ENCYCLOPEDIA BRITANNICA
Reflected propagation
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.
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.
(O3), triatomic allotrope of oxygen (a form
of oxygen in which the molecule contains three atoms instead of two as in the
common form) that accounts for the distinctive odour of the air after a
thunderstorm or around electrical equipment. The odour of ozone around
electrical machines was reported as early as 1785; ozone's chemical
constitution was established in 1872. Ozone is an irritating, pale blue gas
that is explosive and toxic, even at low concentrations. It occurs naturally in
small amounts in the Earth's stratosphere, where it absorbs solar ultraviolet
radiation, which otherwise could cause severe damage to living organisms on the
Earth's surface. Under certain conditions, photochemical reactions between
nitrogen oxides and hydrocarbons in the lower atmosphere can produce ozone in
concentrations high enough to cause irritation of the eyes and mucous
membranes.
Ozone usually is manufactured by passing an electric
discharge through a current of oxygen or dry air. The resulting mixtures of
ozone and original gases are suitable for most industrial purposes, although
purer ozone may be obtained from them by various methods; for example, upon
liquefaction, an oxygen-ozone mixture separates into two layers, of which the
denser one contains about 75 percent ozone. The extreme instability and
reactivity of concentrated ozone makes its preparation both difficult and
hazardous.
Ozone is 1.5 times as dense as oxygen; at –112° C
(–170° F) it condenses to a dark blue liquid, which freezes at -251.4° C (-420°
C). The gas decomposes rapidly at temperatures above 100 °C (212° F) or, in the
presence of certain catalysts, at room temperatures. Although it resembles
oxygen in many respects, ozone is much more reactive; hence, it is an extremely
powerful oxidizing agent, particularly useful in converting olefins into
aldehydes, ketones, or carboxylic acids. Because it can decolorize many
substances, it is used commercially as a bleaching agent for organic compounds;
as a strong germicide it is used to sterilize drinking water as well as to
remove objectionable odours and flavours. See also ozonosphere.