will stop distinct programming for LW broadcasts in 2024 in an effort to transition listeners to other means of listening. A closure date for LW broadcasts has not yet been announced.[12] The radio teleswitch service for electricity meters is broadcast with the long wave signal. In October 2024 the Radio Teleswitch Taskforce said that it will end on 30 June 2025.[13]

Carrier frequencies

[edit]

With the adoption of the Geneva Frequency Plan of 1975, long-wave carrier frequencies are exact multiples of 9 kHz; ranging from 153 to 279 kHz. One exception was a French-language station, Europe 1 in Germany, which retained its prior channel spacing until the long-wave service was terminated in 2019. Other exceptions are all Mongolian transmitters, which are 2 kHz above the internationally recognized channels.[clarification needed]

Until the 1970s, some long-wave stations in northern and eastern Europe and the Soviet Union operated on frequencies as high as 433 kHz.[14]

Some radio broadcasters, for instance Droitwich transmitting station in the UK, derive their carrier frequencies from an atomic clock, allowing their use as frequency standards. Droitwich also broadcasts a low bit-rate data channel, using narrow-shift phase-shift keying of the carrier, for Radio Teleswitch Services.

Long-distance reception

[edit]

Because long-wave signals can travel very long distances, some radio amateurs and shortwave listeners engage in an activity called DXing. DXers attempt to listen in to far away transmissions, and they will often send a reception report to the sending station to let them know where they were heard. After receiving a report, the sending station may mail the listener a QSL card to acknowledge this reception.

Reception of long-wave signals at distances in excess of 17,000 kilometres (11,000 mi) have been verified.[15]

List of long-wave broadcasting transmitters

[edit]
See also: List of longwave radio broadcasters
Height diagram of the antenna towers and antenna masts of long-wave broadcasting stations

See also

[edit]

Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Longwave

View on Grokipedia
from Grokipedia
Longwave, also known as long-wave or LW, refers to the portion of the radio spectrum in the low frequency (LF) range from 30 kHz to 300 kHz, corresponding to wavelengths greater than 1,000 meters.[1] This band is characterized by efficient ground-wave propagation, allowing signals to travel hundreds or thousands of kilometers over the Earth's surface with minimal attenuation, particularly at night when sky-wave interference is reduced.[2] In the context of broadcasting, longwave specifically denotes the allocated band of 148.5–283.5 kHz for amplitude-modulated (AM) transmissions in ITU Region 1, which encompasses Europe, Africa, the Middle East, and parts of Asia.[3] Historically, longwave broadcasting emerged in the early 20th century as one of the first practical methods for wide-area radio communication, enabling transcontinental transmissions and national coverage before the widespread adoption of medium-wave and shortwave bands. Its advantages include reliable reception in remote or rural areas, penetration through buildings and foliage, and stability during ionospheric disturbances compared to higher frequencies.[2] Today, longwave remains in use primarily in Europe for public service broadcasting, with notable stations including the BBC's Radio 4 on 198 kHz, which provides news, drama, and cultural programming across the United Kingdom and neighboring countries from the Droitwich transmitting station.[4] Other active broadcasters operate on frequencies such as 252 kHz (Algeria's Radio Algérie), though the number of stations has declined due to the rise of digital alternatives like DAB and online streaming.[5] Beyond broadcasting, longwave serves critical non-entertainment functions, including time signal dissemination and navigation aids. The German DCF77 station at 77.5 kHz, operated by the Physikalisch-Technische Bundesanstalt (PTB), transmits precise time codes derived from atomic clocks, enabling synchronization for radio-controlled clocks and scientific instruments across Europe with an accuracy of up to 1 millisecond.[6] In aviation and maritime sectors, non-directional beacons (NDBs) in the 190–535 kHz range provide low-frequency navigation signals for aircraft and ships, though these are increasingly supplemented or replaced by GPS.[7] Despite its enduring reliability for emergency communications and wide-area coverage, longwave broadcasting faces obsolescence; for instance, the BBC plans to terminate Radio 4's longwave service in 2026 as listener migration to FM, DAB, and digital platforms accelerates.[8]

