Low frequency
Low frequency (LF), designated as band 5 by the International Telecommunication Union (ITU), encompasses radio frequencies from 30 to 300 kHz, corresponding to wavelengths between 1 and 10 kilometers.[1][2] This range enables effective ground-wave propagation, allowing signals to travel hundreds to thousands of kilometers over the Earth's surface, particularly over seawater due to its conductivity, with minimal attenuation during daytime and potential sky-wave enhancement at night via ionospheric reflection.[3][4] Key applications of LF include long-wave amplitude-modulated radio broadcasting, primarily in Europe and Asia for medium-range coverage unaffected by the ionospheric disruptions that impact higher frequencies.[3] Standard time signal services, such as those operating at 60 kHz and 77.5 kHz, disseminate precise timing for synchronization of clocks and scientific instruments, leveraging the band's reliability for stable, long-distance transmission.[5] Navigation aids and military communications also utilize LF for its robustness in over-the-horizon propagation, supporting aircraft, maritime, and select subsurface operations where higher frequencies fail.[3] Amateur radio experimentation in this band further explores diffraction and waveguide effects for extended reach.[6]Fundamentals
Definition and Frequency Range
Low frequency (LF), as designated by the International Telecommunication Union (ITU), encompasses radio frequencies from 30 kHz to 300 kHz. This range corresponds to wavelengths between 10 km and 1 km, calculated using the speed of light (approximately 300,000 km/s) divided by frequency.[3] The LF band lies above very low frequency (VLF, 3–30 kHz) and below medium frequency (MF, 300–3,000 kHz) in the radio spectrum classification.[7] These boundaries stem from ITU recommendations for standardizing telecommunications nomenclature, facilitating global spectrum allocation and interference management. Frequencies below 30 kHz, such as ELF (3–30 Hz) or VLF, exhibit even longer wavelengths and distinct propagation traits, while exceeding 300 kHz shifts into shorter-wave bands with different applications.[3][8]Physical and Electromagnetic Characteristics
Low-frequency (LF) electromagnetic waves span the radio spectrum band from 30 kHz to 300 kHz, as designated by the International Telecommunication Union (ITU) in its standardized nomenclature for frequency allocations. This range positions LF waves immediately above very low frequency (VLF, 3–30 kHz) and below medium frequency (MF, 300 kHz–3 MHz), distinguishing them by their intermediate wavelengths and propagation behaviors within the broader electromagnetic spectrum.[3] The wavelengths of LF waves, determined by the formula λ = c / f where c is the speed of light (2.99792458 × 10^8 m/s) and f is frequency, extend from approximately 10 km at 30 kHz to 1 km at 300 kHz.[2] These long wavelengths impart physical properties such as pronounced diffraction, allowing waves to bend over terrain obstacles and the Earth's curvature more effectively than higher-frequency signals, which contributes to their utility in non-line-of-sight applications.[9] As transverse electromagnetic waves, LF signals feature oscillating electric (E) and magnetic (B) fields mutually perpendicular to each other and to the propagation direction, propagating at near-light speed in free space with minimal dispersion under ideal conditions.[9] Electromagnetically, LF waves carry low photon energies (E = h f, with Planck's constant h ≈ 6.626 × 10^{-34} J·s), rendering them non-ionizing and incapable of breaking molecular bonds, unlike ultraviolet or higher frequencies.[10] Their low frequencies result in correspondingly long oscillation periods (from about 33 μs at 300 kHz to 33 ms at 30 kHz), which facilitate stable phase coherence over distance but demand large-scale antennas for efficient radiation due to the quarter-wavelength rule (λ/4 ≈ 750 m to 2.5 km). In conductive media like seawater or soil, LF waves exhibit skin depths on the order of tens to hundreds of meters, enabling partial penetration for applications such as submarine communication, though with exponential attenuation governed by the material's conductivity and permittivity.[11] Atmospheric and ionospheric interactions minimally attenuate LF waves during ground-wave propagation, as their wavelengths exceed typical turbulence scales, preserving signal integrity over hundreds of kilometers without significant multipath fading.[12] However, free-space path loss follows the inverse-square law (L ∝ 1/d^2), compounded by ground conductivity variations that can enhance or degrade effective range based on terrain—e.g., seawater paths yield ranges up to 1,000 km or more at 100 kHz under optimal conditions.[3] Polarization is typically vertical for ground-wave modes to maximize coupling with the Earth's surface, minimizing tilt losses.[9]Historical Development
