The frequency spectrum from 3-30 MHz has been very important both nationally and internationally ever since Marconi's 1901 demonstration of communications across the Atlantic opened a new field of thought regarding the Earth's surrounding atmosphere and the nature of long-distance transmission of radio waves. For half a century, theories were formulated and measurements made that proved the existence of elevated reflecting layers which made up the ionosphere. The region below 90 km is called the D region, the region between 90 km and about 160 km is called the E region, and the region above 160 km is called the F region. Skywave propagation in the frequency range from 3 to 30 MHz depends primarily on ionospheric reflections from the E and F regions. The D region weakens HF waves through partial absorption. The most significant feature of propagation in this frequency range is that transmission of useful signals over large distances with low power and simple antennas is possible for many hours of the day. Transmissions, however, are not always reliable because of changing ionospheric conditions. Ionization density varies daily, seasonally, and over the 11-year sunspot cycle.(1) Until the introduction of submarine cables, microwave links, and satellite communications, nearly all long-distance services depended upon ionospheric propagation. Today, the HF spectrum still accounts for about one-quarter of the frequency assignments worldwide.
One problem regarding the use of HF spectrum lies in the fact that HF bands have become congested. Interference problems increase yearly to such an extent that certain HF sub-bands are regarded as not useful for high-reliability, high-priority communications links. The users of the HF spectrum operate in a dynamic manner and change their equipment parameters (power, antenna radiation patterns, etc.), as needed, to meet operational needs and to accommodate the time-varying changes in the ionospheric channel.
The HF spectrum is also characterized as having more blocks of allocations per megahertz than any other portion of the electromagnetic spectrum. In just 27 MHz of spectrum, the HF band is partitioned into approximately 101 sub-bands or "blocks" and has nine different radio service allocations. This concept of block allocations dedicates a band of contiguous frequencies to a certain type of radio service with a somewhat uniform usage and technical standards. This block allocation system in the United States apportions spectrum for exclusive use by the Federal or non-Federal Government or for shared use between the two.
Technology has, for the most part, helped achieve expansion of the very high frequency (30-300 MHz), ultrahigh frequency (300-3000 MHz), and super high frequency (3-30 GHz) ranges through spectrum-efficient techniques. However, for the most part, the HF spectrum continues to be the most crowded and contentious of all the frequency bands. Radio amateurs, aircraft, ship, international broadcasting, and long-distance commercial communications providers use HF. Conflict emerges when the number of bands assigned for a specific radio service are deemed by ITU members as insufficient or unduly restrictive. It is generally understood by many that the HF band is fully allocated among various radio services. Any gain for one radio service is at another radio service's expense. This is commonly referred to as a "zero sum" outcome.(2)
While little use is made of HF radio systems in the United States and other developed countries for domestic communications, lesser developed countries still find HF applications cost-effective for some of their domestic radiocommunications needs, such as for national broadcasting, mobile and fixed point-to-point radiocommunications. This has led to a conflict over allocating the HF band internationally: the developed countries want more use of the band for international broadcasting and long-distance mobile communications, while lesser developed countries want to retain it for their domestic radiocommunications operations.
In the United States, the HF band is divided into 101 sub-bands including four exclusive Federal bands and 20 exclusive non-Federal bands. The fixed and mobile services share the four exclusive Federal bands, a total of 1,170 kHz or approximately 4 percent of the HF band. Of the 20 exclusive non-Federal bands, which total 4,905 kHz or approximately 18 percent of the HF band, 10 are for the amateur services, 3 for the fixed service, 5 for the land mobile service, 1 for the mobile service, and 1 shared by the fixed and mobile services. The remaining 77 sub-bands are shared by Federal and non-Federal users and amount to approximately 78 percent of the HF band.