Fundamentals

Definition and Frequency Range

Longwave refers to a portion of the electromagnetic radio spectrum characterized by wavelengths longer than 1,000 meters, which corresponds to frequencies below 300 kHz. This segment falls within the low frequency (LF) band as defined by the International Telecommunication Union (ITU), spanning 30–300 kHz and encompassing wavelengths from 10 km to 1 km. Longwave signals are distinguished from higher-frequency bands like medium wave (300 kHz–3 MHz) by their extended wavelengths, which enable unique propagation characteristics, though detailed behaviors are addressed elsewhere.[3] The ITU allocates the primary longwave band internationally as 30–300 kHz, with secondary uses extending into adjacent low-frequency segments. For broadcasting purposes, longwave is specifically designated in ITU Region 1 (Europe, Africa, and parts of Asia) from 148.5–283.5 kHz, often narrowed to 153–279 kHz for operational channels spaced at 9 kHz intervals.[3] In contrast, Regions 2 (Americas) and 3 (Asia-Pacific excluding parts of Region 1) do not allocate this sub-band for broadcasting, reserving it primarily for non-broadcast services such as radionavigation and maritime mobile.[3] These allocations ensure global coordination to minimize interference across services. National regulatory bodies adapt ITU guidelines to local needs; for instance, the U.S. Federal Communications Commission (FCC) restricts non-broadcast operations in the low-frequency range, authorizing fixed, mobile, radionavigation, and aeronautical radionavigation services in the 90–110 kHz and 160–190 kHz bands on a primary or secondary basis.[9] In these U.S. bands, federal radionavigation services hold priority, with non-federal uses requiring coordination and subject to case-by-case authorization for applications such as offshore radiolocation. Note that the LORAN-C system, previously operational in the 90-110 kHz band, was discontinued in 2010, leading to increased opportunities for non-federal uses such as radiolocation (as of March 2025).[9][10] Broadcasting is explicitly excluded from these low-frequency allocations in the United States, aligning with the absence of a dedicated longwave broadcast band in ITU Region 2.[9] The relationship between wavelength (λ) and frequency (f) in longwave is governed by the formula λ = c / f, where c is the speed of light (3 × 10^8 m/s) and f is expressed in hertz.
λ=cf \lambda = \frac{c}{f}
For example, a frequency of 300 kHz yields a wavelength of 1 km (λ = 3 × 10^8 / 3 × 10^5 = 1,000 m), while 200 kHz corresponds to 1.5 km (λ = 3 × 10^8 / 2 × 10^5 = 1,500 m). These conversions illustrate the kilometric scale typical of longwave, influencing antenna design and regulatory spacing.

Physical Properties and Comparisons

Longwave signals, operating in the low-frequency (LF) band of 30–300 kHz, necessitate large antenna structures due to their extended wavelengths, which range from 1 to 10 km. A quarter-wavelength monopole antenna at 200 kHz measures approximately 375 m in height, typically realized through tall vertical masts or umbrella configurations to approximate resonance and optimize performance.[11] These designs are vertically polarized to support efficient groundwave and skywave propagation over intermediate distances of 500–1500 km. Electrically short antennas, common in constrained applications, suffer from low radiation resistance—often below 10 ohms—resulting in poor efficiency, as a significant portion of input power dissipates as heat rather than radiated energy. The low frequencies of longwave reduce atmospheric absorption relative to higher bands, enabling more stable signal propagation, though attenuation strongly depends on ground conductivity, with poorer performance over low-conductivity terrains like ice or sand. Longwave exhibits excellent penetration through obstacles, including buildings and foliage, as its longer wavelengths facilitate diffraction and reduced scattering losses compared to shorter wavelengths in medium-wave or VHF bands.[11] In comparison to the medium-wave (MW) band (300–3000 kHz), longwave achieves broader coverage via groundwave propagation, often extending hundreds of kilometers farther over conductive ground, but offers inferior directivity owing to the practical challenges of achieving high-gain patterns with massive LF antennas. Relative to shortwave (HF, 3–30 MHz), longwave provides more consistent daytime groundwave reliability, as HF skywave suffers heavy D-region absorption during daylight, though longwave skywave remains less dependable for long-distance links under similar conditions. Compared to very low frequency (VLF, 3–30 kHz), longwave demands less extreme infrastructure—such as enormous buried arrays or trailing wires—while yielding shorter effective ranges due to increased surface wave attenuation over distance.[12][11] The longwave noise environment is dominated by atmospheric noise from sferics (lightning-generated pulses), which propagate efficiently at LF and exhibit diurnal and seasonal peaks tied to global thunderstorm activity, alongside man-made interference from power lines and switching equipment. These factors elevate the noise floor, yielding signal-to-noise ratios typically 10–20 dB lower than in VHF bands, where galactic and receiver noise predominate over atmospheric contributions.[11]

Propagation

Groundwave Mechanism

Groundwave propagation in the longwave band enables signals to follow the curvature of the Earth's surface primarily through diffraction and surface conduction currents, allowing reliable communication over hundreds to thousands of kilometers during daytime when skywave interference is minimal due to ionospheric absorption. This mode is particularly effective at low frequencies (30–300 kHz) because the long wavelengths interact strongly with the ground, inducing currents that guide the wave along the surface without significant elevation.[13] Key factors influencing groundwave range include soil conductivity, terrain irregularities, transmitted power, and frequency. Seawater, with a conductivity of approximately 4 S/m, supports ranges exceeding 1000 km due to low attenuation, while dry land, at about 0.001 S/m, limits propagation to 100–200 km from energy losses in the soil. Terrain features such as hills introduce shadowing effects, reducing field strength by more than 10 dB in obstructed paths by blocking the surface wave. Range scales with the square root of transmitted power and inversely with the square root of frequency (approximately proportional to $ f^{-0.5} $) for conductivity-limited paths, emphasizing the advantage of lower frequencies in longwave applications. The Sommerfeld-Norton theory provides the foundational model for groundwave attenuation over a homogeneous flat Earth, extended to spherical geometry for practical use. In this framework, the attenuation for the surface wave over a conducting medium yields the exponential decay of field strength as e−αde^{-\alpha d} with distance dd; the full solution involves complex integrals resolved numerically for arbitrary parameters.[13] Norton's extension incorporates Earth's curvature via a numerical distance parameter, enabling predictions of field strength variations.[13] In practice, a 1 MW transmitter at 200 kHz over seawater can achieve reliable groundwave coverage of 1500–2000 km, leveraging the high conductivity for minimal loss. Historical non-directional beacons (NDBs) operating in the longwave band have demonstrated ranges up to 500 km over land under favorable conditions, such as moderate conductivity and flat terrain, supporting aviation navigation.