Early Experiments and Adoption
Guglielmo Marconi initiated wireless telegraphy experiments in late 1894, building on Heinrich Hertz's 1887 demonstration of electromagnetic waves, initially employing short wavelengths equivalent to higher frequencies for short-range transmission.[13] To extend ranges beyond line-of-sight limitations, Marconi progressively increased antenna lengths and adopted longer wavelengths—corresponding to lower frequencies—enabling signals to propagate farther via ground waves, with successful tests reaching several kilometers by 1897.[14] In the early 1900s, researchers like Michael Pupin advanced tuned circuits operable at low frequencies, prompting Marconi to conduct experiments from 1899 to 1901 that incorporated resonance for selectivity amid interference, facilitating reliable over-water communication.[15] Reginald Fessenden furthered low-frequency applications by developing continuous-wave alternators in 1900, capable of generating tones at frequencies around 50–100 kHz, which he used in 1906 for the first amplitude-modulated voice transmissions across the Atlantic from Brant Rock, Massachusetts, demonstrating LF's potential for intelligible signals over long distances despite atmospheric noise.[16] Adoption accelerated post-1912 with the Titanic disaster highlighting radio's lifesaving role, leading to international mandates for shipboard wireless using frequencies in the 300–500 kHz range—overlapping low-frequency bands—for distress calls and navigation.[17] By the mid-1920s, LF systems were integrated into aviation, with the first low-frequency radio ranges operational in 1928, employing directional antennas at 200–400 kHz to guide aircraft along airways via intersecting signal beams, marking early infrastructural use for air traffic control.[18] These developments underscored LF's reliability for ground-wave propagation in non-line-of-sight scenarios, though limited by large antenna requirements and susceptibility to ionospheric variability.[17]Expansion in Broadcasting and Navigation
In the interwar period, low-frequency (LF) radio expanded significantly in broadcasting to meet demands for wide-area coverage, leveraging ground-wave propagation for reliable, long-distance signal travel with minimal fading. The British Broadcasting Corporation (BBC) pioneered regular longwave transmissions in 1925 from its Daventry station operating around 200 kHz, which provided national reception throughout the United Kingdom and into parts of Europe, marking a shift from medium-wave limitations in rural and nighttime propagation.[19] [20] This approach proved effective for single high-power transmitters to serve large populations, with similar expansions in continental Europe; for instance, Finland established a 197 kHz longwave station in Lahti in 1928, initially at 25 kW and upgraded to 40 kW the following year to enhance coverage.[21] By the 1930s, LF longwave networks grew across Europe, Northern Africa, and Asia for AM audio broadcasting, prioritizing stability over the higher fidelity of shorter waves, though power requirements and large antennas constrained further proliferation.[22] Parallel to broadcasting, LF signals became foundational for radio navigation, particularly in aviation, where their ground-wave reliability enabled precise directional guidance over continental distances. The Low Frequency Radio Range (LFR) system, prototyped in 1926 by the U.S. Bureau of Standards, entered operational service in 1928 with stations transmitting four orthogonal audio-modulated beams (typically at 200–400 kHz) to define airways for instrument flight.[18] [23] This expansion rapidly scaled; by 1930, dozens of LFR stations interconnected to form the backbone of North American airways, supporting en route navigation, approaches, and early air traffic control, with effective ranges up to 100–200 miles daytime and farther at night via skywave.