Hubbry Logo
Medium frequencyMedium frequencyMain
Open search
Medium frequency
Community hub
Medium frequency
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Contribute something
Medium frequency
Medium frequency
Medium frequency
View on Wikipedia
from Wikipedia

Medium frequency
Frequency range
0.3 to 3 MHz
Wavelength range
1000 to 100 m
MF's position in the electromagnetic spectrum.

Medium frequency (MF) is the ITU designation[1][2] for radio frequencies (RF) in the range of 300 kilohertz (kHz) to 3 megahertz (MHz). Part of this band is the medium wave (MW) AM broadcast band. The MF band is also known as the hectometer band as the wavelengths range from ten to one hectometers (1000 to 100 m). Frequencies immediately below MF are denoted as low frequency (LF), while the first band of higher frequencies is known as high frequency (HF). MF is mostly used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, and transoceanic air traffic control.

Propagation

[edit]

Radio waves at MF wavelengths propagate via ground waves and reflection from the ionosphere (called skywaves).[3] Ground waves travel just above the earth's surface, following the terrain. At these wavelengths, they can bend (diffract) over hills, and travel beyond the visual horizon, although they may be blocked by mountain ranges. Ground waves are progressively absorbed by the Earth, so the signal strength decreases exponentially with distance from the transmitting antenna. Typical MF radio stations can cover a radius of several hundred kilometres/miles from the transmitter, with longer distances over water and damp earth.[4] MF broadcasting stations use ground waves to cover their listening areas.

MF waves can also travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are refracted back to Earth by layers of charged particles (ions) in the ionosphere, the E and F layers. However, at certain times the D layer (at a lower altitude than the refractive E and F layers) can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is heavily ionised, such as during the day, in summer and especially at times of high solar activity.

At night, especially in winter months and at times of low solar activity, the ionospheric D layer can virtually disappear. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away as the signal will be refracted by the remaining F layer. This can be very useful for long-distance communication, but can also interfere with local stations. Because of the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasting stations several hundred miles apart. On nights of good skywave propagation, the signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency. The North American Regional Broadcasting Agreement (NARBA) sets aside certain channels for nighttime use over extended service areas via skywave by a few specially licensed AM broadcasting stations. These channels are called clear channels, and the stations, called clear-channel stations, are required to broadcast at higher powers of 10 to 50 kW.

Uses and applications

[edit]
Mast radiator of a commercial MF AM broadcasting station, Chapel Hill, North Carolina, USA

A major use of these frequencies is AM broadcasting; AM radio stations are allocated frequencies in the medium wave broadcast band from 526.5 kHz to 1606.5 kHz[5] in Europe; in North America this extends from 525 kHz to 1705 kHz[6] Some countries also allow broadcasting in the 120-meter band from 2300 to 2495 kHz; these frequencies are mostly used in tropical areas. Although these are medium frequencies, 120 meters is generally treated as one of the shortwave bands.

There are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz. These include, as examples, the French MRCC on 1696 kHz and 2677 kHz, Stornoway Coastguard on 1743 kHz, the US Coastguard on 2670 kHz and Madeira on 2843 kHz.[7] RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz.[8] Non-directional navigational radio beacons (NDBs) for maritime and aircraft navigation occupy a band from 190 to 435 kHz, which overlaps from the LF into the bottom part of the MF band.

2182 kHz is the international calling and distress frequency for SSB maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band. 500 kHz was for many years the maritime distress and emergency frequency, and there are more NDBs between 510 and 530 kHz. Navtex, which is part of the current Global Maritime Distress Safety System occupies 518 kHz and 490 kHz for important digital text broadcasts. Lastly, there are aeronautical and other mobile SSB bands from 2850 kHz to 3500 kHz, crossing the boundary from the MF band into the HF radio band.[9]

An amateur radio band known as 160 meters or 'top-band' is between 1800 and 2000 kHz (allocation depends on country and starts at 1810 kHz outside the Americas). Amateur operators transmit CW morse code, digital signals and SSB and AM voice signals on this band. Following World Radiocommunication Conference 2012 (WRC-2012), the amateur service received a new allocation between 472 and 479 kHz for narrow band modes and secondary service, after extensive propagation and compatibility studies made by the ARRL 600 meters Experiment Group and their partners around the world. In recent years, some limited amateur radio operation has also been allowed in the region of 500 kHz in the US, UK, Germany and Sweden.[10]

Many home-portable or cordless telephones, especially those that were designed in the 1980s, transmit low power FM audio signals between the table-top base unit and the handset on frequencies in the range 1600 to 1800 kHz.[11]

Antennas

[edit]
Ferrite loopstick receiving antenna used in AM radios
Cage T antenna used by amateur radio transmitter on 1.5 MHz.