Skywave and Ionospheric Effects

Skywave propagation in the longwave band (30–300 kHz) relies on reflections from the ionospheric E and F layers, enabling multi-hop paths that extend signal range significantly beyond line-of-sight limitations.[11] These layers refract low-frequency waves back to Earth due to their electron densities exceeding the plasma frequency threshold for the signal frequency, with the E layer (90–130 km altitude) supporting single-hop distances up to approximately 2,000 km and the F layer (130–500 km) allowing hops exceeding 4,000 km in the 1F2 mode, where a single reflection from the F2 sub-layer facilitates jumps of 2,000 km or more.[11] Multi-hop propagation, involving successive reflections between the ionosphere and Earth's surface, can achieve transcontinental distances, though each hop introduces potential losses and phase shifts.[2] The ionosphere's influence on longwave skywave is profoundly shaped by solar activity, which modulates the critical frequency of the E layer (foE) to approximately 3–5 MHz, far above longwave frequencies, ensuring reliable reflection via lower-order modes rather than penetration.[14] Seasonal variations in ionization density further affect propagation, with higher summer electron densities enhancing reflection efficiency, while geomagnetic disturbances in auroral zones—such as polar cap absorption events—can disrupt signals by increasing D-region ionization and causing temporary blackouts or enhanced absorption.[2] A key factor in skywave reliability is signal fading arising from multipath interference, where multiple propagation paths (e.g., direct skywave and groundwave or varying hop combinations) arrive out of phase, leading to constructive or destructive interference patterns.[11] This multipath effect is particularly pronounced at night, contributing to signal fluctuations of 10–50% in amplitude.[11] Ionospheric absorption, which attenuates skywave signals, is quantified by the formula
A=∫κ ds A = \int \kappa \, ds
where AA is the total absorption, κ\kappa is the absorption coefficient (dependent on electron density and collision frequency in the ionosphere), and dsds is the differential path length through the absorbing region.[11] In the D layer, κ\kappa peaks due to high collision rates with neutral particles, making absorption highly sensitive to electron density variations. Diurnal variations dominate longwave skywave behavior, with the D layer (50–90 km altitude) acting as a primary absorber during daylight hours when solar Lyman-alpha radiation ionizes it to electron densities of 10810^8–10910^9 m−3^{-3}, resulting in up to 90% signal absorption and confining propagation largely to stable groundwaves.[11] At night, the D layer dissipates rapidly after sunset, reducing absorption dramatically (by 2–3 orders of magnitude in reflection coefficient), allowing skywaves to dominate and enable reception over distances up to 2,400 km or more, though with inherent variability from residual multipath and layer height changes (e.g., E-layer reflection at ~90 km nighttime versus 70 km daytime).[15] This nighttime enhancement supports transcontinental links but introduces challenges like 30 dB amplitude swings from interference.[15]

Non-Broadcast Applications

Navigation and Beacon Systems

Non-directional beacons (NDBs) serve as essential ground-based aids for aeronautical navigation, transmitting omnidirectional signals that enable aircraft to determine relative bearings. These systems operate primarily in the low-frequency (LF) and medium-frequency (MF) bands, with ICAO standards defining the core operational range as 190 to 535 kHz, of which the longwave (LF) portion spans 190 to 300 kHz. Aeronautical NDBs, or A-NDBs, occasionally utilize even lower frequencies around 175 to 190 kHz for specific applications in challenging environments. The beacons emit a continuous unmodulated carrier wave, amplitude-modulated with either a 400 Hz or 1020 Hz tone for identification purposes, including a two- or three-letter Morse code identifier transmitted at intervals. Power levels typically range from 25 W to 2000 W, supporting effective ranges of 50 to 300 km during daylight via groundwave propagation, though distances can extend further at night due to skywave effects.[16][17] Aircraft equipped with automatic direction finders (ADFs) receive NDB signals and automatically orient a sensing antenna toward the station, providing pilots with a magnetic bearing for en-route navigation, holding patterns, or non-precision instrument approaches. ICAO Annex 10, Volume I, outlines technical specifications, including a minimum field strength of 70 to 120 µV/m, a protection ratio of at least 15 dB against interference, and modulation depths of 10% to 30% for the carrier. These standards ensure compatibility across global aviation operations, with NDBs often co-located at airports or along airways to define waypoints. Their simplicity and low cost have historically made them valuable backups to satellite-based systems, particularly in areas with poor GPS reception.[16][18] In maritime contexts, NDBs function similarly to their aeronautical counterparts, aiding vessel positioning near coasts and offshore installations, with signals integrated into broader navigation frameworks like historical systems such as Loran-C operating around 100 kHz. A prominent example is the Decca Navigator, a hyperbolic positioning system that employed sequential transmissions across four longwave bands—70-72 kHz, 84-86 kHz, 112-115 kHz, and 126-129 kHz—to derive lines of position via phase comparisons between master and slave stations, achieving accuracies of 50 to 800 m for ships traveling up to 50 knots. This system, recognized by the International Maritime Organization (IMO), supported global maritime routes until its obsolescence.[16][19] The reliance on NDBs has diminished significantly since the 2010s with the widespread adoption of Global Navigation Satellite Systems (GNSS) like GPS, leading to phased decommissioning worldwide. In the United States, the Federal Aviation Administration (FAA) ceased publishing new NDB instrument approaches and has decommissioned the majority of its NDB network by 2020, retaining only select units in remote or oceanic regions for contingency use as of 2025. For instance, ongoing proposals target the removal of Alaska-based NDBs due to equipment failures and redundancy with GNSS, though some persist to support operations in GNSS-denied environments. Their groundwave propagation enhances reliability over water and rugged terrain, but overall, NDB infrastructure continues to contract globally.[20][21]

