Since its radio waves pass through walls or desktops, ________-capable devices are very popular.

In atmospheric propagation the electromagnetic wave travels through the air along a single path from transmitter to receiver. The propagation path can follow a straight line, or it can curve around edges of objects, such as hills and buildings, by ray diffraction. Diffraction permits mobile phones to work even when there is no line-of-sight transmission path between the phone and the base station.

Atmospheric attenuation is not significant for radio frequencies below 10 gigahertz. Above 10 gigahertz under clear air conditions, attenuation is caused mainly by atmospheric absorption losses; these become large when the transmitted frequency is of the same order as the resonant frequencies of gaseous constituents of the atmosphere, such as oxygen (O2), water vapour (H2O), and carbon dioxide (CO2). Atmospheric attenuation does not change gradually across the spectrum; there exist short spectral “windows,” which specify frequency bands where transmission occurs with minimal clear-air absorption losses. Additional losses due to scattering occur when airborne particles, such as water droplets or dust, present cross-sectional diameters that are of the same order as the signal wavelengths. Scattering loss due to heavy rainfall is the dominant form of attenuation for radio frequencies ranging from 10 gigahertz to 500 gigahertz (microwave to submillimetre wavelengths), while scattering loss due to fog dominates for frequencies ranging from 103 gigahertz to 106 gigahertz (infrared through visible light range).

For low radio frequencies, terrestrial antennas radiate electromagnetic waves that travel along the surface of the Earth as if in a waveguide. The attenuation of surface waves increases with distance, ground resistance, and transmitted frequency. Attenuation is lower over seawater, which has high conductivity, than over dry land, which has low conductivity. At frequencies below 3 megahertz, surface waves can propagate over very large distances. Ranges of 100 km (about 60 miles) at 3 megahertz to 10,000 km (6,000 miles) at 1 kilohertz are not uncommon.

Sometimes part of the transmitted wave travels to the receiver by reflection off a smooth boundary whose edge irregularities are only a fraction of the transmitted wavelength. When the reflecting boundary is a perfect conductor, total reflection without loss can occur. However, when the reflecting boundary is a dielectric, or nonconducting material, part of the wave may be reflected while part may be transmitted (refracted) through the medium—leading to a phenomenon known as refractive loss. When the conductivity of the dielectric is less than that of the atmosphere, total reflection can occur if the angle of incidence (that is, the angle relative to the normal, or a line perpendicular to the surface of the reflecting boundary) is less than a certain critical angle.

Common forms of reflected wave propagation are ground reflection, where the wave is reflected off land or water, and ionospheric reflection, where the wave is reflected off an upper layer of the Earth’s ionosphere (as in shortwave radio; see below The radio-frequency spectrum: HF).

Some terrestrial radio links can operate by a combination of atmospheric wave propagation, surface wave propagation, ground reflection, and ionospheric reflection. In some cases this combining of propagation paths can produce severe fading at the receiver. Fading occurs when there are significant variations in received signal amplitude and phase over time or space. Fading can be frequency-selective—that is, different frequency components of a single transmitted signal can undergo different amounts of fading. A particularly severe form of frequency-selective fading is caused by multipath interference, which occurs when parts of the radio wave travel along many different reflected propagation paths to the receiver. Each path delivers a signal with a slightly different time delay, creating “ghosts” of the originally transmitted signal at the receiver. A “deep fade” occurs when these ghosts have equal amplitudes but opposite phases—effectively canceling each other through destructive interference. When the geometry of the reflected propagation path varies rapidly, as for a mobile radio traveling in an urban area with many highly reflective buildings, a phenomenon called fast fading results. Fast fading is especially troublesome at frequencies above one gigahertz, where even a few centimetres of difference in the lengths of the propagation paths can significantly change the relative phases of the multipath signals. Effective compensation for fast fading requires the use of sophisticated diversity combining techniques, such as modulation of the signal onto multiple carrier waves, repeated transmissions over successive time slots, and multiple receiving antennas.

The very low frequency to medium frequency (VLF-MF) bands extend from 3 kilohertz to 3 megahertz, or wavelengths of 100 km 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.

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 about 50 to 300 km (about 30 to 200 miles) 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 km (D layer), 110 km (E layer), 200 km (F1 layer), and 320 km (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 km. 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.

Schematic diagram showing the propagation of high-frequency (shortwave) radio waves by reflection off the ionosphereSpecific ionization conditions vary greatly between day (left) and night (right), causing radio waves to reflect off different layers of the ionosphere or transmit through them, depending upon their frequency and their angle of transmission. Under certain conditions of location, ionization, frequency, and angle, multiple “skips,” or reflections between ionosphere and Earth, are possible. At night, with no intervening layers of the ionosphere present, reflection off the F layer can yield extremely long transmission ranges.

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.

The very high frequency to ultrahigh frequency (VHF-UHF) bands are in the wavelength range of 10 metres to 10 cm (33 feet to 4 inches), extending from 30 megahertz to 3 gigahertz. Some of these bands are used for broadcast services such as FM radio (in the United States, 88–108 megahertz), 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 (GPS; 1.575-gigahertz uplink and 1.227-gigahertz downlink). In the North American over-the-air digital broadcast system, a television equipped with a QAM (quadrature amplitude modulation) tuner can decode digital signals, which are broadcast within each 6-megahertz-wide band already assigned to that station—i.e., a station that now broadcasts analog signals on channel 7, which operates from 174 to 180 megahertz, uses the same bandwidth to broadcast digital signals.

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 km (200 to 300 miles) and carries up to 100 voice channels.

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 radio navigation—applications in which atmospheric attenuation is not a factor.