Transmitting antennas commonly used on this band include monopole mast radiators, top-loaded wire monopole antennas such as the inverted-L and T antennas, and wire dipole antennas. Ground wave propagation, the most widely used type at these frequencies, requires vertically polarized antennas like monopoles.

The most common transmitting antennas, monopoles of one-quarter to five-eighths wavelength, are physically large at these frequencies, 25 to 250 metres (82 to 820 ft) requiring a tall radio mast. Usually the metal mast itself is energized and used as the antenna, and is mounted on a large porcelain insulator to isolate it from the ground; this is called a mast radiator. The monopole antenna, particularly if electrically short requires a good, low resistance Earth ground connection for efficiency since the ground resistance is in series with the antenna and consumes transmitter power. Commercial radio stations use a ground system consisting of many copper cables, buried shallowly in the earth, radiating from the base of the antenna to a distance of about a quarter wavelength. In areas of rocky or sandy soil where the ground conductivity is poor, above-ground counterpoises are sometimes used.

Lower power transmitters often use electrically short quarter wave monopoles such as inverted-L or T antennas, which are brought into resonance with a loading coil at their base.

Receiving antennas do not have to be as efficient as transmitting antennas since in this band the signal-to-noise ratio is determined by atmospheric noise. The noise floor in the receiver is far below the noise in the signal, so antennas small in comparison to the wavelength, which are inefficient and produce low signal strength, can be used. The weak signal from the antenna can be amplified in the receiver without introducing significant noise. The most common receiving antenna is the ferrite loopstick antenna (also known as a ferrite rod aerial), made from a ferrite rod with a coil of fine wire wound around it. This antenna is small enough that it is usually enclosed inside the radio case. In addition to their use in AM radios, ferrite antennas are also used in portable radio direction finder (RDF) receivers. The ferrite rod antenna has a dipole reception pattern with sharp nulls along the axis of the rod, so that reception is at its best when the rod is at right angles to the transmitter, but fades to nothing when the rod points exactly at the transmitter. Other types of loop antennas and random wire antennas are also used.

See also

[edit]

References

[edit]

Further reading

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

Medium frequency

View on Grokipedia
from Grokipedia
Medium frequency (MF) refers to the portion of the radio-frequency spectrum designated by the (ITU) as spanning from 300 kHz to 3 MHz, corresponding to hectometric waves with wavelengths between 1,000 meters and 100 meters. This band, numbered 6 in the ITU nomenclature, lies between (LF) and (HF) and is characterized by ground-wave propagation that enables reliable over-the-horizon communication, particularly at night when ionospheric reflection can extend range. The MF band is primarily allocated for (AM) medium-wave broadcasting, with key sub-bands such as 526.5–1,605 kHz dedicated to terrestrial radio services across ITU Regions 1, 2, and 3. It supports international broadcasting standards, allowing stations to reach audiences over hundreds of kilometers, though interference from sky-wave can affect nighttime reception in some areas. Beyond broadcasting, MF frequencies are essential for non-directional beacons (NDBs) used in aviation navigation, where low- or medium-frequency signals provide bearing information for en route and during approaches. In maritime applications, the MF band facilitates ship-to-shore and ship-to-ship communications, including distress and safety signals under the Global Maritime Distress and Safety System (GMDSS), with dedicated frequencies for emergency calls and navigational aids. Additionally, it supports coast-to-sea voice and data exchanges, as well as automatic direction-finding systems for vessels, enhancing safety in offshore operations. Overall, the band's versatility stems from its balance of propagation characteristics, making it a cornerstone for legacy and ongoing radiocommunication services worldwide.