Time and Frequency Signals

Longwave transmissions serve as a reliable medium for disseminating precise time and frequency standards, enabling synchronization across vast distances with minimal infrastructure. These signals, generated from atomic clocks, provide traceable references to Coordinated Universal Time (UTC) and are essential for applications requiring high stability, such as scientific instrumentation and critical infrastructure. Primary stations operate in the low-frequency band, leveraging groundwave and skywave propagation for coverage primarily in the Northern Hemisphere.[22] Key stations include DCF77 in Germany, MSF in the United Kingdom, and WWVB in the United States. DCF77, operated by the Physikalisch-Technische Bundesanstalt (PTB), broadcasts at 77.5 kHz from Mainflingen with a 50 kW semiconductor transmitter since 1998, having commenced official operations on January 1, 1959.[23][24] MSF, managed by the National Physical Laboratory (NPL), transmits at 60 kHz from Anthorn, Cumbria, with an effective radiated power of 15 kW in an omnidirectional pattern.[25] WWVB, under the National Institute of Standards and Technology (NIST), operates at 60 kHz from Fort Collins, Colorado, delivering 70 kW effective radiated power via dual antennas erected in 1962.[26] These stations maintain carrier frequencies with uncertainties below 1 part in 10^12, derived from ensemble atomic clocks synchronized to UTC.[22][27] Signal formats employ modulation techniques to encode time information alongside the carrier. DCF77 uses 85% amplitude modulation for binary-coded decimal (BCD) transmission of minutes, hours, day, month, year, and leap second announcements every minute, supplemented by phase shifts of ±13° at second marks for enhanced precision; it also includes bits for solar geomagnetic data.[28][29] MSF similarly encodes date and time in BCD via on-off keying and phase modulation, with the end-of-minute marker accurate to ±1 millisecond relative to UTC(NPL).[25] WWVB broadcasts a 60-bit frame per minute using pulse-width modulation in its legacy format, upgraded in 2012 with binary phase-shift keying (BPSK) for robust noise rejection, achieving UTC uncertainty of about 100 microseconds after path delay correction.[22] Over typical reception distances up to 1000 km, these signals deliver timing accuracy of ±1 millisecond, supporting reliable synchronization despite propagation variations.[30][31] Receivers for these signals are integrated into radio-controlled clocks, which automatically adjust for daylight saving time and leap seconds; European models predominantly tune to 77.5 kHz for DCF77, while North American and some global devices use 60 kHz for WWVB or MSF.[32] Beyond consumer use, the signals synchronize telecommunications networks, electrical power grids, and financial systems, where sub-millisecond precision mitigates errors in data exchange and grid stability.[22] As of 2025, all three stations remain fully operational, with DCF77 and MSF providing continuous service across Europe (up to 2100 km at night for DCF77) and WWVB covering North America.[33][34] MSF experiences planned maintenance outages, such as in March and July-August 2025, but maintains high availability.[34] WWVB's 2012 digital enhancements continue to improve reception in noisy environments.[22] Coverage remains limited in the Southern Hemisphere due to the northern locations of transmitters, prompting reliance on alternative global systems like GPS for equatorial regions. Low-power experimental transmissions for time signaling also occur in amateur radio contexts.[22]

Submarine and Military Communications

Longwave signals in the low-frequency (LF) band (30–300 kHz) offer a key advantage for submarine communications due to their ability to penetrate seawater to shallow depths, enabling higher data rates compared to extremely low-frequency (ELF) systems while still allowing one-way broadcasts to submerged vessels.[35] This penetration is governed by the skin depth formula δ=2ωμσ\delta = \sqrt{\frac{2}{\omega \mu \sigma}}, where ω=2πf\omega = 2\pi f is the angular frequency, μ\mu is the magnetic permeability, and σ\sigma is the conductivity of seawater (approximately 4 S/m).[35] For example, at 200 kHz in seawater, the skin depth is roughly 0.5 m, limiting effective reception to near-surface operations but sufficient for tactical updates when submarines are at periscope depth.[36] VLF signals (3–30 kHz) achieve deeper penetration (up to 20–30 m), but LF longwave supports faster transmission for non-emergency messages.[37] Major military powers operate dedicated LF and VLF fixed stations for submarine communications. The U.S. Navy maintains high-power facilities such as the Cutler station in Maine, operating at 24 kHz VLF with LF extensions for broader coverage, transmitting to submerged submarines across global oceans.[37] NATO's Allied Submarine Broadcast System includes VLF transmitters at Anthorn (UK), Tavolara (Italy), and Rhauderfehn (Germany), operating around 20–24 kHz to support allied naval forces with secure, one-way messaging to submerged assets, though the broader system incorporates LF capabilities in the 30–300 kHz range.[38] Russia employs ELF systems like ZEVS at 82 Hz for deep-submerged strategic communications but relies on LF longwave for tactical operations, including high-capacity stations such as Hercules and Ram-M for faster message relay to surface or shallow-diving submarines.[39][40] These systems primarily function through one-way broadcasts, such as the U.S. Emergency Action Messages (EAMs), which deliver encrypted command and control instructions to nuclear-capable forces, including submarines.[37] Data rates typically range from 50 to 300 baud, enabling text-based updates but requiring compression for efficiency; for instance, VLF/LF broadcasts often operate at 50 baud as the backbone of submarine command networks.[38] Transmitters use immense power levels of 100 kW to 1 MW, paired with massive antennas spanning 2–4 km, such as trideco or umbrella arrays, to achieve global reach despite the low frequencies.[37] As of 2025, while satellite systems provide backups for higher-speed links, LF longwave remains essential for its resistance to electromagnetic pulse (EMP) effects in contested environments, as low frequencies suffer less attenuation from nuclear-induced disruptions.[41] Declassified Cold War programs, like the U.S. Navy's Project Sanguine (proposed in 1968 as an ELF precursor), underscored the strategic value of such resilient networks for penetrating deep-water barriers during crises.[42] Modernization efforts, including contracts for VLF/LF antenna upgrades, ensure continued viability amid evolving threats.[43]

