The Radio-Frequency Spectrum

Before 1930 the radio spectrum above 30 megahertz was virtually empty of man-made signals. Today, civilian radio signals populate the radio spectrum in eight frequency bands, ranging from very low frequency (VLF), starting at 3 kilohertz, and extending to extremely high frequency (EHF), ending at 300 gigahertz.

It is frequently convenient to express radio frequencies in terms of wavelength, which is the ratio between the speed of light through a vacuum (approximately 300 million metres per second) and the radio frequency. The wavelength of a VLF radio wave at 3 kilohertz is thus 100 kilometres, while the wavelength of an EHF radio wave at 300 gigahertz is only 1 millimetre. An important measure of the efficiency with which a transmitting antenna delivers its power to a remote receiving antenna is the effective isotropic radiated power (EIRP), measured in watts per metre squared. To achieve high EIRP the antenna dimensions should be several times larger than the largest transmitted wavelength. For frequencies below the medium frequency (MF) band, where wavelengths range upward from 100 metres, this is usually not practical; in these cases transmitters must compensate for low EIRP by transmitting at higher power. This makes frequency bands up through high frequency (HF) unsuitable for such applications as handheld personal radios, radio pagers, and satellite transponders, in which small antenna size and power efficiency are essential.

Two radio links can share the same frequency band or the same geographic area of coverage, but they cannot share both without interference. Therefore, international use of the radio spectrum is tightly regulated by the international Telecommunication Union (ITU). Each radio link is assigned a specific frequency band of operation, a specific transmitter radiation pattern and a maximum transmitter power. For example, a broadcast radio or television station may be authorized to broadcast only in certain directions and only at certain times of the day. Frequency bandwidths are also allocated, ranging from 300 hertz for radiotelegraphs to 10 kilohertz for voice-grade radiotelephones and more than 500 megahertz for multichannel digital radio relays in the telephone network.

VLF-MF

The very low frequency to medium frequency (VLF-MF) bands extend from three kilohertz to three megahertz, or wavelengths of 100 kilometres to 100 metres. These bands are used for low-bandwidth analog services such as long-distance radio navigation, maritime telegraph and distress channels, and standard AM radio broadcasting. Owing to insufficient available bandwidth, they are unsuitable for broadband telecommunication services such as television and FM radio. Because of the high conductivity of salt water, maritime radio transmissions at VLF can propagate via surface waves for thousands of kilometres.

HF

High-frequency (HF) radio is in the 100- to 10-metre wavelength band, extending from 3 megahertz to 30 megahertz. Much of the HF band is allocated to mobile and fixed voice communication services requiring transmission bandwidths of less than 12 kilohertz. International (shortwave radio) broadcasting also is conducted in the HF band; it is allocated to seven narrow bands between 5.9 megahertz and 26.1 megahertz.

The primary mode of propagation for HF radio transmissions is reflection off the ionosphere, a series of ionized layers of the atmosphere ranging in altitude from 50 to 300 kilometres above the Earth. Ionization is caused primarily by radiation from the Sun, so that the layers vary in height and in reflectivity with time. During the day the ionosphere consists of four layers located at average altitudes of 70 kilometres (D layer), 110 kilometres (E layer), 200 kilometres (F₁ layer), and 320 kilometres (F₂ layer). At night the D and E layers often disappear, and the F₁ and F₂ layers combine to form a single layer at an average altitude of 300 kilometres. Reflective conditions thus change with time. During the day an HF radio wave can reflect off the E, F₁, or F₂ layers. At night, however, it can reflect only off the high-altitude F layer, creating very long transmission ranges. (The D layer is nonreflecting at HF frequencies and merely attenuates the propagating radio wave.) In the lower HF band, transmission ranges of many thousands of kilometres can be achieved by multiple reflections, called skips, between the Earth and layers of the ionosphere.

Strong ionospheric reflections occur only below a maximum usable frequency (MUF), which is determined by the zenith angle of the incident ray and by the ionization density of the reflecting layer. In general, the MUF is higher at larger zenith angles and higher ionization densities. During the peaks of the 11-year sunspot cycle, solar ultraviolet radiation produces the highest ionization densities. These sunspot peaks can last several days or months, depending on the persistence of sunspot visibility, producing a sporadic E layer that often can be used for multiple-skip communications by amateur radio operators at frequencies up to 144 megahertz—well into the VHF band.

VHF-UHF

The very high frequency to ultrahigh frequency (VHF-UHF) bands are in the wavelength range of 10 metres to 10 centimetres, extending from 30 megahertz to 3 gigahertz. Some of these bands are used for broadcast services such as FM radio, VHF television (54–88 megahertz for channels 2–6, 174–220 megahertz for channels 7–13), and UHF television (frequency slots scattered within 470–806 megahertz). The UHF band also is used for studio and remote-pickup television relays, microwave line-of-sight links (1.7–2.3 gigahertz), and cellular telephony (806–890 megahertz). Parts of the band are used for radio navigation applications, such as instrument landing systems (108–112 megahertz), military aircraft communications (225–400 megahertz), air traffic control radio beacons (1.03–1.09 gigahertz), and the satellite-based Navstar global positioning system (1.575-gigahertz uplink and 1.227-gigahertz downlink).

Powerful UHF transmitters can achieve beyond-the-horizon transmission ranges by scattering transmitted energy off layers of the troposphere (the lowest layer of the atmosphere, where most clouds and weather systems are contained). Unlike signals in the longer-wavelength HF band, for which layers in the atmosphere appear as relatively smooth reflective surfaces, signals in the shorter-wavelength UHF band reflect off small irregularities in the atmospheric layers as if these irregularities were randomly oriented granular reflectors. The reflectors disperse the propagating UHF signal in many directions, so that only a fraction of the transmitted signal power may reach the receiver. In addition, owing to unpredictable disturbances in atmospheric conditions, significant fading can occur over a given path, at a given time, and at a given radio frequency. For this reason a tropospheric scatter relay typically uses combinations of space, time, and frequency diversity techniques. A typical relay links two large terminals across spans of 320 to 480 kilometres (200 to 300 miles) and carries up to 100 voice channels.

SHF-EHF

The superhigh frequency to extremely high frequency (SHF-EHF) bands are in the centimetre to millimetre wavelength range, which extends from 3 gigahertz to 300 gigahertz. Typical allocated bandwidths in the SHF band range from 30 megahertz to 300 megahertz—bandwidths that permit high-speed digital communications (up to 1 gigabit per second). In addition to degradation from fading and from atmospheric attenuation radio waves in the SHF-EHF band undergo high penetration losses as they propagate through the exterior walls of buildings. Because of the severe atmospheric attenuation, and in particular rainfall scattering losses, the EHF band is currently the least populated radio band for terrestrial communication. However, it has been used for intersatellite communication and satellite radionavigation—applications in which atmospheric attenuation is not a factor.

UNIT 16