Currently, approximately 30 Federal agencies and numerous state and local government agencies and private sector entities hold HF assignments or licenses. Over 80,000 assignment records are reflected in the Government Master File (GMF) and the FCC Master Frequency List. The largest holder of HF frequency assignments in the Federal Government is the Department of the Navy with over 16,000 assignments, followed by the USCG with approximately 8,700 assignments. The Departments of the Army, Air Force, Justice and the Federal Aviation Administration (FAA) follow with approximately 8,200, 4,700, 1,800, and 1,200, respectively. The private sector has over 26,000 frequency licenses held by various entities such as private citizens, businesses, utilities, state and local governments, etc.(3)
In terms of the total amount of HF spectrum allocated, the shared fixed and mobile allocated service is the largest followed by maritime mobile, amateur, fixed, broadcasting, aeronautical mobile, mobile, radio astronomy, and standard frequency and time signal services. The fixed and mobile services comprise 9,094 kHz of spectrum in 22 sub-bands with over 34,000 assignments. The maritime mobile service has over 32,000 assignments in 12 sub-bands that total 4,808 kHz. In 3,550 kHz, the amateur service supports approximately 400,000 licensed amateur radio operators in the United States in 10 sub-bands.(4) The fixed services are difficult to characterize because of close pairing with mobile assignments in multiple station configurations. In 15 sub-bands, the fixed service has over 8,700 assignments in 3,357 kHz of allocated spectrum. The broadcasting service has 2,930 kHz of allocated spectrum in eight sub-bands containing over 3,100 assignments. Broadcast use of the bands normally is on a scheduled basis and requires extensive global coordination to minimize interference on a given frequency. The aeronautical mobile service has 21 sub-bands and over 5,900 assignments in 2,026 kHz of allocated spectrum. The mobile service, for purposes of this report, comprises the one mobile and five land mobile bands with over 2,500 assignments in 985 kHz of allocated spectrum. In 170 kHz of allocated spectrum, the radio astronomy service contains about 35 assignments, while the standard frequency and time signal service has over 2,100 assignments in 80 kHz of allocated spectrum. Figure 3-1 depicts the U.S. HF frequency band allocations and their relative sizes.
The HF spectrum is finite and cannot be expanded to accommodate other users or additional radio services. For radio services seeking expansion to satisfy their spectrum requirements, it is usually at the expense of another radio service. In the NTIA Requirements Study, the fixed service was the only significant radio service where no additional spectrum requirement was expressed in the HF band. This portion of the study will examine the U.S. use of the HF spectrum and its spectrum flexibility. It is important for the reader to know that operations in the radio services of the HF band support numerous public benefits such as public safety, emergency medical assistance communications, weather observation reporting, environmental resource management, etc. There are no current plans to auction any HF spectrum.
In the HF band, there are 15 sub-bands allocated to the fixed service, 12 of which are shared between the Federal and non-Federal users. The remaining three are allocated for non-Federal use. An examination of this band revealed that the 12 shared sub-bands have over 8,700 assignments in 3,167 kHz of allocated spectrum. Further, a high degree of spectrum flexibility is evident in each frequency block. For instance, Federal usage in the 5005-5060 kHz band shows 446 assignments for the fixed service, while 177 assignments are maritime mobile, 283 assignments for mobile, 29 for aeronautical mobile, 10 for experimental, and 4 for radiodetermination. Figure 3-2 graphically depicts the general distribution of radio services using the 5005-5060 kHz fixed band. Other fixed service bands in this group of 12 bands generally resemble the extent of usage by other radio services. Table 3-1 below reflects the flexibility usage of the other 12 bands. The three fixed service bands allocated exclusively for non-Federal use contain approximately 100 assignments within 190 kHz of spectrum. Of the approximately 100 assignments, about 40 are held by Federal agencies. As can be seen in the fixed service bands, a significant amount of sharing by other radio services takes place among Federal users. Examination of the non-Federal licenses revealed little flexibility in the fixed service band usage where fixed operations and associated mobiles tended to be the licensed operations.
FIXED RADIO SERVICE BAND (kHz) |
5005-
|
9040-
|
9900-
|
11400-
|
12050-
|
15600-
|
17410-
|
18030-
|
18900-
|
19800-
|
21850-
|
22855-
|
| Fixed | 442 | 1093 | 239 | 580 | 832 | 1124 | 349 | 138 | 468 | 108 | 95 | 181 |
| Maritime Mobile | 340 | 321 | 16 | 167 | 333 | 411 | 83 | 9 | 66 | 10 | 0 | 40 |
| Aeronautical Mobile | 32 | 145 | 17 | 207 | 114 | 258 | 93 | 25 | 94 | 5 | 3 | 15 |
| Mobile | 120 | 128 | 15 | 114 | 171 | 130 | 48 | 13 | 88 | 0 | 0 | 1 |
| Experimental | 10 | 57 | 6 | 8 | 33 | 56 | 20 | 10 | 22 | 2 | 6 | 14 |
| Broadcasting | 0 | 0 | 0 | 4 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Radiolocation | 4 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
This radio service is used primarily for air traffic control and has two categories: aeronautical mobile (R) and aeronautical mobile (OR) services. The aeronautical mobile (R) service is intended for communications with aircraft or between aircraft relating to safety and regularity of flight, primarily along national or international civil air routes. The aeronautical mobile (OR) service is for communications with aircraft or between aircraft outside national or international civil air routes. As a general rule, HF frequencies are not used for aeronautical mobile (R) communications in the domestic services when aircraft are within the airspace of the conterminous United States. The need to use HF was eliminated through successful implementation of VHF air traffic control communications. The use of the aeronautical mobile (OR) HF frequencies is primarily to satisfy military aeronautical communications requirements.