Amateur Radio and Low-Power Operations

In the United States, amateur radio operators engage in low-power longwave activities under FCC Part 15 rules, which permit unlicensed operations in the 160–190 kHz band (known as LowFER) with a maximum input power of 1 watt and a transmission line length not exceeding 15 meters.[44] This allocation supports hobbyist experimentation without requiring a license, focusing on beacon transmissions and short-range communications. Internationally, licensed amateur bands include the 135.7–137.8 kHz segment (2200-meter band) in ITU Region 1, where operations are secondary and subject to power limits such as 1 watt effective radiated power (EIRP) to avoid interference with primary users like power line carriers. In the U.S., the FCC expanded access to this band for licensed amateurs in 2017, allowing fixed operations with similar low-power constraints and requiring coordination with the Utilities Technology Council to protect infrastructure. Low-power (QRP) techniques dominate longwave amateur operations due to regulatory limits and propagation challenges, emphasizing efficient antenna designs like small loops or magnetic beacons that fit practical spaces while achieving adequate radiation efficiency.[45] Operators commonly use continuous wave (CW) modes for their narrow bandwidth and robustness in noisy low-frequency environments, often at slow speeds of 1–10 words per minute to enhance readability over distances affected by ground losses.[45] Experimental digital modes, such as slow-speed variants of radioteletype or phase-shift keying, are also employed for data transmission, paired with QRP rigs outputting under 5 watts to maximize signal-to-noise ratios in groundwave propagation, which typically limits reliable contacts to 10–100 kilometers via earth conduction.[45] Amateur communities centered on longwave experimentation include ARRL-supported LowFER groups, where enthusiasts share designs for homebrew transmitters and receivers, fostering activities like beacon monitoring and propagation testing.[45] These groups promote earth-mode experiments, utilizing direct ground conduction for non-radiating signals that travel through soil, enabling communications in areas with high atmospheric noise.[45] As of 2025, longwave amateur activities have seen growth through affordable software-defined radios (SDRs), which facilitate digital signal processing for improved reception of weak signals and integration with computer-based decoding tools. Events such as special operating sessions on the 2200-meter band continue to engage operators, building on the 2017 FCC expansions that have encouraged broader participation in low-power experimentation.[46]

Historical Non-Broadcast Uses

In the early 1900s, Guglielmo Marconi pioneered ship-to-shore wireless communications using low frequencies, often in the MF range (300–600 kHz), with later adoption of longwave (below 300 kHz) for extended distances, to enable maritime operators to exchange telegrams with coastal stations over distances of several hundred kilometers.[47] These systems relied on spark-gap transmitters and large antennas aboard ships and at shore facilities, marking a significant advancement in naval safety and commerce by reducing isolation at sea.[48] During World War I, Marconi's technology evolved into wireless direction finding (WDF) systems, which used longwave signals to locate enemy vessels and aircraft, such as German U-boats and Zeppelins, by triangulating transmissions from high-power stations like Norddeich.[49] British and Allied forces deployed over a dozen Marconi-equipped WDF stations to intercept and plot positions, contributing to naval intelligence efforts that intercepted millions of messages without relying on code-breaking.[49] Transatlantic experiments further demonstrated longwave's potential for point-to-point telegraphy through Ernst Alexanderson's alternator systems, introduced in 1918 at the 200 kW NFF station in New Brunswick, New Jersey, operating at around 200 kHz.[50] These mechanical generators produced continuous waves for Morse code transmission, achieving reliable skywave propagation over 5,000 km to European receivers, even when undersea cables were severed during the war.[51] The alternators enabled daily transoceanic traffic volumes exceeding 100,000 words, supporting military and commercial needs until vacuum-tube alternatives emerged.[50] Other applications included railroad signaling in the 1920s, where experimental wireless systems at approximately 150 kHz facilitated train-to-station communications for safety and dispatching, as tested by early adopters like the Pennsylvania Railroad.[52] In the 1940s, precursors to modern navigation aids, such as low-frequency LORAN experiments, operated in the 100–200 kHz band to provide hyperbolic positioning over extended ranges in the Pacific theater, addressing the limitations of higher-frequency systems during World War II.[53] These historical uses declined post-World War II as radar and shortwave technologies offered superior reliability and portability, rendering alternators and early longwave setups obsolete for most applications.[50] The last operational Alexanderson alternators were phased out by the 1990s, with remaining installations dismantled amid the shift to solid-state electronics.[54]