[24] Maritime applications followed, including low-power non-directional beacons (NDBs) in the LF/MF overlap starting in the 1930s for homing, though LF's dominance waned postwar with VHF omnidirectional ranges (VOR).[25] Post-World War II innovations sustained LF navigation utility, notably the Decca Navigator system deployed from 1946 at 85–127.5 kHz, which used phase-comparison hyperbolic positioning for accurate coastal and oceanic fixes, achieving sub-kilometer precision over hundreds of miles and serving military, shipping, and aviation until GPS emergence in the 1990s.[26] These developments underscored LF's persistence in specialized roles due to its penetration through terrain and ionospheric stability, despite inefficiencies compared to higher bands.[27]Decline and Specialized Persistence
Longwave broadcasting, operating in the low frequency band of 30-300 kHz, experienced peak usage in Europe during the mid-20th century for national coverage due to reliable groundwave propagation over long distances. However, from the late 1970s onward, the number of longwave stations dwindled from dozens to a handful, driven by the rise of medium wave, FM, and digital audio broadcasting technologies that offered superior audio quality, spectrum efficiency, and smaller infrastructure requirements.[19] In the United Kingdom, the BBC's longwave service for Radio 4, which began in 1925, faces termination alongside the Radio Teleswitch service by June 30, 2025, as digital alternatives like DAB and internet streaming provide equivalent or better reliability for off-peak programming and utility signals.[19][20] Significant shutdowns marked this decline, including Ireland's RTÉ 252 station, which ceased operations on April 14, 2023, after delays since 2014, citing high maintenance costs for its 210-meter mast and limited audience amid digital shifts.[28] Similarly, aviation's Low Frequency Radio Range (LFR) system, deployed from 1928 to guide aircraft via directional beams, was phased out by the 1970s, replaced by VHF omnidirectional ranges (VOR) that supported higher precision and aircraft densities.[18][29] These transitions reflected broader inefficiencies of LF, such as the need for massive antennas—often exceeding 200 meters—and low data rates, rendering it obsolete for mass media and civil navigation as spectrum demands grew for mobile and satellite services.[19] Despite broadcasting's retreat, LF persists in specialized applications leveraging its unique propagation traits, including deep ground penetration and ionospheric stability for non-line-of-sight communication. Time signal stations remain operational, such as Germany's DCF77 at 77.5 kHz, which synchronizes millions of radio-controlled clocks with atomic precision, transmitting since 1959 for applications in utilities and instrumentation where GPS vulnerability to jamming necessitates robust alternatives.[30] Maritime and navigation aids continue limited LF use for long-range distress signals and buoys, benefiting from surface wave propagation unaffected by atmospheric noise peaks.[31] Military applications sustain LF viability, particularly for submerged submarine communications, where frequencies around 30-300 kHz penetrate seawater better than higher bands, enabling one-way broadcasts from shore stations to vessels at periscope depth or below, as exemplified by systems prioritizing reliability over bandwidth in strategic deterrence scenarios.[32] These niche roles underscore LF's endurance where coverage trumps capacity, insulated from commercial pressures by governmental or institutional mandates, though even here, very low frequency (VLF) extensions below 30 kHz increasingly compete for ultra-reliable links.[30]Propagation Characteristics
Ground Wave and Surface Wave Propagation
Ground wave propagation refers to the transmission of radio signals along the Earth's surface, primarily effective in the low frequency (LF) band of 30 to 300 kHz, where waves diffract and follow the planetary curvature to achieve ranges beyond line-of-sight.[33] This mode relies on the induction of currents in the ground by vertically polarized electromagnetic fields, which sustain propagation through continuous re-radiation from the surface.[34] In LF applications, such as long-range navigation and broadcasting, ground waves provide stable, daytime coverage with minimal atmospheric interference, contrasting with higher-frequency skywave variability.