Definition and Characteristics

Frequency Range and Designations

The medium frequency (MF) band is defined by the (ITU) in its Radio Regulations as the portion of the spanning 300 kHz to 3 MHz. This designation, established under Article 2, Section I, subdivides the overall into nine progressive bands to facilitate international coordination and allocation of frequencies for various services. The MF band serves as a critical segment for regulated radio communications, with its boundaries precisely set to avoid overlap with adjacent allocations while supporting distinct operational needs. Regional implementations of the MF band exhibit variations, particularly for (AM) broadcasting. In , the (FCC) allocates the primary AM broadcast band from 535 kHz to 1705 kHz, utilizing 10 kHz channel spacing to accommodate stations across this range. In contrast, the European Conference of Postal and Telecommunications Administrations (CEPT) designates the medium wave band for AM broadcasting from 526.5 kHz to 1606.5 kHz, employing 9 kHz spacing to align with denser channel arrangements in the region. These differences reflect harmonized yet localized adaptations within the broader ITU framework to optimize spectrum use. The ITU has periodically refined MF band allocations through world radiocommunication conferences to address evolving service requirements. Notably, the World Radiocommunication Conference (WRC-12) introduced specific updates, including the exclusive worldwide allocation of the 495-505 kHz segment to the maritime mobile service for distress, safety, and calling functions, enhancing global maritime communications without interference from other services. Such revisions ensure the band's continued relevance while maintaining compatibility with international standards. The MF band is positioned between the low frequency (LF) band, which extends from 30 kHz to 300 kHz, and the high frequency (HF) band above 3 MHz.

Wavelength and Physical Properties

The wavelength λ\lambda of electromagnetic waves in the medium frequency (MF) band is calculated using the formula λ=cf,\lambda = \frac{c}{f}, where cc is the speed of light in vacuum, exactly 299 792 458299\,792\,458 m/s, and ff is the frequency in Hz. For the MF range of 300 kHz to 3 MHz, this yields wavelengths from approximately 1,000 m at 300 kHz to 100 m at 3 MHz. These longer wavelengths impart distinct physical properties to MF signals compared to higher-frequency bands. The extended wavelengths enable efficient ground wave propagation, where signals can diffract and follow the Earth's curvature, supporting reliable over-the-horizon communication. However, practical implementation requires proportionally larger antennas for optimal performance, as resonant structures like quarter-wave monopoles can exceed 250 m in height at the band's lower frequencies to achieve effective . MF signals also demonstrate reduced in conductive media, such as or moist , where high ground conductivity (σ\sigma) and (ϵr\epsilon_r) minimize loss through shallower skin depths—ranging from about 0.4 m over to over 30 m on poor at 300 kHz. This property enhances signal strength over such terrains relative to less conductive environments. In terms of energy distribution, MF broadcasting employs moderate power levels, with AM transmitters authorized up to 50 kW for Class A and B stations to achieve wide coverage. In free space, the direct and reflected components of the space wave exhibit quasi-optical behavior, propagating primarily along line-of-sight paths with allowing modest extension beyond the horizon.

Historical Development

Early Uses in Radio

The pioneering adoption of medium frequency (MF) radio began in the early , with achieving a landmark transatlantic transmission on December 12, 1901, from Poldhu, , to Signal Hill, Newfoundland. Using a operating at a nominal frequency of approximately 700 kHz, Marconi and his assistant George Kemp detected faint signals representing the letter "S," demonstrating the potential for long-distance wireless communication over roughly 2,100 miles despite daytime propagation challenges. This experiment highlighted MF's suitability for reliable signal propagation via ground waves and early ionospheric reflections, marking a shift from short-range to oceanic-scale applications. Building on such advancements, Reginald Fessenden conducted groundbreaking experiments in amplitude modulation (AM) in 1906, transitioning radio from Morse code to voice transmission. On December 24, 1906, from his station in Brant Rock, Massachusetts, Fessenden broadcast the first audio program—including violin music, a Bible reading, and a phonograph record—to ships at sea and nearby receivers, using continuous-wave alternator technology to modulate voice onto a carrier wave. These efforts, though initially at lower frequencies, established AM as a foundational technique for MF communications, enabling clearer and more versatile signaling that would soon be adapted to the 300–3000 kHz band for practical use. During , MF radio saw extensive military deployment for tactical signaling and , leveraging its ground-wave propagation for stable, medium-range coverage up to several hundred kilometers over varied terrain. British and Allied forces employed portable sets like the Marconi 52M, operating in the 732 kHz to 2 MHz range, to coordinate artillery fire, troop movements, and , with power outputs of 40 watts supporting reliable links in frontline conditions. Direction-finding systems using MF signals also emerged, allowing precise location of enemy transmitters by triangulating bearings from multiple receivers, a technique critical for intelligence and despite jamming vulnerabilities. In the pre-broadcast era of the , MF facilitated vital maritime communications, particularly ship-to-shore telephony centered on the 500 kHz international calling and distress frequency. Following post-war developments in vacuum-tube technology, systems enabled voice exchanges between vessels and coastal stations, with 500 kHz serving as the initial calling channel before shifting to assigned working frequencies for conversations, enhancing safety through direct distress reporting. This application, formalized after the Titanic disaster, saved numerous lives by allowing rapid coordination with shore authorities, underscoring MF's role in reliable over-water voice links before widespread .