Broadcasting

Historical Development

The origins of longwave broadcasting trace back to early 20th-century experiments in wireless voice transmission. In December 1906, Canadian inventor Reginald Fessenden conducted the first successful long-distance broadcast of human voice and music from Brant Rock, Massachusetts, using a high-frequency alternator operating at approximately 100 kHz to reach ships at sea up to several hundred miles away. This breakthrough demonstrated the potential of continuous-wave transmission on low frequencies for audio content, laying foundational groundwork for broadcasting beyond Morse code. Commercial longwave broadcasting emerged in Europe during the 1920s, driven by the need for wider coverage amid growing interference on higher frequencies. Stations began operating in the allocated 200 kHz band, with the BBC launching its 5XX transmitter at Daventry in 1925 at 25 kW, providing national coverage across the United Kingdom on a wavelength of 1,500 meters. This marked one of the first dedicated longwave broadcast services, enabling reliable reception over large areas via groundwave propagation.[55] By the late 1920s, similar initiatives proliferated in continental Europe, establishing longwave as a primary medium for public and entertainment programming. The 1930s saw rapid European expansion, with investments in high-power infrastructure to enhance signal reach and quality. The BBC upgraded its network by replacing Daventry with the Droitwich transmitter in 1934, operating at 150 kW (initially planned for 200 kW) on 200 kHz to serve the National Programme continent-wide. Concurrently, commercial ventures like Radio Luxembourg initiated transmissions in 1933 from Junglinster using a 200 kW longwave setup on 1,191 meters, targeting English-speaking audiences in Britain with sponsored content and pioneering advertising formats. Technological advances fueled this growth: early reliance on high-power alternators, such as the Alexanderson type, gave way to more efficient vacuum-tube transmitters by the early 1930s, enabling higher modulation fidelity and reduced interference. Germany also advanced synchronized broadcasting networks during this decade, coordinating multiple transmitters on shared frequencies for uniform national coverage.[56][57] World War II disrupted progress, with most European longwave stations imposing blackouts from 1939 to 1945 to avoid aiding enemy forces through signal triangulation and navigation. The BBC, for instance, suspended Droitwich's domestic service until 1941, redirecting it to propaganda broadcasts. Post-war reconstruction sparked a boom in the 1950s, as nations rebuilt and expanded networks; Europe hosted over 30 major longwave transmitters by mid-decade, supporting public service and international programming amid rising radio ownership.[58][59] Longwave broadcasting peaked in the 1970s and 1980s through high-power stations like those of the BBC, RTL, and Europe 1, which delivered news, music, and talk amid limited FM penetration in rural areas. However, the rise of FM and VHF technologies from the 1990s onward—offering superior audio quality and local coverage—initiated an early decline, shifting audiences and reducing longwave's role in mainstream broadcasting.[60][59]

Technical Specifications and Frequencies

The longwave broadcasting band is allocated in ITU Region 1 from 148.5 kHz to 283.5 kHz for sound broadcasting services. In Europe, operational channels span 153 kHz to 279 kHz with 9 kHz spacing, yielding 15 channels such as 153, 162, 171, 180, 189, 198, 207, 216, 225, 234, 243, 252, 261, 270, and 279 kHz. Usage in Asia and the Middle East follows similar patterns but is sparser, with representative stations operating on frequencies like 162 kHz in countries such as Saudi Arabia and the UAE. In ITU Region 2, covering the Americas including the United States, no dedicated longwave band exists for broadcasting, with the spectrum reserved primarily for non-broadcast services like aeronautical and maritime navigation.[3][5][3] Longwave signals employ narrowband amplitude modulation designated as A3E, consisting of double-sideband transmission with a full carrier to ensure compatibility with legacy receivers. The audio bandwidth is restricted to 3–5 kHz to conserve spectrum and limit interference within the narrow channels. Carrier powers for transmitters typically range from 100 kW to 1500 kW, enabling extensive groundwave coverage. Adjacent-channel synchronization, often via phase-locking of carriers, is implemented to suppress beat interference and maintain signal quality across shared propagation paths.[61][5] Antenna systems for longwave broadcasting feature tall guyed masts to counteract low radiation efficiency at these frequencies, where wavelengths exceed 1 km. A prominent example is the 412 m mast at Hellissandur, Iceland, previously supporting longwave transmissions. For electrically short monopoles (height $ l \ll \lambda $), radiation resistance is approximated as
Rrad≈40π2(lλ)2 Ω, R_{\mathrm{rad}} \approx 40 \pi^2 \left( \frac{l}{\lambda} \right)^2 \, \Omega,
illustrating the inefficiency of compact antennas and the necessity for elevated structures or loading coils to enhance performance. International standards for longwave broadcasting are outlined in ITU-R recommendations, including BS.560, which specifies radio-frequency protection ratios (e.g., 27–40 dB co-channel in LF) to manage interference between stations. Digital modulation trials using DRM30, a variant of Digital Radio Mondiale adapted for bands below 30 MHz, were conducted in the 2010s, demonstrating potential for improved audio quality and data services; however, adoption remains rare as of 2025, with analog AM persisting due to infrastructure and receiver compatibility.[62][61]