[35] Surface wave propagation constitutes the dominant long-distance component of ground waves in LF, characterized by waves that tightly couple to the conductive boundary layer of the Earth, experiencing exponential attenuation with distance but benefiting from lower frequencies' reduced losses.[34] Unlike the space wave portion (direct and ground-reflected rays, limited to optical horizons), surface waves penetrate slightly into the ground and atmosphere, enabling diffraction over terrain irregularities.[36] Empirical models, such as those developed by the ITU for MF/LF broadcasting, predict field strengths based on this mechanism, with vertical polarization optimizing efficiency over horizontal due to minimized ground tilt and absorption.[37] Key factors influencing propagation distance include ground conductivity (σ), permittivity (ε), and frequency; over seawater (σ ≈ 4 S/m), attenuation is low, supporting LF ranges up to 1000 km or more with sufficient power, while arid land (σ < 10^{-3} S/m) limits effective distance to under 200 km.[34] [9] Time-varying elements like soil moisture and temperature further modulate losses, with higher humidity enhancing conductivity and extending range by 10-20% seasonally.[38] NTIA ground-wave models validate these effects for frequencies below 0.5 MHz, estimating median distances of approximately 300 km over mixed terrain at modest power levels (e.g., 1 kW), scaling logarithmically with transmitter output.[39] Polarization mismatch or elevated antennas can degrade performance by 3-6 dB, underscoring the need for low-angle radiation patterns in LF systems.[40]Skywave Propagation and Ionospheric Interactions
Skywave propagation refers to the reflection or refraction of radio waves by the ionosphere, enabling signals to travel beyond the horizon via multiple hops between the Earth's surface and ionospheric layers. In the low frequency (LF) band of 30–300 kHz, skywave modes are secondary to dominant groundwave propagation but become viable under specific conditions, particularly at night when ionospheric absorption diminishes.[41][42] The ionosphere's D-layer, located at altitudes of approximately 75–95 km, plays a primary role in LF skywave interactions by causing significant non-deviative absorption of signals during daylight hours due to collisions between electrons and neutral particles. This absorption increases inversely with frequency, rendering daytime skywave propagation negligible for LF signals, as most energy is dissipated before reaching higher reflective layers like the E (90–150 km) or F (150–500 km) regions. At night, the D-layer dissipates rapidly after sunset, reducing absorption and allowing LF waves to refract off the E- and F-layers, where electron densities create refractive indices that bend waves back toward Earth.[43][44] Nighttime LF skywave exhibits variability influenced by ionospheric dynamics, including multipath interference from multiple reflection paths, leading to fading with amplitudes that can fluctuate significantly—up to several decibels over short periods—as observed in 40 kHz signals where nighttime amplitudes vary appreciably compared to stable daytime skywave remnants. Solar activity modulates these effects: during solar minima, lower electron densities in the F-layer support more consistent reflections for LF, while geomagnetic storms can enhance absorption or scattering via auroral ionization. Seasonal variations in D-region electron density, inferred from LF propagation data, show higher winter absorption due to nocturnal recombination differences, impacting reliable skywave range.[45][46][47] Wave hop theory models LF skywave as a series of guided modes between the ionosphere and ground, extending into shadow zones with each hop covering thousands of kilometers, though attenuation per hop limits practical multi-hop distances to 2000–3000 km for typical LF powers. Unlike higher-frequency HF bands, LF skywaves experience less critical frequency dependence but are constrained by the ionosphere's lower boundary height, typically 70–100 km at night, which determines the maximum skip distance via the secant law relating virtual height to range. Empirical models for LF/MF bands incorporate skywave field strengths of 0.1–1 mV/m at 1000 km nighttime distances, aiding navigation systems like LORAN-C historically reliant on hybrid ground-skywave paths.[42][48][49]Penetration and Attenuation Properties