Standardization and Band Allocation

The standardization of medium frequency (MF) allocations began with international efforts to resolve interference issues arising from the rapid growth of radio broadcasting in the early 20th century. The 1927 International Radiotelegraph Conference in Washington, D.C., marked a foundational step by establishing the first global table of frequency distributions, allocating the band corresponding to wavelengths of approximately 200 to 550 meters (roughly 545 to 1,500 kHz) primarily for broadcasting services to harmonize usage across nations. This conference, attended by representatives from 50 countries, also created the International Consultative Committee for Radio (predecessor to ITU-R) to oversee ongoing technical coordination. Building on this, the (ITU) was formally established in 1932 through the Madrid Conference, which unified earlier telegraph and radiotelegraph conventions into a single framework for global , including MF bands for commercial and maritime applications. Regional agreements further refined these allocations; the North American Regional Broadcasting Agreement (NARBA) of 1950, signed by the , , , and other nations, standardized the MF broadcasting band at 540 to 1,600 kHz with 10 kHz channel spacing to minimize cross-border interference and support clear-channel operations. Subsequent World Radiocommunication Conferences (WRCs) have updated MF provisions to accommodate technological advancements, including support for digital broadcasting transitions. World Radiocommunication Conference (WRC-03) adopted Resolution 543, providing provisional RF protection ratios for digital sound in the MF and HF bands to facilitate compatibility with analog systems. WRC-23 further addressed MF usage through Resolution 366, aimed at improving the utilization and channelization of maritime radiocommunications in the MF and HF bands. Under the current (edition of 2024), the MF band (300 to 3,000 kHz) is primarily allocated to in Regions 1 and 3 (, , , and ) from 526.5 to 1,605 kHz, and in Region 2 () from 535 to 1,605 kHz, with provisions for aeronautical and maritime mobile services. Specific frequencies include 2,182 kHz, designated worldwide as the international distress and calling for maritime radiotelephony, requiring continuous monitoring by vessels. The band 500 to 505 kHz is allocated to the maritime mobile service for safety communications, with secondary low-power use permitted for operations in certain regions to support experimental and emergency activities. Regional variations persist to optimize local usage; for instance, in (ITU Region 3), the broadcasting band extends from 531 to 1,620 kHz with 9 kHz channel spacing, differing from the 10 kHz spacing in Region 2, to accommodate denser station populations while adhering to ITU coordination requirements.

Propagation Mechanisms

Ground Wave Propagation

Ground wave propagation represents the primary mode for medium frequency (MF) signals during daytime, enabling reliable communication by following the curvature of the 's surface. This mechanism involves surface waves that are induced by the interaction of the with the ground, primarily vertically polarized, and propagate as currents within the conducting . The waves diffract around the 's curvature, extending beyond the optical horizon, with their long wavelengths (100 to 1000 meters) facilitating this "hugging" effect along the terrain. Attenuation of these surface waves is significantly influenced by the conductivity of the ground over which they travel. Higher conductivity, as found in (typically 1 to 5 S/m), results in lower losses and extended ranges, often up to 1000 km for MF signals under optimal conditions. In contrast, over land with lower conductivity (e.g., 0.001 to 0.01 S/m for average ), attenuation increases, limiting reliable . over obstacles, such as hills or irregular , further shapes the signal path, with methods like the Millington approach used to model mixed land-sea transitions. exhibits diurnal stability, remaining relatively consistent during daylight hours with minimal over conductive paths, though seasonal variations up to 15 dB may occur over land. Limitations include its dominance only during daytime, as nighttime ionospheric interference can mask signals, and progressive fading over land beyond 200–300 km due to cumulative attenuation and terrain effects.