The aeronautical mobile service has 21 bands totaling 2,026 kHz shared between the Federal and non-Federal users. Eleven bands are designated as aeronautical mobile (R) and the remaining 10 are designated aeronautical mobile (OR). Federal and non-Federal aeronautical stations that operate in the aeronautical mobile (R) service within the United States and Possessions are normally authorized by the FAA and the FCC. Frequencies in the bands allocated exclusively to the (OR) service are used primarily for the satisfaction of military aeronautical requirements. The aeronautical mobile (R) bands have 1,181 kHz of HF spectrum while the aeronautical mobile (OR) bands have 845 kHz of HF spectrum. There are over 5,800 assignments in the HF aeronautical mobile bands: over 1,100 in the (R) bands and over 4,600 in the (OR) bands. Less flexibility is shown in the aeronautical mobile bands than the fixed service primarily because of the operational and safety requirements associated with national and international aviation.
The maritime mobile bands are used primarily to communicate between and among fixed coast stations, ships and other offshore vessels. Specific uses include the command, control, and communications of U.S. Navy and USCG ships and vessels, distress and safety communications, treaty and law enforcement, drug interdiction, geological survey operations, and national marine fishery operations.
The maritime mobile service has 4,808 kHz of spectrum allocated in the 12 shared HF bands. Stations use this spectrum either exclusively to the maritime mobile service or on a shared basis with other radio services. The Federal agencies have approximately 13,000 assignments in the maritime mobile bands while there are almost 10,000 assignments for non-Federal users. Several international plans detail the specific uses of certain maritime mobile HF frequencies. National planning for the maritime mobile bands closely follows international use.(5)
Amateurs have been active in radio since its earliest days. The amateur service is the oldest radio service and pre-dates regulation of radiocommunications.(6) The amateur service allows its users to provide a unique service to the public while enjoying a popular, technical hobby. Radio amateurs have made significant contributions to the field of radio propagation, HF single-sideband radio, HF data communications systems, packet radio protocols and communications satellite design. Further, amateur radio continues to play an important role in disaster-relief communications where amateurs provide radiocommunications independent of the telephone network or other radio services, particularly in the first few days before relief agencies are at the scene and have set up disaster telecommunications services. It is estimated that there are in excess of 650,000 amateur radio operators in the United States (7) and over 2.4 million worldwide.
In the HF band, the amateur operator has a choice of narrow frequency bands each with different propagation properties for long-distance communications. The amateur operator can follow the changing maximum usable frequency (MUF) as propagation conditions change in the amateur bands and still be able to communicate. Having a good selection of frequencies is critical to maintain reliable communications for both voice and data operations in the HF band. All U.S. amateur and amateur-satellite allocations in the HF band are allocated on a primary basis for exclusive non-Federal use only.
The use of HF frequencies for the broadcasting service in the United States includes: i) operations by U.S.-based broadcasters who are trying to reach non-U.S. audiences; ii) operations by both U.S.-based and foreign-based broadcasters who are trying to reach U.S. audiences. Broadcasting services that are intended to be transmitted across international borders are HF broadcast operations subject to the ITU Radio Regulations. For decades, nations have made increasing use of the electromagnetic spectrum to conduct public diplomacy by broadcasting speech and music directly to receivers throughout the world. There is a total of 2,930 kHz of HF spectrum presently allocated in the United States for the broadcasting service within eight HF sub-bands. These bands are allocated on a primary basis to both Federal and non-Federal users.(8)
Rounding out the other radio services that have HF allocations are the standard frequency and time signal service, radio astronomy, land mobile, and on a secondary basis, the radiolocation service, with an allocation at 3230-3400 kHz. The standard frequency and time signal service is allocated on a shared, primary basis at 4995-5005, 9995-10005, 14990-15010, 19990-20010 and 24990-25010 kHz. The radio astronomy service is allocated on a primary basis to Federal and non-Federal users at 13360-13410 and 25550-25670 kHz. The land mobile service is allocated exclusively to non-Federal users on a primary basis at 25010-25070, 25210-25330, 26175-26480, 27410-27540, and 29700-29800 kHz.