Low-frequency (LF) electromagnetic waves, spanning 30 to 300 kHz, exhibit enhanced penetration through lossy and obstructive media relative to higher-frequency signals, owing to their longer wavelengths (1–10 km) that facilitate diffraction around obstacles and larger skin depths in conductive materials. The skin depth, defined as the distance at which the wave amplitude attenuates to 1/e (approximately 37%) of its surface value, scales inversely with the square root of frequency and directly with the resistivity of the medium, given by δ ≈ √(ρ / (π f μ)), where ρ is resistivity, f is frequency, and μ is magnetic permeability.[50] This results in LF waves diffracting effectively over terrain irregularities, foliage, and urban structures, with minimal scattering losses compared to VHF or UHF bands.[51] In the atmosphere, LF signals experience negligible gaseous absorption, with attenuation typically below 0.001 dB/km under standard conditions, as molecular absorption lines (e.g., from water vapor or oxygen) occur at much higher frequencies above 20 GHz.[52] Ionospheric D-layer absorption during daylight can introduce 10–50 dB of additional loss for skywave paths, varying with solar activity and frequency within the LF band, but groundwave and direct paths remain largely unaffected.[53] Penetration into seawater is limited by its high conductivity (≈4 S/m), yielding skin depths of approximately 1.3 m at 30 kHz and 0.4 m at 300 kHz, leading to rapid exponential attenuation (α ≈ 1/δ nepers/m) that restricts reliable communication to near-surface submerged assets.[54] In contrast, soil penetration varies widely with moisture and composition; dry soils (σ ≈ 0.001–0.01 S/m) allow skin depths of 10–50 m, enabling modest subsurface propagation, while wet or clay-rich soils (σ > 0.1 S/m) reduce this to 1–5 m with attenuation rates of 0.2–2 dB/m.[55] Through building materials like concrete or brick, LF waves incur 5–15 dB loss per wall versus 20–40 dB for UHF, supporting indoor reception of distant broadcasts.[51]| Medium | Typical Conductivity (S/m) | Skin Depth at 100 kHz (m) | Attenuation Rate (dB/m) |
|---|---|---|---|
| Seawater | 4 | ≈0.8 | ≈8.7 |
| Wet Soil | 0.1 | ≈2.5 | ≈0.9 |
| Dry Soil | 0.01 | ≈8 | ≈0.1 |
| Atmosphere | ≈10^{-12} | >>1000 km | <0.001/km |
Antenna and Equipment Design
Transmission Antenna Challenges and Types
Transmission antennas for low frequency (LF) signals, spanning 30 to 300 kHz with wavelengths of 1 to 10 km, face fundamental challenges due to their electrically short dimensions relative to the operating wavelength.[57] A resonant quarter-wavelength monopole at the LF band's lower end would exceed 2.5 km in height, rendering full-size designs structurally and economically unfeasible for most transmitters.[57] Instead, practical antennas operate far below resonance, exhibiting inherently low radiation resistance proportional to the square of the electrical length (ka << 1, where k is the wave number and a is the antenna's effective radius), which severely limits radiation efficiency. This inefficiency arises primarily from the mismatch between the antenna's high capacitive reactance and the transmitter's output impedance, necessitating bulky loading coils or networks that introduce additional ohmic losses and narrow the operational bandwidth.[58] Ground losses further degrade performance, as LF fields couple strongly to the soil, requiring extensive radial systems—often thousands of buried wires spanning hundreds of meters—to achieve acceptable efficiency levels above 50% in high-power setups.[59] High voltages at the antenna top, exceeding 100 kV for kilowatt-level transmissions, also pose insulation and corona discharge challenges, compounded by environmental factors like weather-induced detuning.[60] Conventional LF transmission antennas predominantly employ vertical monopole configurations mounted on guyed masts or towers, typically 150 to 412 meters tall for longwave broadcasting stations operating around 200 kHz.[60] Top-loading elements, such as capacitive hats or radial wires forming umbrella structures, increase the antenna's effective capacitance and height, thereby boosting radiation resistance and reducing reliance on base inductors for tuning.[60] Sectionalized designs with insulated segments enable precise control of current distribution along the radiator, optimizing the vertical radiation pattern for ground-wave coverage while suppressing skywave interference.[60] For enhanced directivity or redundancy, arrays of multiple such monopoles are used, fed with phase-shifted currents to steer nulls toward undesired directions, though this increases complexity and cost.[60] While research into compact alternatives like mechanical or time-varying antennas promises higher efficiency for portable applications, operational LF systems rely on these large-scale vertical structures due to their proven ability to handle megawatt powers with efficiencies up to 80% under optimal conditions.[58]Reception Antenna Configurations