Sky Wave and Ionospheric Effects

In medium frequency (MF) propagation, sky waves are refracted or reflected back to primarily by the E layer of the at night, with the D layer playing a negligible role in reflection due to its lower . The E layer, situated at approximately 90-140 km altitude, enables this mechanism for frequencies between 300 kHz and 3 MHz when the D layer dissipates after sunset. The skip distance, representing the minimum range at which sky waves return to the surface, can be approximated as dskip≈htan⁡θd_{\text{skip}} \approx \frac{h}{\tan \theta}, where hh is the ionospheric reflection height (around 100 km for the E layer) and θ\theta is the angle of incidence. This geometry results in single-hop distances typically exceeding 1000 km under favorable conditions. Diurnal variations significantly influence MF sky wave reliability, as the D layer—formed by solar ionization during daylight—absorbs MF signals, rendering sky wave propagation ineffective and limiting coverage to ground waves. At night, the absence of the D layer minimizes absorption, allowing E-layer reflection to support long-distance jumps of over 1000 km, often via multi-hop paths where signals bounce repeatedly between the ionosphere and . Solar activity further modulates these effects; during solar peaks, increased ionization raises the maximum usable frequency (MUF), enhancing reflection efficiency for MF bands, while solar minima reduce it, potentially shortening usable ranges. Nighttime sky wave propagation introduces challenges such as multi-hop , arising from multipath interference as signals arrive via differing ionospheric paths, leading to signal amplitude fluctuations that follow a in mid-latitudes. Co-channel interference also intensifies, as distant stations become receivable over 1000+ km, causing overlap with local signals and complicating reception in shared frequency allocations. These issues complement daytime reliance on propagation for consistent short-range coverage.

Primary Applications

AM Broadcasting

Amplitude modulation (AM) broadcasting utilizes the medium frequency (MF) band, primarily between 526.5 kHz and 1606.5 kHz in 1 and 3, and 540 kHz to 1700 kHz in Region 2, to transmit audio signals for , , and to wide audiences. This allocation enables reliable signal propagation via ground waves during the day and sky waves at night, supporting both local and extended coverage. In , channels are spaced at 9 kHz in ITU Regions 1 and 3 or 10 kHz in Region 2 to minimize interference, allowing for efficient spectrum use within the MF range. Transmitter power levels typically range from 0.25 kW for smaller stations to 50 kW for high-power facilities, depending on class and regulatory limits, which balance coverage with interference control. The reaches up to 100% for voice and music signals, ensuring full variation without , though root-mean-square levels often average 20-40% for typical programming. Coverage patterns rely on for daytime local reception, extending 50-200 km based on power, terrain, and soil conductivity, providing primary service areas for urban and suburban listeners. At night, sky wave via ionospheric reflection enables regional or national reach, often exceeding 1,000 km, though it introduces variable interference from distant stations. Digital enhancements, such as HD Radio's all-digital mode (authorized by the FCC for full-time use since October 2020) and (DRM), are deployed on select MF stations to improve audio quality and add data services; hybrid modes maintain compatibility with analog receivers. AM broadcasting reached its zenith in the mid-20th century, dominating from the 1920s through the 1950s with widespread adoption for news, music, and drama programming. Its prominence declined post-1960s due to the superior fidelity of FM and the rise of television, leading to reduced analog AM usage in developed markets in favor of digital alternatives. However, AM persists strongly in developing regions, where affordable receivers and robust support essential services like alerts and rural information dissemination.

Maritime and Aeronautical Communication

Medium frequency (MF) bands have historically played a critical role in maritime communications, particularly for distress signaling and navigational warnings. The frequency of 2182 kHz served as the primary international distress and calling channel for voice radiotelephony, enabling ships to transmit urgency and safety messages over medium ranges. However, under the Global Maritime Distress and Safety System (GMDSS), mandatory watchkeeping on 2182 kHz was phased out globally on February 1, 1999, with the U.S. Coast Guard terminating its monitoring in 2013. Further SOLAS amendments adopted in 2019 and effective January 1, 2024, modernize GMDSS by recognizing additional recognized mobile satellite services for Sea Areas A3 and A4, and removing requirements for certain legacy equipment while maintaining MF for redundancy in A1/A2 areas, prioritizing digital systems. NAVTEX, a narrow-band direct-printing service, continues to operate in the MF band to disseminate navigational and meteorological warnings, using 518 kHz for international English-language broadcasts and 490 kHz for local languages in specific regions. This system provides automated, one-way text messages receivable up to 400 nautical miles, enhancing safety without requiring two-way interaction. Integration of (DSC) has further modernized MF maritime operations, allowing for automated distress alerts on frequencies like 2187.5 kHz, which transmit predefined digital messages including position data to coast stations and nearby vessels. DSC operates alongside traditional MF channels, enabling rapid alerting in Area A2 (up to 150 nautical miles offshore) where VHF coverage is limited, and supports follow-on voice communications on associated working frequencies. SOLAS amendments effective , 2024, further modernize GMDSS by recognizing additional recognized mobile satellite services for Areas A3 and A4, and removing requirements for certain legacy equipment while maintaining MF for redundancy in A1/A2 areas. In aeronautical applications, the MF band from 2850 kHz to 3000 kHz supports en-route communications between and ground stations, particularly for high-altitude or oceanic flights requiring reliable medium-range links. Additionally, non-directional beacons (NDBs) in the MF range (typically 190–1750 kHz) serve as backups to VHF-based systems like VOR and ILS, providing low-precision navigation aids in remote or low-visibility conditions where higher-frequency signals may fail. Despite transitions to VHF and digital satellite systems under GMDSS and aviation modernization, MF retains value for long-range coverage in polar and remote areas, where ionospheric propagation extends signal reach beyond VHF limitations. In maritime contexts, MF DSC and persist in A2 sea areas for redundancy, while aeronautical MF supports contingency operations in regions with sparse infrastructure.