Reception Characteristics

Longwave broadcast signals are typically received using simple loop antennas, often featuring a ferrite core with an inductance of 200–500 μH to enhance sensitivity in the 153–279 kHz band.[63] These antennas pair with receivers incorporating high Q filters, where Q values exceed 100, providing sharp selectivity to isolate stations amid adjacent channel interference.[63] In Europe, where longwave broadcasting remains prevalent, portable receiver sets with built-in ferrite loop antennas are widely used for everyday listening, offering compact mobility without sacrificing performance.[64] Coverage for longwave signals relies primarily on groundwave propagation during the day, extending 500–1000 km over land depending on soil conductivity and transmitter power, delivering reliable service to regional audiences.[65] At night, skywave propagation via ionospheric reflection can reach beyond 2000 km, enabling distant reception but introducing fading with signal variations of 20–40 dB due to multipath interference between ground and sky components.[65] Medium-wave bleed-over from higher-frequency stations further complicates nighttime listening, often overpowering weaker longwave signals within 2000 km.[65] Listeners in rural areas benefit from stable groundwave signals with minimal disruption, allowing clear reception of local stations even on basic equipment.[66] In urban environments, however, reception suffers from prevalent noise, including a characteristic 50/60 Hz hum generated by power lines and electrical infrastructure, which elevates the noise floor and masks weaker broadcasts.[66] To mitigate such interference, enthusiasts employ phasing antennas, which combine signals from multiple loops to create deep nulls—up to 30–40 dB attenuation—targeting specific noise sources or unwanted stations.[67] As of 2025, modern software-defined radio (SDR) receivers have become popular for longwave DXing, incorporating digital noise reduction algorithms that suppress atmospheric and man-made interference by 10–20 dB, improving signal-to-noise ratios for faint skywave paths.[68] Complementary mobile apps, such as those integrating real-time ionosonde data, assist in tracking ionospheric conditions to predict optimal propagation windows, enabling hobbyists to log distant European longwave stations from afar.[69]

Current Transmitters and Global Status

In 2025, longwave broadcasting persists with a small number of high-power transmitters primarily serving Europe, North Africa, and Asia, focusing on national public service content in analog AM format. Key operational facilities include the BBC's Radio 4 transmission on 198 kHz from sites in Scotland and England at up to 400 kW, providing 24-hour programming to the UK and parts of northern Europe.[70] Poland's Polskie Radio 1 on 225 kHz from Solec Kujawski at 1200 kW (daytime), 700 kW (nighttime) delivers nationwide coverage, while Romania's Antena Satelor on 153 kHz from Bod at 200 kW supports rural listeners with reduced hours since August 2025.[5] In North Africa, Algeria's Radio Algérienne operates on 153 kHz from Kénadsa at 2000 kW (reactivated mid-September 2025) and on 252 kHz from Tipaza at 750 kW (occasional), while Morocco's Radio Méditerranée Internationale (Médi 1) transmits on 171 kHz from Nador at 1600 kW during prime hours, targeting regional and Mediterranean reception.[5] Low-power or occasional operations include Italy's AM-Italia on 207 kHz at 1 kW and Sweden's Kinna 261 on 261 kHz at 0.05 kW (testing since October 2025).
StationFrequency (kHz)LocationPower (kW)OperatorCoverage Focus
BBC Radio 4198UK (multiple sites)400British Broadcasting CorporationUK national, northern Europe
Polskie Radio 1225Solec Kujawski, Poland1200 (day), 700 (night)Polskie RadioPoland national
Antena Satelor153Bod, Romania200Societatea Română de RadiodifuziuneRomania rural (reduced hours)
Radio Méditerranée Internationale (Médi 1)171Nador, Morocco1600 (day)Société d'Études de Radiodiffusion et de TélévisionMorocco, Mediterranean
Radio Algérienne, Chaîne 1153Kénadsa, Algeria2000Radiodiffusion Télévision AlgérienneAlgeria national (reactivated Sep 2025)
Globally, approximately 10 high-power longwave transmitters remain active for broadcasting purposes, a sharp decline from around 40 in 2000, reflecting a broader shift toward FM, DAB, and digital platforms.[60] All operate in traditional AM analog mode, with no widespread adoption of digital modulation, preserving compatibility for legacy receivers in remote areas. These stations primarily cater to rural and older demographics in Europe and Asia, where modern alternatives like DAB face coverage gaps or equipment barriers.[5] Recent closures underscore the diminishing infrastructure: Denmark's DR Kalundborg site on 243 kHz at 300 kW ended operations in December 2023, Ireland's RTÉ Radio 1 on 252 kHz ceased in April 2023 (frequency now used by Algeria), and Iceland's RÚV transmission on 189 kHz shut down in September 2024.[60] France's Allouis facility, once a major 200 kW broadcaster on 162 kHz, transitioned away from program services by 2016, though a time signal persists.[5] The BBC's longwave service, operational since 1925, faces potential shutdown in 2026 amid cost pressures and spectrum reallocation, despite its role in emergencies such as providing resilient news during the 2022 Ukraine conflict, where analog radio proved vital amid digital disruptions.[71] Relays and backups continue for crisis scenarios, including potential EU-wide phase-out discussions targeting 2030 to free spectrum for other uses.[70] The listener base centers on niche audiences, with estimates suggesting up to 1.5 million regular users in the UK alone, mainly older individuals and those in rural or maritime settings lacking alternatives; Europe-wide figures hover around 10-20 million, bolstered by long-distance reception appealing to international DX enthusiasts.[71] This sustained but shrinking role highlights longwave's enduring value in underserved contexts, even as global trends favor spectrum-efficient technologies.[5]