Reception antennas for low-frequency (LF) signals in the 30–300 kHz range must address the electrical smallness of practical structures relative to wavelengths of 1–10 km, resulting in low radiation resistance and high reactance that limit efficiency. Configurations prioritize magnetic field detection to mitigate electric field noise from local sources, with passive and active designs dominating due to size constraints. Full-size dipoles or verticals are infeasible for most users, as a quarter-wavelength monopole at 100 kHz would exceed 750 meters in height.[61]%20LF%20Antennas.pdf) Ferrite rod antennas, consisting of a coil wound around a high-permeability ferrite core, concentrate magnetic flux to boost sensitivity, making them suitable for LF reception where core losses remain low up to several MHz. These compact devices, often 10–30 cm long, exhibit a figure-8 directional pattern with deep nulls perpendicular to the rod axis, enabling interference rejection by rotating the antenna. They are standard in commercial LF receivers, such as those for time signals like DCF77 at 77.5 kHz, and provide effective coupling for signals down to 50 Hz with proper winding (e.g., 100–500 turns of fine wire).[62][63][64] Small loop antennas, either air-core or ferrite-filled, form a parallel resonant circuit tuned with a variable capacitor to the operating frequency, converting magnetic field variations into voltage via Faraday's law. Air-core loops (e.g., 1–2 m diameter) offer broadband response but require larger size for gain, while ferrite variants achieve similar performance in portable form factors. Directionality aids in signal-to-noise improvement, with nulls used to suppress man-made noise like power-line hum.[65][66] Active configurations incorporate a low-noise preamplifier at the element to overcome inherent low signal levels from short antennas, such as vertical electric field probes (e.g., 1–2 m whips) or miniaturized loops. These designs, like the AMRAD active LF antenna using a short vertical with JFET amplification, extend usable range into MF and HF while maintaining LF coverage, with gain tailored to avoid overload from strong local signals. Impedance matching via transformers or networks is essential, as passive LF antennas present high capacitive reactance (often thousands of ohms).[67][68][69] For specialized high-sensitivity applications, such as weak-signal monitoring, larger untuned loops or traveling-wave antennas like shortened Beverages provide pattern control and low-angle reception, though LF implementations demand lengths of hundreds of meters and elevated terminations to minimize ground losses. These prioritize signal-to-noise ratio over compactness, with spatial filtering via arrays for further noise reduction.[70]Efficiency and Size Limitations
Low frequency antennas face severe size constraints due to the band's long wavelengths, ranging from 1 km at 300 kHz to 10 km at 30 kHz, which demand resonant structures on the scale of hundreds of meters to kilometers for optimal performance—such as quarter-wavelength monopoles exceeding 250 m in height—rendering full-size designs structurally impractical and prohibitively expensive for most applications.[71][72] Practical transmitting antennas are thus electrically short relative to the wavelength (ka ≪ 1, where k is the wave number and a is the smallest enclosing sphere's radius), leading to inherently low radiation resistance and efficiency, as shorter antennas capture and radiate less of the electromagnetic energy effectively.[73][74] To mitigate detuning, these short antennas incorporate loading coils or capacitors, but this elevates the antenna's quality factor (Q), narrows bandwidth, and amplifies ohmic losses, further degrading radiation efficiency—often to below 1% for compact designs—necessitating high transmitter input powers to achieve viable effective radiated power (ERP).