Technical Implementation

Antenna Design

Antenna design for medium frequency (MF) signals, spanning 300 kHz to 3 MHz, is constrained by the relatively long wavelengths, which dictate large physical scales for efficient radiation and reception. Quarter-wave monopole antennas, a fundamental transmitting type, require heights of 75 to 250 meters to achieve resonance across this band, as the quarter-wavelength at 3 MHz is approximately 25 meters while at 300 kHz it extends to 250 meters. These structures provide omnidirectional coverage with a gain of about 5.15 dBi but demand substantial support infrastructure due to their size. To mitigate height requirements, top-loaded configurations such as T-antennas or umbrella antennas are employed, reducing effective height to less than one-eighth (e.g., 10 to 30 meters at 1 MHz) by adding capacitive elements at the top that increase and effective . Umbrella antennas, featuring a central mast with radially extending wires forming an inverted , improve efficiency for short radiators. Base loading coils are integrated at the antenna base for tuning shorter monopoles, compensating for inductive reactance to resonate the structure at the desired MF frequency and minimizing losses in the feed system. Efficiency in MF transmitting antennas is heavily influenced by ground plane implementation, where radial wire systems—typically 120 buried conductors each at least a quarter-wavelength long—provide a low-loss return path for ground currents, boosting by up to 144 mV/m per kW with 40 radials versus significantly lower values with fewer. The Q-factor, defined as Q=fresΔfQ = \frac{f_{\text{res}}}{\Delta f} where fresf_{\text{res}} is the resonant and Δf\Delta f is the 3 dB bandwidth, typically ranges from 100 to 300 for MF antennas, reflecting narrow bandwidths inherent to short, loaded designs that prioritize over wide tuning. For reception in the MF band, loop antennas offer inherent through their figure-eight , enabling nulling of interference when oriented perpendicular to the desired signal direction. Ferrite rod core antennas, a compact variant of magnetic loop designs, enhance sensitivity by concentrating the via the core's high permeability (up to μ = 600 for suitable materials), with output voltage scaling as Vo=E[Q](/page/Q)heV_o = E [Q](/page/Q) h_e where heh_e is effective and is the coil factor. Core size critically affects performance: longer rods (e.g., length-to-diameter ratios of 15–20) yield higher apparent permeability (μ_rod ≈ 120) and sensitivity, though excessive length introduces losses that cap practical at 150–200 for stable operation up to 2 MHz.