In recent years, many developed countries have reduced fixed service operations in the HF band and rely on satellite, microwave, and submarine cables for long-distance communications to support their news services, radiotelexes, and diplomatic communications. However, HF communications still remains to support these functions, but as backup communications systems. At recent WRC's, more and more HF spectrum from the fixed service has been re-allocated to broadcasting, aeronautical mobile, and maritime mobile services. As this trend continues, lesser developed countries have become concerned about the amount of HF spectrum available in the fixed services for their domestic radiocommunications.
NTIA noted in its NTIA Requirements Study that the fixed service was the only significant radio service where no additional spectrum requirement was expressed in the HF band. Because of the domestic and international radiocommunications uses of the HF band, a review of U.S. neighboring countries' uses was conducted to see the impact any re-allocations done by the United States may have on them.
The ITU International Frequency List contains just over 2 million HF frequency registrations of which over 1 million (51%) are for Region 2 countries.(9) The United States has over 340,000 HF frequency registrations in Region 2. Of the 15 U.S. allocated fixed service bands in the HF spectrum, 11 of them correspond to the ITU frequency allocations for Region 2.
Any U.S. re-allocations to HF spectrum will affect at least our neighboring countries to the north and south. Careful planning will be needed to avoid interference between systems needing regional/global allocations and those operating in bands allocated domestically. The following figures reveal the distribution of HF assignments in the fixed service allocations of Region 2 countries that correspond to U.S. fixed service allocations.
Figure 3-3. Region 2 HF Registrations in the 9040 - 9400 kHz Band.
(1)Not all HF frequencies are equal. The ionosphere is the thin air that is electrically charged (ionized) by radiation from the sun. When ionized by solar radiation, the ionosphere can refract radio waves. If the wave is bent enough, it returns to Earth. This is the way long-distance radio signals travel. Alternatively, the radio wave travels into space. HF communications of up to 4000 kilometers are possible with one skip of the ionosphere; worldwide communications using several skips can take place if conditions are right. Two major factors determine the skip propagation possibilities between two points: the frequency in use and the level of ionization. The higher the frequency of the radio wave, the less it is bent by the ionosphere. The highest frequency at which the ionosphere bends radio waves back to a desired location on Earth is called the maximum usable frequency (MUF). The MUF for communications between two points depends on solar radiation strength and the time of day. The level of ionization is greatest during the day and during the summer months. The amount of radiation coming from the sun varies through the day, season and year. Sunspots vary in number and size over an 11-year cycle and more sunspot activity usually means more ionization of the ionosphere. As a result, the MUF tends to also be higher. When sunspots are low, radiation, and thus the MUF, is lower. The most heavily used portion of the HF band is between 3-11 MHz and the upper portions of the HF band are used heavily during high sunspot activity.
(2)This zero sum situation was evident at the WARC-79 when those advocating growth of the HF broadcast bands found strong opposition from lesser developed countries who feared a reduction of the fixed service bands. For the lesser developed countries, the fixed service bands carried radio teletype and utility services important and profitable.
(3)The FCC Master Frequency List does not reflect the hundreds of thousands of U.S. licensed amateurs authorized to operate in the HF amateur bands.
(4)Telephone Interview with Paul L. Rinaldo, Manager, Technical Relations, The American Radio Relay League (Jul. 31, 1996).
(5)Exceptions are indicated in the National Table of Frequency Allocations (Chapter 4).
(6)International Telecommunication Union, Excerpts of ITU Radiocommunication Texts Concerning the Amateur Service and Amateur-satellite Service, Foreword, 1995.
(7)Telephone Interview with Paul L. Rinaldo, Manager, Technical Relations, The American Radio Relay League (Jul. 31, 1996).
(8)Unlike users of other radiocommunications services, broadcasters, in particular HF broadcasters, do not control the receiver end of the communications circuit. HF broadcasters do control the transmitter end of the communications circuit and, in accordance with Recommendation 515 (WARC HFBC-87), have made progress in making HF broadcast transmitters convertible to single-sideband (SSB) operation. No such progress has been made, or is in the prospect, in making inexpensive SSB receivers broadly available throughout the world.
(9)ITU Radiocommunication Bureau, International Frequency List (Edition 13.6), March 16, 1994.