[71][75] Fundamental bounds on efficiency, derived from conductivity, frequency, and electrical surface area (k²S), confirm that η_r decreases inversely with shrinking size, with approximations like η_r max ≈ [1 + 3π/(δ k S)]⁻¹ highlighting the trade-off for low-frequency small antennas.[73] Commercial longwave stations, such as those operating in the 150-300 kHz range, commonly employ input powers exceeding 100 kW—sometimes reaching megawatts—to compensate, as seen in military or navigation transmitters where ERP must support ground-wave ranges of thousands of kilometers despite these losses.[71][72] Reception antennas face fewer size demands, as reciprocity allows smaller loops or ferrites to suffice for signal capture, though noise susceptibility remains high; however, transmitting efficiency limitations dominate LF system design, constraining portable or low-power uses to short ranges or experimental modes like those under FCC Part 15 rules, where 1 W input yields minimal ERP due to antenna mismatch.[75][72] These constraints underscore LF's niche persistence in specialized roles, where propagation advantages outweigh efficiency drawbacks, rather than broad commercial viability.[71]Regulatory Allocations
International ITU Framework
The International Telecommunication Union (ITU), through its Radiocommunication Sector (ITU-R), manages the global radio-frequency spectrum via the Radio Regulations, a treaty ratified by member states and revised at World Radiocommunication Conferences (WRC), with the latest edition incorporating outcomes from WRC-23 effective January 1, 2024.[76][77] Article 5 of the Regulations outlines the Table of Frequency Allocations, assigning spectrum bands to radiocommunication services on a primary or secondary basis, either worldwide or by ITU Region (Region 1: Europe, Africa, Middle East; Region 2: Americas; Region 3: Asia-Pacific).[77] This framework prioritizes interference-free operations, with primary services protected from secondary ones, and mandates coordination for cross-border use.[76] In the low frequency (LF) band of 30–300 kHz, allocations emphasize long-range, low-data-rate applications due to natural propagation limits, with worldwide primary assignments to fixed and mobile services (excluding aeronautical mobile, denoted as OFM) across most sub-bands, supplemented by radionavigation and maritime mobile.[77] Time signal and standard frequency services operate primarily in 30–70 kHz, limited to specific stations like those for precise timing dissemination.[77] Radionavigation holds primary status in segments such as 70–130 kHz and 130–200 kHz, supporting non-directional beacons and direction-finding.[77] Aeronautical and maritime radionavigation are primary in 200–285 kHz (with regional primacy for aeronautical in Regions 2 and 3) and 285–300 kHz, where emissions must avoid interference to legacy radiobeacons via narrow-band techniques per footnote 5.73.[77] Broadcasting receives a secondary allocation in 148.5–255 kHz across all regions, enabling longwave AM services in parts of Europe and Asia, though subordinated to primary navigation uses.[77][78] Amateur operations are secondary in 135.7–137.8 kHz worldwide, restricted to 1 W effective isotropic radiated power (e.i.r.p.) under footnote 5.67A to minimize interference.[77] Regional footnotes, such as 5.70 for African alternatives, allow limited variances, but states must align national tables with international allocations while notifying ITU of assignments exceeding specified power or bandwidth limits.[79][77]| Sub-band (kHz) | Primary Services | Secondary Services | Key Notes/Footnotes |
|---|---|---|---|
| 30–50 | FIXED, MARITIME MOBILE, Time Signal | Standard Frequency and Time Signal | Maritime limited to coast stations (5.57)[77] |
| 50–70 | FIXED, MARITIME MOBILE, Time Signal | Standard Frequency and Time Signal | Specific coordination conditions (5.58)[77] |
| 70–130 | FIXED, MARITIME MOBILE | RADIONAVIGATION | Worldwide[77] |
| 130–200 | FIXED, MARITIME MOBILE, RADIONAVIGATION | - | Supports direction-finding[77] |
| 135.7–137.8 | FIXED, MARITIME MOBILE | Amateur | 1 W e.i.r.p. limit (5.67A)[77] |
| 148.5–255 | - | BROADCASTING | All regions, secondary to navigation[77] |
| 200–285 | MARITIME MOBILE, RADIONAVIGATION | AERONAUTICAL RADIONAVIGATION (primary in Regions 2/3) | Regional variations[77] |
| 285–300 | MARITIME RADIONAVIGATION, AERONAUTICAL RADIONAVIGATION | - | No interference to beacons; narrow-band allowed (5.73)[77] |