Receivers and Interference Mitigation

Medium frequency (MF) receivers primarily employ superheterodyne architectures to achieve high selectivity and sensitivity in the 300 kHz to 3 MHz band. In these designs, the incoming RF signal is mixed with a to produce a fixed (IF), commonly 455 kHz for AM broadcast applications, allowing for efficient amplification and filtering through multiple IF stages. This IF choice facilitates the use of crystal filters for sharp selectivity, rejecting while preserving audio bandwidth up to 10 kHz. Modern MF receivers increasingly integrate (DSP) to enhance selectivity beyond traditional analog methods. DSP enables adaptive filtering, such as dynamic bandwidth adjustment from 3 kHz in high-noise environments to 10 kHz for clear signals, and multi-stage noise rejection algorithms that detect and suppress co-channel or adjacent-channel interferers with up to 50 dB . These techniques, often implemented in (SDR) frameworks using integrated chips from manufacturers like , provide programmable response to varying propagation conditions without hardware modifications. Automatic gain control (AGC) is essential in MF receivers to manage the wide dynamic range of signals, typically spanning 80-100 dB due to fading and varying transmitter distances. AGC circuits adjust amplifier gain based on detected signal strength, compressing output amplitude variations to prevent overload in strong signals and maintain detectability in weak ones, with attack times of 60-100 ms optimized for AM modulation to avoid distortion from low-frequency components. Distributed across RF and IF stages, AGC ensures consistent signal-to-noise ratios, reducing the required dynamic range of subsequent demodulators. A primary interference source in MF reception is man-made noise from electrical appliances and power distribution systems, which generates impulsive broadband emissions peaking in urban and business areas. For instance, power lines at 115-250 kV contribute noise levels up to 20-30 dB above thermal noise at 0.5 MHz, degrading signal and increasing bit rates in digital modes. These emissions, often from arcing or switching in appliances like fluorescent lamps and computers, propagate efficiently via ground waves, overwhelming weak MF signals. Mitigation strategies focus on shielding, filtering, and directional techniques to isolate desired signals. using conductive enclosures or Faraday cages attenuates external RF interference by 40-60 dB in the MF range, protecting receiver front-ends from coupled noise. Notch filters, tunable to specific interferer frequencies like 10 kHz-wide bands for adjacent channels, suppress unwanted signals by 30-50 dB while preserving the . Directional nulling, implemented via DSP or phased arrays, creates spatial nulls toward noise sources, achieving 20-40 dB rejection in multi-element receiver systems. Regulatory standards from the FCC and ITU limit spurious emissions to minimize receiver interference in the MF band. ITU Recommendation SM.329 specifies attenuation of at least 43 + 10 log P (dB) for spurious emissions in 300-3000 kHz, with absolute limits of -13 dBm for powers up to 50 W, ensuring protection for co-primary services like . FCC Part 15 enforces similar constraints on unintentional radiators, for example in the AM broadcast band (0.535-1.605 MHz) limiting field strengths to approximately 15,000-45,000 / f(kHz) μV/m at 30 meters to curb man-made noise injection. Synchronous detection addresses selective in MF AM signals by regenerating the carrier using a , reducing distortion from ionospheric multipath by up to 20 dB compared to envelope detection. This technique locks onto the carrier , demodulating sidebands independently to maintain audio during deep fades.