References

  1. long wave noun - Definition, pictures, pronunciation and usage notes
  2. [PDF] Longwave Propagation - A Brief Review. - DTIC
  3. Frequency Bands allocated to Terrestrial Broadcasting Services - ITU
  4. About Radio 4 - BBC
  5. 153-279 kHz: Longwave Radio stations in Asia, Europe and North ...
  6. DCF77 - PTB.de - Physikalisch-Technische Bundesanstalt
  7. Long Wave RadioBy Ross Goodall, WD4NJV - nofars.net -
  8. What is happening to Radio 4 on LW? - BBC
  9. [PDF] FCC ONLINE TABLE OF FREQUENCY ALLOCATIONS
  10. [PDF] handbook the ionosphere and its effects on radiowave propagation
  11. [PDF] Report ITU-R BS.2144
  12. [PDF] FCC/OET R86-1, Modern Methods for Calculating Ground-Wave ...
  13. Derivation of global ionospheric Sporadic E critical frequency (foEs ...
  14. Exploiting LF/MF signals of opportunity for lower ionospheric remote ...
  15. [PDF] Handbook on Radio Frequency Spectrum Requirements for Civil ...
  16. [PDF] FAA Order 6740.6 - U.S. National Aviation Standard for the NDB ...
  17. ENR 4.1 Navigation Aids – En Route - Federal Aviation Administration
  18. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/AssemblyDocuments/A.816(19
  19. [PDF] PBN NAS Navigation Strategy - Federal Aviation Administration
  20. Decommissioning of Alaska NDB/DME proposed - AOPA
  21. Radio Station WWVB | NIST
  22. https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time/dcf77/dcf77-transmitting-facilities.html
  23. [PDF] Special Topic 50 years of time dissemination with DCF77
  24. [PDF] NPL Time & Frequency Services, “MSF 60 kHz Time and Date Code”
  25. WWVB Antenna Configuration and Power | NIST
  26. DCF77 carrier frequency - PTB.de
  27. DCF77 time code - PTB.de - Physikalisch-Technische Bundesanstalt
  28. DCF77 phase modulation - PTB.de
  29. DCF77 Time Server - Meinberg
  30. Decoding The MSF-60 Radio Time Signal | TimeTools Ltd
  31. Help with WWVB Radio Controlled Clocks | NIST
  32. Reach of DCF77 - PTB.de - Physikalisch-Technische Bundesanstalt
  33. Time and frequency MSF radio signal - NPL
  34. VLF communications - Australian Space Academy
  35. https://engx.theiet.org/f/wiring-and-regulations/21915/low-frequency-radio-transmission
  36. Very Low Frequency (VLF) - United States Nuclear Forces - Nuke
  37. [PDF] VLF/LF Submarine Communications - Nuclear Information Service
  38. http://rusnavy.com/science/electronics/rv6.htm
  39. ZEVS, The russian 82 Hz ELF transmitter - VLF
  40. [PDF] The ULF/ELF/VLF Electromagnetic Fields Generated in a Sea ... - DTIC
  41. USN ELF Communications System - Navy Radio
  42. Long Wave Secures $84 Million Navy Contract for Very Low ...
  43. 47 CFR 15.217 -- Operation in the band 160-190 kHz. - eCFR
  44. LF (Low Frequency) - ARRL
  45. 630-Meter Special Operating Event on November 11 ... - ARRL
  46. 5. Radio at Sea (1891-1922) - Early Radio History
  47. Marconi Station Site (U.S. National Park Service)
  48. Code Breaking and Wireless Intercepts - Marconi Heritage
  49. 8. Alternator-Transmitter Development (1891-1922)
  50. Alexanderson Radio Alternator | Invention & Technology Magazine
  51. [PDF] Experimental Wireless Stations Edelman 1920 - World Radio History
  52. [PDF] The Development of Loran4 - Time and Frequency Division
  53. Alexanderson alternator - Radio Story
  54. Daventry Transmitter opens providing national radio coverage - BBC
  55. Droitwich - BBCeng.info
  56. [PDF] THE STATION THE STARS - World Radio History
  57. Broadcasting in Britain: War-time Broadcasting - Transdiffusion
  58. Radio - Broadcasting, Technology, Europe | Britannica
  59. Long wave radio fans mourn fading frequencies - BBC
  60. [PDF] DRM Introduction and Implementation Guide - Digital Radio Mondiale
  61. BS.560 : Radio-frequency protection ratios in LF, MF and HF ... - ITU
  62. [PDF] LOOP ANTENNAS DESIGN & THEORY - World Radio History
  63. Best portable radio for LW and NDB reception? - The SWLing Post
  64. None
  65. Power Line Noise - ARRL
  66. [PDF] Active Antenna Phasing System - DX Engineering
  67. Longwave Software Defined Radio - LloydM Home
  68. Propagation - RadioHobbyist.org
  69. BBC Radio 4 begins information campaign to transition listeners ...
  70. [PDF] Still-Speaking-to-the-Nations-Report-on-BBC-Longwave
User Avatar
No comments yet.