References

  1. https://www.itu.int/dms_pubrec/itu-r/rec/v/r-rec-v.431-8-201508-i!!pdf-e.pdf
  2. https://www.itu.int/en/ITU-R/terrestrial/broadcast/Pages/bands.aspx
  3. https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_4.1.html
  4. https://resources.pcb.cadence.com/blog/2022-an-overview-of-frequency-bands-and-their-applications
  5. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-C/part-73/subpart-A/section-73.14
  6. https://physics.nist.gov/cgi-bin/cuu/Value?c
  7. https://www.everythingrf.com/community/itu-frequency-bands
  8. https://its.ntia.gov/publications/download/99-368_ocr.pdf
  9. https://www.fcc.gov/media/radio/am-clear-regional-local-channels
  10. https://ieee-aess.org/post/blog/history-column-marconi-radio
  11. https://ethw.org/Milestones:Reception_of_Transatlantic_Radio_Signals%2C_1901
  12. https://ethw.org/Milestones:First_Wireless_Radio_Broadcast_by_Reginald_A._Fessenden%2C_1906
  13. https://www.britannica.com/biography/Reginald-Aubrey-Fessenden
  14. https://blogs.mhs.ox.ac.uk/innovatingincombat/files/2013/03/Army-radio-communication-in-the-Great-War_V2.pdf
  15. https://earlyradiohistory.us/sec013.htm
  16. http://vtda.org/pubs/BSTJ/vol09-1930/articles/bstj9-3-407.pdf
  17. https://jproc.ca/radiostor/preserving_500khz.html
  18. https://www.itu.int/en/history/Pages/RadioConferences.aspx?conf=4.39
  19. https://search.itu.int/history/HistoryDigitalCollectionDocLibrary/4.39.43.en.100.pdf
  20. https://publications.gc.ca/collections/collection_2024/isde-ised/co41-1/Co41-1-1D-eng.pdf
  21. https://www.itu.int/dms_pub/itu-r/oth/0C/0A/R0C0A00000F00120PDFE.pdf
  22. https://www.itu.int/en/ITU-R/conferences/wrc/2023/Pages/default.aspx
  23. https://transition.fcc.gov/oet/spectrum/table/fcctable.pdf
  24. https://www.ntia.gov/sites/default/files/2023-11/4_2021_edition_rev_2023.pdf
  25. http://www.eletrica.ufpr.br/armando/index_arquivos/R-HDB-59-2014-PDF-E.pdf
  26. https://hamradio.engineering/ground-wave-path-loss/
  27. https://www.sws.bom.gov.au/Category/Educational/Other%20Topics/Radio%20Communication/Intro%20to%20HF%20Radio.pdf
  28. https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/ionospheric-layers-regions-d-e-f1-f2.php
  29. https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/skywaves-skip-distance-zone.php
  30. https://peer.asee.org/the-d-and-f-ionosphere-layers-why-are-am-broadcast-signals-mostly-local.pdf
  31. https://www.fcc.gov/media/radio/am-groundwave-field-strength-graphs
  32. https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BS.2004-1995-PDF-E.pdf
  33. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-C/part-73/subpart-A/section-73.21
  34. https://www.itu.int/dms_pub/itu-r/opb/rep/R-REP-BS.2433-2018-PDF-E.pdf
  35. https://www.fcc.gov/media/radio/am-stations-at-night
  36. https://hdradio.com/all-digital-am/
  37. https://www.drm.org/about-drm/drm-technology/
  38. https://www.federalregister.gov/documents/2020/12/03/2020-25252/all-digital-am-broadcasting-revitalization-of-the-am-radio-service
  39. https://www.radioconnection.com/reference-library/history-of-radio-2/
  40. https://www.wshu.org/vintage-radio/2016-03-16/vintage-radio-the-life-decline-and-possible-rebirth-of-am
  41. http://econweb.umd.edu/~sweeting/sweeting_radio_final.pdf
  42. https://www.press.org/newsroom/radio-making-difference-developing-countries
  43. https://www.govinfo.gov/content/pkg/FR-2013-07-15/pdf/2013-16801.pdf
  44. https://www.imo.org/en/OurWork/Safety/Pages/RadioCommunications-Default.aspx
  45. https://www.navcen.uscg.gov/navtex-maritime-safety-broadcasts
  46. https://www.nhc.noaa.gov/pdf/TAFB_navtex.pdf
  47. https://www.navcen.uscg.gov/digital-selective-calling
  48. https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html
  49. https://www.icao.int/sites/default/files/FSMP/Doc.9718-Vol-I-AdvanceUneditedVersion-2021.pdf
  50. https://gmdsstesters.com/radio-survey/general/frequencies-for-the-global-maritime-distress-and-safety-system-gmdss.html
  51. https://ieeexplore.ieee.org/document/959524
  52. https://www.researchgate.net/publication/3040957_Short_Medium_Frequency_AM_Antennas
  53. http://www.hatdaw.com/papers/Dawson%20&%20Lockwood%20-%20MW%20Ground%20System.pdf
  54. https://www.osengr.org/Articles/Loop-Antennas.pdf
  55. https://www.electronics-notes.com/articles/antennas-propagation/ferrite-rod-bar-antenna/basics-tutorial.php
  56. https://digital-library.theiet.org/doi/full/10.1049/joe.2013.0126
  57. https://www.sciencedirect.com/topics/computer-science/superheterodyne-receiver
  58. https://circuitdigest.com/article/superheterodyne-am-receiver
  59. https://www.kintronic.com/wp-content/uploads/2016/01/NAB-AM-Receiver-Paper.pdf
  60. https://www.qsl.net/va3iul/Files/Automatic_Gain_Control.pdf
  61. https://its.ntia.gov/publications/download/ot-74-38.pdf
  62. https://www.analog.com/media/en/training-seminars/tutorials/mt-095.pdf
  63. https://www.gpsworld.com/innovation-null-steering-antennas/
  64. https://www.itu.int/dms_pubrec/itu-r/rec/sm/R-REC-SM.329-8-200004-S!!PDF-E.pdf
  65. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15/subpart-C/section-15.209
  66. https://swling.com/blog/wp-content/uploads/2009/08/syncdetectionarrl.pdf
Add your contribution
Related Hubs
Contribute something
User Avatar
No comments yet.